Stellar, Solar and Laboratory Spectra: The History of Lockyer's Proto-elements

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Stellar, Solar andLaboratory Spectra: TheHistory of Lockyer's Proto-elementsMatteo Leone a & Nadia Robotti aa Department of Physics , University ofGenoa , via Dodecaneso 33, Genoa, 16100,ItalyPublished online: 05 Nov 2010.

To cite this article: Matteo Leone & Nadia Robotti (2000) Stellar, Solar andLaboratory Spectra: The History of Lockyer's Proto-elements, Annals of Science,57:3, 241-266, DOI: 10.1080/00033790050074156

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A n n a l s o f S c i e n c e , 57 (2000), 241± 266

Stellar, Solar and Laboratory Spectra:The History of Lockyer’s Proto-elements

M a t t e o Le o n e and N a d i a R o b o t t i

Department of Physics, University of Genoa via Dodecaneso 33, 16100 Genoa,

Italy

Received 19 January 1999; revised version accepted 30 August 1999

SummaryUntil now studies on the historical development of atomic spectroscopy have

focused on three main aspectsÐ its ® rst applications as a method of chemical

analysis, the formulation of spectral laws (the empirical laws of Balmer, Rydberg,

Kayser± Runge, etc.), and the rise of the old quantum theory. These developments

of spectroscopy were based on the same assumption: the invariance of the atomic

spectrum after ® xing the chemical element (if any degree of change was expected,

this was considered the result of magnetic or electric ® elds). This paper shows that

running alongside these lines of research there was another, no less important area

of study based on the negation of this assumption. The focus of these latter studies

was the behaviour of atomic spectra as a function of diŒerent working conditions.

English astrophysicist Sir Norman Lockyer played a central role in this research.

While studying the eŒect of temperature on spectral lines and comparing stellar

spectra with laboratory spectra, he discovered a new class of spectral lines (the

ènhanced lines’ ). In an attempt to explain the origin of these lines, Lockyer, at the

beginning of 1897 (a few months before J. J. Thomson’s discovery of the electron),

hypothesized the `dissociation ’ of the chemical elements into simpli® ed forms of

matter, named proto-elements. Lockyer’s studies are mentioned by Thomson in the

paper which introduces the corpuscle (or electron) for the ® rst time. In this paper

we have tried to reconstruct the history of Lockyer’s proto-elements and highlight

the links between them and Thomson’ s corpuscle. Our analysis has shown that not

only the famous studies on discharges in rare® ed gases but also Lockyer’s stellar

spectroscopy played an important role in the work of J. J. Thomson, convincing

him that the atom was complex and divisible.

Contents1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2412. The mutability of spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2433. The complexity of the atom .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2454. Ènhanced lines ’ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2465. A new eŒect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2546. Objections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2567. Lockyer’s proto-elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2578. Proto-elements and corpuscles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2629. Epilogue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265

1. Introduction

Spectra are the languages of atoms; each element always speaks its own

characteristic and invariable language, the same on earth and in the farthest

Annals of Science ISSN 0003-379 0 print/ISSN 1464-505X online ’ 2000 Taylor & Francis Ltdhttp://www.tandf.co.uk/journals

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242 Matteo Leone and Nadia Robotti

stars. It is this very invariability that enables us to determine the wavelengths

of the lines with extraordinary precision and that promises a great certainty and

success to the synthesis of numerical values.1

This was how in 1900 Rydberg summed up the proposition about the singleness

of atomic spectra that provided the basis for spectroscopy in the last decades of the

nineteenth century. This assumption, which was derived from KirchhoŒ’s discovery

(1859) that emission and absorption spectra were identical, enabled spectroscopy to

become on the one hand a powerful instrument of chemical analysis and on the other

an original method of classifying stars.2 At the same time, the availability of precise

spectral measurements enabled ® rst Balmer and then Rydberg, Kayser and Runge to

formulate the mathematical laws the atomic spectra seemed to obey.

These studies shared the primary goal of a precise classi® cation of atomic spectra,

independently of the physical conditions in which they were manifest.

Nevertheless, as we shall try to show in this paper, from 1873 onwards another no

less important type of study ran alongside it, aimed at studying the behaviour of

atomic spectra in diŒerent working conditions. English astrophysicist Sir Norman

Lockyer played a leading role in this research. In 1879 Lockyer proposed a

dissociation of elements into simpli® ed forms of matter identi® able via spectral

analysis ( ® rst dissociation hypothesis).

In the following years, studying the eŒect of temperature on spectral lines and

comparing stellar spectra with laboratory spectra, he discovered a new class of

spectral lines (the ènhanced lines’ ). In an attempt to explain the origin of these lines,

Lockyer, at the beginning of 1897 (i.e. a few months before J. J. Thomson’ s discovery

of the electron), discovered that such lines enabled him to formulate a new

dissociation criterion of chemical elements into simpli® ed forms of matter called

proto-elements (second dissociation hypothesis). Lockyer’ s studies are mentioned by

Thomson in the paper which introduces the corpuscle (or electron) for the ® rst time.

The success of the explanatory model proposed by Lockyer was spectacular. His

dissociation hypothesis predicted events that no theory had so far been able to

foresee. It successfully explained the diŒerences between the chromospheric spectrum

and Fraunhofer’s spectrum; it successfully created a framework for stellar

thermometry based on the extension of the ultraviolet component ; and so on. His

model also explained the diŒerent shift of lines H and K of calcium with respect to

other spectral lines as a result of pressure.

Not all aspects of Lockyer’s hypothesis were, however, con® rmed. As dissociation

was conceived as a `fragmentation ’ into comparable masses, the hypothesis on the

one hand explained the thermometric classi® cation of the stars in terms of variations of

atomic weight, while on the other it forced Lockyer to make use of ad hoc hypotheses

to explain the irregularities his interpretations required, or alternatively `skim over ’

the lack of a chemical con® rmation of the dissociation process.

A historical analysis of the English astrophysicist ’s research shows that the

successes and limits of the dissociation hypothesis were a direct consequence of an

1 J. R. Rydberg, `La distribution des raies spectrales ’ , Congre’ s International de Physique, Rapports(Paris, 1900), 200± 24.

2 On the role played by Bunsen in developing spectra studies in collaboration with KirchhoŒsee FrankA. J. L. James, `The Establishment of Spectro-Chemical Analysis as a Practical Method of QualitativeAnalysis, 1854± 1861 ’ , Ambix, 30 (1983), 30± 53. See also Frank A. J. L. James, `The Practical Problems of`̀ New’ ’ Experimental Science: Spectro-chemistry and the Search for Unknown Chemical Elements inBritain 1860± 1869 ’ , British Journal for the History of Science, 21 (1988), 181± 94.

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Stellar, Solar and Laboratory Spectra 243

original working technique, based on constant comparison between observed and

laboratory data. At the base of this methodology was frequent recourse to principles

of natural continuity and uniformity. Nevertheless, as often happens when reasoning

is based on analogy, Lockyer confounded analogy with equivalence, interpreting

àtomic dissociation ’ as a fragmentation of masses similar to molecular dissociation.

As we shall show, while this interpretation was to be abandoned, it nevertheless

played a key role in the ® rst studies of the subatomic components of matter.

2. The mutability of spectra

Since the second half of the nineteenth century, it had been known that several

elements seemed to give diŒerent line spectra according to the working temperature.3

The intensity of the lines varied with temperature in a complicated manner, and if the

temperature was su� ciently high, new lines, previously absent from the spectrum,

might appear. The main way to vary the temperature was to use two diŒerent

spectrum-generating devices, the electric arc and the spark.4 The spark was known to

produce higher temperatures than the arc. In the former, a direct current is applied

across two poles made of the material whose spectrum is under observation. In the

spark method, an electric spark is produced when a high potential is discharged from

the induction coil of a condenser across a spark gap.

Before 1897 the evidence, which did not ® t in with the assumption of spectral

uniqueness, had been explained away: some attempted to explain these phenomena

in terms of impurities ; other researchers looked for an explanation in the `bell

hypothesis ’ (or, the idea that the appearance of the new lines was due to the greater

intensity of collisions among particles owing to the higher source temperature). From

1873 onwards another possible interpretation of these phenomena was put forward

by English astrophysicist Norman Lockyer (1836± 1920), based on the analysis of

laboratory and solar spectra.5

It had been known for some time that the spectrum obtained for various salts

diŒered when using a ¯ ame or an electric arc and sparkÐ a ¯ ame gives rise to band

spectra, while sparks or the arcs generate line spectra. This phenomenon was

explained in 1862 (by Mitscherlich and, independently, by Roscoe and Clifton), as the

dissociation of salt into its elements as a result of the higher temperature of the arc

and spark. Band spectra are therefore related to the molecule, while line spectra refer

to their constituent elements. The change from band to line spectra is clearly caused

by the dissociation of molecules into atoms owing to the higher temperature involved.

By analogy, Lockyer suggested in 1873 that the new lines seen in the transition

from a lower to a higher temperature source (for example in passing from arc to

3 Cf., for example, A. J. A/ ngstrom, Recherches sur le Spectre Solaire, Uppsala, 1868. For a detailedanalysis of spectroscopical researches in the XIX century see William McGucken, Nineteenth-CenturySpectroscopy (Baltimore and London, 1969), 48± 9.

4 For a description of the mechanisms of spectrum production see E. C. R. Baly, Spectroscopy, 3 vols(London, 1905), I ; (London, 1912), II ; (London, 1929), III.

5 Norman Lockyer, `Researches in Spectrum Analysis in connection with the spectrum of the sun ’ ,Philosophical Transactions, 164 (1873± 74). LockyerÐ founder of Nature and discoverer of the helium inthe sun’s atmosphereÐ had been a pioneer in the application of the spectroscope to the sun and stars. In 1868he discovered (together with P. J. C. Janssen) a spectroscopic method of observing solar prominenceswithout the aid of an eclipse (on his solar researches see the paragraph `New Lines ’ in this paper). Hisinterests ranged from the constitution of matter to archaeology (he is regarded as the `founder ’ of so± calledarchaeo-astronomy) and from astronomy to golf rules. While many of his astrophysical researches were inadvance of his time, the speculative and conjectural style of his writings did not allow his theories wideacceptance among his contemporaries.

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spark) were caused by a dissociation of the atom induced by temperature Ð a

dissociation that led to `simpli® ed forms of matter’ .

In 1879 he presented a complete version of his original dissociation hypothesis and

formulated his `basic lines ’ theory. His idea was that these simpler forms of matter

were common to the diŒerent chemical elements and that the presence of coincident

lines (the `basic lines ’ ) in diŒerent spectra was evidence of such simpli® ed forms.

Furthermore, the hotter the source, the more simpli® ed the spectrum as a result of the

presence of these common lines.6

Lockyer’s hypothesis stood more or less unchanged until late 1896. In the

meantime, however, evidence was building up against the accuracy of the empirical

data on which this idea was based. The coincidences between the lines common to the

various elements (as seen in the solar spectrum) turned out to be accidental and due

to the poor resolution of the spectroscopes used.7 An important role was also ascribed

to impurities present. For example in 1880 G. D. Liveing and J. Dewar showed that

Lockyer’ s samples did not contain pure magnesium but a compound of magnesium

and hydrogen.8 Lockyer also posited that each elementary substance can be

decomposed into as many simpler substances as there are lines in that element’s

spectrum. This meant, for example, that iron would have a molecular structure so

complex that it would include at least 1200 diŒerent simpler substances Ð a result that

seemed unacceptable.9 During the 1890s further data derived from the analysis of the

solar spectrum and used to support the dissociation theory were brought into

question. For example, the unexplained lines in sunspots turned out to be due not to

an alleged dissociation of elements but to the presence of vanadium and other

ordinary elements.10

In spite of the deepening discredit into which the `basic lines ’ theory was falling,

Lockyer’ s dissociation hypothesis continued to have a following among chemists and

spectroscopists, 11 even though it was rejected by most academics. Arthur Schuster

(1851± 1934) was an example of such scepticism among scientists. In 1897 he wrote as

follows in an appendix to an article by Lockyer:

Had Mr Lockyer con® ned himself to bringing forward his hypothesis as one

which is legitimate, consistent and deserving of attention, many of us would, I

think, have agreed that he has made a good case. But he claims his theory as the

only one that can explain the facts, and dismisses as unphilosophical the only

alternative which he discusses. I think that in this he has gone too far ¼ 12

6 For a discussion of the ® rst dissociation hypothesis see W. H. Brock, `Lockyer and the chemists : The® rst dissociation hypothesis ’ , Ambix, 16 (1969), 81± 99.

7 Cf. C. A. Young, `Spectroscopic notes, 1879± 1880 ’ , American Journal of Science, 20 (1880), 353± 8 ;G. D. Liveing and J. Dewar, Òn the identity of spectral lines of diŒerent elements ’ , Proceedings of theRoyal Society (London ), 32 (1881), 225± 30.

8 G. D. Liveing and J. Dewar, Òn the spectrum of magnesium and lithium’, Proceedings of the RoyalSociety (London ), 30 (1880), 93± 9 ; Ìnvestigations on the spectrum of magnesium. No 1 ’ , Proceedings ofthe Royal Society (London ), 32 (1881), 189± 203. In these articles the two researchers showed that Lockyer’ssamples did not contain pure magnesium but a compound of magnesium and hydrogen.

9 W. N. Hartley, Òn homologous spectra ’ , Journal of Chemical Society, London, 43 (1883), 390± 400.10 A. L. Cortie, Monthly Notices of the Royal Astronomical Society, 58 (1898), 370.11 Cf., for example, William Crookes, Òn the nature and origin of the so-called elements ’ , Report of

the British Association (1886), 558± 76 ; W. C. Roberts-Austen, `Metals at high temperature ’ , Proceedingsof the Royal Institution, 13 (1892), 502± 18 ; J. M. Eder and E. Valenta, Ù$ ber die verschiedenen Spectrendes Quecksilbers ’ , Denkschriften der Kaiserlichen Akad. der Wissen, Vienna, 61 (1894), 401± 30.

12 Arthur Schuster, Òn the chemical constitution of the stars ’ , Proceedings of the Royal Society(London ), 61 (1897), 209± 13, 209. Schuster’s important contributions ranged from spectroscopy (heindependently discovered the Rydberg± Schuster law which relates the diŒerent spectral series of the same

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Dissociation was implicit in Lockyer’s physical view of matter. The originality of

Lockyer’ s work lay in his rejection of the well-rooted paradigm of the uniqueness of

spectra and of the idea of the elementary nature of atoms. For Lockyer the mutability

of spectra signalled the dissociation of atoms, a dissociation that obviously entailed

the non-elemental nature of matter.

3. The complexity of the atom

One of the central controversies in chemistry in the nineteenth century concerned

the elemental nature of atoms and therefore questioned Daltonian atomism, which

de® ned the chemical elements as speci® c and indivisible elementary atoms.13

As we know, in 1815 William Prout suggested that all chemical elements were

composed of hydrogen atoms because the experimental evidence seemed to show that

the atomic weights of all the elements were whole numbers.14 In its original form,

Prout’ s theory became increasingly indefensible. As shown by measurements of

atomic weight by Berzelius, Turner, and Dumas, there was a wealth of ànomalies ’15

and the theory no longer had an experimental basis. In 1860 the credibility of the idea

that atoms were made up of hydrogen took another blow in the form of Stas’s

confutation.16 His measurements showed that the atomic weight of chlorine was 35.5

and not 35, and that Prout’ s law was thus without foundation. This conclusion

enjoyed a general consensus.

The confutation, however, addressed only the original form of Prout’s hypothesis.

The fact that many atomic weights seemed to be signi® cantly close to whole numbers

led various chemists and physicists to suppose there must anyway be a certain

regularity in the structure of the elements. One way to maintain Prout’s theory, in a

modi® ed form, was simply to reduce the size of the basic unit to accommodate the

experimental values. Shortly before Stas published his results, Dumas proposed an

elemental unit that was a quarter or a half of the atomic weight of hydrogen.17 A

similar proposition was made by Marignac.18 A few years later Za$ ngerle even

suggested choosing a mass around a thousand times smaller than that of hydrogen.19

Another way to defend Prout’s hypothesis was to contest the theories on which

experimental chemical puri® cation techniques were based. In 1860 Marignac wrote

that though he was convinced of the accuracy of Stas’ s experimental results, `[there

is no evidence that] the diŒerences observed between his results and those required by

Prout’ s law cannot be explained by the imperfect nature of the experimental

substance), to discharge through rare® ed gases (in 1884 he tried to obtain the charge/mass ratio for cathoderays by using a magnetic ® eld), and to terrestrial magnetism.

13 For a brief history of the concept of the atom in the nineteenth century, see Helge Kragh, `JuliusThomsen and 19th century Speculations on the Complexity of Atoms’ , Annals of Science, 39 (1982), 61± 75.

14 William Prout, Òn the relation between the speci® c gravities of bodies in their gaseous states andthe weights of their atoms’ , Annals of Philosophy, 6 (1815), 321± 30. An analysis of the history and impactof Prout’s hypothesis can be found in W. H. Brock, From Protyle to Proton (Bristol and Boston, 1985).

15 Imre Lakatos describes Prout’s work as à research programme that proceeds in a sea of anomalies ’ .See `Falsi® cationism and the Methodology of Scienti® c Research Programmes ’ , in Criticism and theGrowth of Knowledge, edited by Imre Lakatos and Alan Musgrave (Cambridge, 1970), 138± 40.

16 Jean Stas, `Researches on the mutual relations of atomic weights ’ , Bull. Acad. R. Belg., 10 (1860),208± 336.

17 J. B. Dumas, `Memoire sur les equivalents des corps simple ’ , Annales de Chimie, 55 (1859), 129± 210.18 J. C. G. Marignac, `Commentary on Stas ’ researches on the mutual relations of atomic weights ’ ,

Bibl. Univ., 9 (1860), 97± 107.19 Max Za$ ngerle, UX ber die Natur der Elemente und die Beziehungen der Atomgewichte derselben zu

einander und zu den physikalischen und chimischen Eigenschaften (Munich 1882).

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methods ’ .20 In 1886, W. Crookes observed that `not a few chemists of admitted

eminence consider that we have here [in Prout’s law] an expression of the truth,

masked by residual or collateral phenomena which we have not yet succeeded in

eliminating ’ .21

A third way to save Prout’s theory was to demonstrate its validity by spectrum

analysis. Lockyer’s work22 quoted above and all that follows below must be read in

this light. But as we have already said, Lockyer went beyond a `simple ’ recognition

of the complexity of atoms and also suggested they could be dissociated into other

simpler versions of themselves, thus anticipating what Thomson showed in 1897 with

the discovery of his corpuscle. Despite the contested experimental evidence, Prout’s

theory continued to exert a certain in¯ uence over physicists and chemists till the end

of the nineteenth century. There was no general consensus of opinion however. As J.

H. Scott was to write, ìn that period neither Daltonian atomism nor neo-

Proutianism ¼ enjoyed the status of paradigm’ .23 For some chemists24 atoms were

elementary, while for others they were undivided composite or ontologically indivisible

bodies.25

The in¯ uence of Lockyer’s dissociation hypothesis, which went beyond commonly

accepted ideas, was to remain limited as long as there was no reliable criterion to

demarcate dissociated and non-dissociated atoms. Lockyer found that criterion in

1897, the year in which he stated his second theory of dissociation.

4. Ènhanced lines’

In January 1897 Lockyer reported to the Royal Society on the experiments he had

been carrying out in the laboratory since 1879 on the eŒects of electricity on the

spectra of metals. Lockyer wrote:

In continuation of investigations communicated to the Royal Society in 1879

and 1881, on the eŒect of high-tension electricity on the line spectra of metals,

I have recently used a more powerful current and larger jar surface than that I

formerly employed. The former work consisted in noting (1) the lines brightened

in passing a spark in a ¯ ame charged with metallic vapours, and (2) the lines

brightened on passing from the arc to the spark. It was found, in the case of

iron, that two lines in the visible spectrum at 4924,1 and 5018,6, on Rowland’s

scale, were greatly enhanced in brightness, and were very important in solar

phenomena.26

As can be seen in the former statements, since the 1870s he had been aware that

certain lines were ènhanced ’ when passing from the arc to the spark spectrum. But

20 J. C. G. Marignac (note 18).21 William Crookes (note 11), in which he suggested that `some of the atomic weights represent a merely

average value ’ .22 Crookes was deeply in¯ uenced by Lockyer’s convinction that he could use spectroscopy to

demonstrate the dissociation of elements. In his `Presidential Address to the Chemistry Section of theBritish Association ’ (note 11), Crookes claimed he had demonstrated spectroscopically the dissociation ofyttrium into its basic components. In fact, his spectral results were produced simply by removingimpurities.

23 According to Helge Kragh (note 13), 40.24 The involvement of physicists was to become signi® cant after the discovery of the electron.25 J. H. Scott, `The nineteenth century atom: undivided or indivisible? ’ , Journal of Chemical Education,

36 (1959), 64± 7.26 Norman Lockyer, Òn the iron lines present in the hottest stars. Preliminary note ’ , Proceedings of

the Royal Society (London ), 60 (1897), 475± 76.

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Figure 1. Comparison of metallic lines with lines of `cleivete gases ’ in star of increasingtemperature according to Lockyer. (From Norman Lockyer, Òn the Chemistry ofHottest Stars ’ , Proceedings of the Royal Society (London ), 61 (1897), 172.)

it was only in 1897 that he realized the frequency of the phenomenon and its vital

importance for the study of solar and star spectra. In 1885 Lockyer brought the then

young Arthur Fowler into his laboratory, ® rst as a research student, then as assistant.

Fowler worked there till 1901 when Lockyer resigned from the Royal College of

Science, where Fowler later replaced him. According to Herbert Dingle, ìt was

Lockyer’ s practiceÐ by no means unusual in those timesÐ to publish the work of his

assistants as his own. Fowler wrote the words `̀ My workÐ AF ’ ’ on his own copy of

the article for the Royal Society.’27 Dingle says that Fowler often told him that

Lockyer kept the paper for some time before deciding whether or not to publish it and

that Fowler believed Lockyer was held back by doubts about whether to take

responsibility for it or follow a safer line and let Fowler do it. As regards this delay,

Arthur J. Meadows, author of a biography of Lockyer, remarks that ìf the enhanced

lines were to be related to dissociation, this required a complete alteration in

27 Herbert Dingle, À hundred years of spectroscopy ’ , British Journal for the History of Science, 1 :3(1963), 199± 216. Dingle is convinced of the truth of Fowler’s recollection because of his friendship withhim and also the cold dryness of the article, so unlike the speculative style of Lockyer. Perhaps the lackof a full theoretical explanation delayed his decision but `whatever the case, Lockyer did take responsibilityfor the discovery and provoked clamorous opposition, which seems odd since anyone could quite simplyhave tested the phenomenon for themselves ’ .

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Lockyer’ s conceptual understanding of dissociation. The `̀ basic lines ’ ’ approach [see

`The mutability of spectra’ ] had suggested that diŒerent elements broke down on

heating to the same simpler substances. The `̀ enhanced lines ’ ’ approach implied

rather that each element dissociated into diŒerent, equally unique, forms of the

speci® c element concerned.’28

However, neither Dingle’s nor Meadows’s suppositions about Lockyer’s argu-

ments are supported by Alfred Fowler’s ® rst-hand witness : Ìn view of the importance

of this paper [the paper, dated January 1897, to the Royal Society], and the perfectly

de® nite character of the experimental data, I never understood why Lockyer witheld its

publication for several months after its completion ’ (emphasis added).29

Anyway, in January 1897 Lockyer stated:

The recent work carries these results into the photographic region. The result is

interesting and important, since seven additional lines have been found to have

their brightness enhanced at the highest temperature.26

On the basis of these results obtained by laboratory spectroscopy (see Table 1)

Lockyer concluded that in a space heated to the temperature of the spark and shielded

from lower temperatures, the iron spectrum should comprise exclusively enhanced

lines. One important con® rmation came from stellar spectroscopy. By de® ning the

hottest stars as those with the most extensive ultraviolet spectrum, it was observed

that the iron in them was represented in practice only by the enhanced lines, as those

forming the arc spectrum were almost completely absent.

Table 1. Lines of iron enhanced in the spark. (From Norman Lockyer, Òn the iron linespresent in the hottest Stars. Preliminary note. ’ , Proceedings of the Royal Society (London) 60(1897), 475.)

WavelengthsIntensity in

¯ ame

Intensity in arc(K and R)

(Max. ¯ 10)

Length in arc(L)

(Max. ¯ 10)

Intensity inspark (T)

(Max. ¯ 10)

Intensity inhot spark (L)(Max. ¯ 10)

4233 ± 3 Ð 1 Ð Ð 44508 ± 5 Ð 1 Ð Ð 44515 ± 6 Ð 1 Ð Ð 44520 ± 4 Ð 1 Ð Ð 24522 ± 8 Ð 1 3 Ð 44549 ± 6 Ð 4 5 Ð 64584 ± 0 Ð 2 4 Ð 74924 ± 1 Ð 1 3 6 65018 ± 6 Ð 4 Ð Ð 6

A wavelength/star temperature graph (Figure 1) attached to Lockyer’s paper

shows that the enhanced lines may be absent from the spectrum of a star if the

temperature is too high or too low.

The fact that at low temperatures iron is in any case represented by the spectrum

lines, whereas at high temperatures indications of it seem to disappear completely, led

Lockyer to conclude :

28 Arthur J. Meadows, Science and Controversy : A Biography of Sir Norman Lockyer (London, 1970),168± 9.

29 Alfred Fowler, `Memories of Sir Norman Lockyer ’ , in Life and Work of Sir Norman Lockyer, editedby Mary and Winifred Lockyer (London, 1928), 458.

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¼ this result aŒords a valuable con® rmation of my view, that the arc spectrum

of the metallic elements is produced by molecules of diŒerent complexities, and

it also indicates that the temperature of the hottest stars is su� cient to produce

simpli® cations beyond those which have so far been produced in our

laboratories.

In the following months Lockyer discovered that the results obtained for iron could

also be extended to other alkaline-earth elements, such as calcium and magnesium.30

In calcium the H and K lines are enhanced and in magnesium the line at 4481 A/ is

particularly intense.

So what was revolutionary in Lockyer’s discovery was not simply his observation

that emission spectra studied in the laboratory varied with temperature, but that such

variations could solve certain problems inherent in the interpretation of solar and star

absorption spectra.

The basis for all the observations to follow was the `thermal arc ’ concept

developed by Lockyer in 1887 in the framework of the dissociation theory.31

According to this idea the stars are characterized by a rising followed by a falling

temperature phase and spectrum analysis helps us distinguish between the two. He

drew a temperature/time graph in which the two groups are linked by a thermal curve.

The existence of stars whose temperatures are risingÐ in contrast with the prevailing

belief in one-way evolution (from high-temperature stars to low-temperature

stars)Ð was also argued by other researchers such as Lane and Ritter.32

Lockyer showed that presence and intensity, or absence, of enhanced lines

compared with arc lines makes it possible to create a rapid method of star

temperature classi® cation independently of violet radiation analysis (see Figure 1).

The use of the magnesium spectrum to assess the temperature of stars had already

been suggested in 1894 by two German scientists, Scheiner and Keeler.33 Their

suggestion was criticized and improved upon by Lockyer himself on the basis of more

precise measurements of the spectrum wavelengths:

In the case of the stars so far discussed, the same order of temperature is arrived

at by a consideration of the lines of calcium and magnesium as that deduced in

the ® rst instance from the relative lengths of continuous spectrum, and

afterwards by an inquiry into the presence of the enhanced iron lines. Four

indications of stellar temperatures are therefore now available, namely, the

extent of the continuous radiation, the lines of iron, the lines of calcium, and the

lines of magnesium.34

For a more exact classi® cation of hotter stars, Lockyer used the intensity of the

hydrogen lines and the so-called `cleveite gas ’ . The spectrum of the gas seemed to be

30 Norman Lockyer, Òn the chemistry of the hottest stars ’ , Proceedings of the Royal Society (London ),61 (1897), 148± 209.

31 Norman Lockyer, `Suggestions on the classi® cation of the various species of heavenly bodies ’ ,Proceedings of the Royal Society (London ), 44 (1888), 1± 93.

32 Homer Lane, Òn the theoretical temperature of the sun; under the hypothesis of a gaseous massmaintaining its volume by its internal heat, and depending on the laws of gases as known to terrestrialexperiment’ , American Journal of Science, 50 (1870), 57± 74 ; A. Ritter, Òn the constitution of gaseouscelestial bodies ’ , Astrophysic s Journal, 8 (1898), 293± 315.

33 Cf. J. E. Keeler, Astronomische Nachrichten, 3245. For a brief summary see also `The magnesiumspectrum as a criterion of stellar temperature ’ , Nature, 50 (1894), 364± 5.

34 Norman Lockyer, Òn the chemistry of the hottest stars ’ (note 30), 169.

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made up of helium lines (such as 7066 A/ , D3 and 4473 A/ ) and lines of another

unknown gas (6678, 5016 and 4922 A/ are the predominant lines),35 labelled

provisionally gas X.36 Lockyer was convinced these were two distinct gases but his

diŒusion experiments and those of Runge and Paschen gave con¯ icting results.37 As

Lockyer wrote :

The lines of the cleveite gases do not appear in stellar spectra until the

temperature is high enough to exhibit the enhanced lines of magnesium and iron

with their greatest intensity. At this stage the gas lines appear only very feebly,

but, unlike the metallic lines, they gradually increase in intensity up to the

highest temperature, as represented by Bellatrix.30

To back up his conclusions, Lockyer published a map showing a `perfect continuity ’

in the case of both metallic lines and cleveite lines. Where the two sets of lines

coincide, the stars are placed in the same classi® cation in the temperature scale, but,

as Lockyer observed :

¼ there is a complete inversion in the behaviour of the gas as compared with the

metallic lines. We seem to be in the presence of a chemical change, iron being

® nally replaced by helium.

The results of spectroscopic observations seemed to be supported at both ends of

Lockyer’ s thermal arc. Falling temperature stars showed a progression from helium

and gas X lines to enhanced lines and ® nally arc lines.

Studying the spectra of stars of diŒerent temperatures (using the same enhanced

lines criterion) led the British astrophysicist to another discovery. Lockyer wrote :

It ¼ appears that while calcium remains visible up to the highest temperatures,

the enhanced lines of strontium probably cease to be visible at temperatures

higher than that of c Cygni, while those of barium have not yet been certainly

traced in any of the stars. The order of appearance of the metals of the calcium

group thus conforms with the chemical order.38

This singular regularity was later to be explained by Saha in terms of ionization

potential (see Epilogue).39

The set of results from star spectrum observation was interpreted in the light of

the so-called `law of continuity ’ to which Lockyer made frequent recourse.

35 On the identi® cation of the lines in the stars at very high temperatures see : Norman Lockyer, Ònthe new gas obtained from uraninite ’ , Proceedings of the Royal Society (London ), 58 (1895), 67± 70, 113± 19,192± 5 ; Proceedings of the Royal Society (London ), 59 (1896), 4± 8, 342± 3.

36 Lockyer was later to adopt the name Asterium. Other suggested names included parhelium (Stoney)and Orionium, because some of the spectral lines had been identi® ed in the spectrum of the Orion nebulosa.Cf. Norman Lockyer, Òn the new gas obtained from uraninite ’ (note 35).

37 Today, of course, we know why. There is in fact only one substance, helium, which has twoatomically neutral and independent spectra corresponding to the triplet (parallel electron spin) and singlet(anti-parallel spin) states.

38 Norman Lockyer, `Further observations of enhanced lines ’ , Proceedings of the Royal Society(London ), 61 (1897), 441± 4.

39 See Normal Lockyer, `The present standpoint in spectrum analysis ’ , Nature, 59 (1899), pp. 585± 6.As stated by CliŒord Maier in his doctoral dissertation, The Role of Spectroscopy in the Acceptance of anInternally Structured Atom, 1860± 1920 (University of Wisconsin, 1965), 234 : Ìn some of Lockyer’s laterwritings, he made reference to the fact that diŒerent elements responded diŒerently to the same stimuli. Inthese discussions there was something of a vague portent of the later uses his work would be put to withreference to excitation levels, ionization potentials and the role of electricity. But these notions were presentonly in the most rudimentary form.’

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According to this `law’ the stars carry on the dissociation of the elements, which we

can only observe (partially) on earth at the high temperatures produced by electrical

discharge. Lockyer asked himself : `Does a temperature higher than any applied so far

really act in the same way as those we have applied? Or should we expect some

unforeseen break in the uniformity of natural processes ?’ 40

As observed by W. H. Brock, the argument from continuity `had been deep-

seated in Victorian scienti® c thought ever since the debates over geological

explanation in 1830s. `These debates ¼ were concerned with wider issues than

geology and involved the nature of natural philosophy and its relations with

metaphysics as much as the structure of the earth. It was William Whewell who had

insisted that in the search for the causes of historical events, just as for mechanical

ones, philosophers were bound by the principle of continuity. ’41 As regards the

principle of continuity, and the associated principle of uniformity, `many scientists

interpreted this principle diŒerently from Lockyer ’ .42 However there was `surprisingly

little opposition to the use of the principle of uniformity in astrochemistry ’ , a state

of things ùndoubtedly due to its methodological familiarity ’ .43As remarked by

Brock, `by 1860, Newton’s second rule of reasoning in philosophy44 had become an

unquestioned assumption in scienti® c discourse ’ .

The concept of continuity led Lockyer to consider a `simpli® cation ’ of the

constitution of chemical elements. Later we shall see what Lockyer meant by

`simpli® cation ’ and how he treated the subject, also in light of the results of spectrum

analysis of the chromosphere, during the total solar eclipse on 22 January 1898.

Several discoveries based on observations of the solar spectrum during eclipses

played a decisive role in accepting or rejecting the dissociation theory. Particularly

signi® cant in the last decade of the nineteenth century were the eclipses of 1893, 1896,

and 1898.45 We shall pass over the ® rst two because results were inconclusive owing

to both poor weather and the low dispersion power of available apparatus.

`Considering Rowland’s lines, we may say that on average one line is observed every

6/10 angstroms ¼ The dispersion power with which my photographs were taken does

not allow us to assume a degree of accuracy greater than 5/10 of that unit ¼ So we

cannot make any de® nitive a� rmations about the presence or absence of iron lines

in the photos of the eclipse ’ , wrote Lockyer in a paper on the type of measurements

proposed for the imminent eclipse in India in 1898.46

However, a radical diŒerence between what emerged from the 1893 and 1896

eclipses and what happened in the 1898 event lay in the fact that the latter occurred

after the second theory of dissociation had been published. So it was not just a

question of new facts previously unavailable (although this was at least partly true)

but observations that could now be compared using a new interpretive theory.

40 Cf. Norman Lockyer, Chemistry of the Sun (London, 1887). Lockyer clearly thought the answer tothe last question should be no.

41 W. H. Brock, From Protyle to Proton (note 14), 183± 4.42 Arthur J. Meadows, Science and Controversy: A Biography of Sir Norman Lockyer (note 28), 147.43 W. H. Brock, From Protyle to Proton (note 14), 184.44 According to Newton’s second rule of reasoning: Ìdeoque eŒectum naturalium ejusdem generis eadem

sunt causae ’ . See Isaac Newton, Philosophiae Naturalis Principia Mathematica (London, MDCLXXXVII).45 See, for example, Norman Lockyer, `The total eclipse of the sun, April 16th, 1893. Report and

discussion of the observations relating to solar physics ’ , Philosophical Transactions, 187 (1897), 551± 618;`The solar eclipse of August 9, 1896. Report on the expedition to Kio$ Island ’ , Philosophical Transactions,190 (1898), 1± 21.

46 Norman Lockyer, `The approaching total eclipse of the sun ’ , Nature, 56 (1897), 154± 7, 175± 8,318± 21, 365± 8, 392± 5, 445± 9.

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The most astonishing fact to emerge from studies of the photographs of the eclipse

on 22 January 1898 is the substantial diŒerence between the Fraunhofer spectrum and

the so-called `¯ ash spectrum’ (Figure 2).47

The ¯ ash spectrum consists in the sudden reversal of the Fraunhofer lines from

dark to bright, which occurs at the sun’s rim at the beginning and end of a total

eclipse. This was for a long time interpreted as proof of the existence of a reversing

layer, in which absorption gives rise to the Fraunhofer lines and which was

hypothesized by Frankland and Lockyer in the early stages of research into the solar

spectrum. The ® rst complete observation of the phenomenon was by C. A. Young in

1870, with the spectrograph slit placed at a tangent to the rim of the sun. Developed

separately by Lockyer and Janssen in 1868, this method makes it possible to observe

the spectrum of solar eruptions at any moment, without having to wait for an eclipse.

According to the then accepted model of the make up of the atmosphere, the ¯ ash

spectrum was produced by the layer responsible for the Fraunhofer lines. But,

according to Lockyer, the lines in the ¯ ash spectrum were formed in diŒerent layers

(see Table 2). When an observer looks at the rim of the sun, his line of sight passes

through all the layers of the atmosphere, one after the other,48 so it is be expected that,

unlike the ® rst model, the diŒerent lines in the ¯ ash spectrum are of diŒerent

intensities from those in the Fraunhofer spectrum.

Table 2. Distribution of spectral lines in solar chromosphere with the height. (From NormanLockyer, Captain Chisholm-Batten, A. Pedler, `Total eclipse of the sun, January 22, 1898 ’ ,Philosophical Transactions, 197 (1901), 201.)

LengthHeight

Lines of arcs (in miles) (in secs of arc)

Ca(K) 130 ° 6000 13 ± 3Hydrogen 112 ° 4500 10 ± 0He 4471 ± 25 105 ° 4000 8 ± 9He 4026 ± 3 ; Sr 4077 ± 9, 4215 ± 66 86 ° 2700 6 ± 0Ca 4226; Sc 4247

7212 2000 4 ± 4

Mg ultra-violet tripletFe triplet (4045)Strongest arc lines (4307 ± 96, 4325 ± 92, &c.) 60 ° 1450 3 ± 2Al 3944 ± 16 and 3961 ± 67Fe enhanced lines 4584, 4233

51 ° 1100 2 ± 4Mn quartet (4030 ± 9, &c)Fe enhanced quartet (4523 ± 0, &c) and manyother lines

40 ° 650 1 ± 4

Carbon ¯ uting and many lines, includingsome arc lines of iron

35 ° 475 1 ± 05

The 1898 eclipse con® rmed Lockyer’ s prediction. J. Evershed’s observations, and

Lockyer’ s own too,49 showed that there was a signi® cant diŒerence in the intensity of

the lines of the two spectra and, furthermore, as Lockyer observed :

47 For a detailed discussion of this hypothesis see Norman Lockyer, `The sun and stars ’ , Nature, 33(1886), 399± 403, 426± 72, 499± 502, 540± 3 ; Nature, 34 (1886), 41± 5, 205± 7, 227± 30, 280± 2.

48 J. Evershed, `Wavelength determinations and general results obtained from a detailed examinationof spectra photographed at the solar eclipse of January 22, 1898 ’ , Proceedings of the Royal Society(London ), 68 (1901), 6± 9.

49 Norman Lockyer, Captain Chisholm-Batten and A. Pedler, `Total eclipse of the sun, January 22,1898. Ð Observations at Viziadrug ’ , Philosophical Transactions, 197 (1901), 151± 208.

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Figure 2. Spectrum of the chromosphere as photographed during an eclipse (January 22, 1898)compared with Fraunhofer lines. (From Norman Lockyer, Captain Chisholm-Batten, A.Pedler, `Total Eclipse of the Sun, January 22, 1898’ , Philosophical Transactions, 197(1901), 203.)

¼ comparing the wave-length values of the ¯ ash spectra with Rowland’s wave-

lengths of the solar lines, it is at once evident that practically all the strong dark

solar lines are present in the ¯ ash as bright lines ; and all the bright lines in the

¯ ash, excepting hydrogen and helium, coincide with dark lines having an

intensity greater than three on Rowland’s scale. The relative intensities of the

lines in the two spectra are, however, widely diŒerent, many conspicuous ¯ ash

lines coinciding with weak solar lines, and some of the strong solar lines being

represented by weak lines in the ¯ ash spectrum.50

To sum up, the spectrum photographs show that the chromosphere spectra are

diŒerent from Fraunhofer spectra. The discrepancy between chromospheric and

Fraunhofer lines had already been noted at previous eclipses. However, such

diŒerences could now be interpreted in terms of the second theory of dissociation.

Lockyer then discovered that the enhanced lines were more intense than the

Fraunhofer spectrum. Lockyer observed :

An examination of the eclipse photographs shows that the temperature of the

most luminous vapours at the sun’ s limb is not far from that produced by an

electric spark of very high tension, the lines, which we have seen to be enhanced

on passing from the arc to such a spark, being present. The chromosphere, then,

is certainly not the origin of the Fraunhofer lines, either as regards intensity or

number.51

And in the light of dissociation theory, the greater intensity of the enhanced lines was

evidently seen as a signal of higher temperatures.

Lockyer also found that `some of the lines are seen to be relatively much brighter

in the upper strata than in the lower. Chief among these lines are those of hydrogen,

helium and calcium (H and K), but there is an additional line at wave-length 4686.2

or thereabouts, which behaves in the same way.’52

As regards solar physics, he was therefore led to think that the temperature of the

chromosphere was higher (because of the presence of the hydrogen and helium lines

50 Norman Lockyer, Inorganic Evolution (London, 1900), 42.51 The famous k 4686 line, ionized helium, was of vital importance for the a� rmation of Bohr’s model

(1913) of the hydrogen and hydrogenoid spectra. For the controversy between Bohr and Fowler on howto interpret the line, see Nadia Robotti, `The spectrum of Zeta Puppis and the historical evolution ofempirical data ’ , Historical Studies in the Physical Sciences, 14 :1, 123± 45.

52 Hermann Vogel, `Lockyer’s dissociation theory ’ , Nature, 27 (1883), 233.

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and the more intense enhanced lines) than that of the photosphere. This went against

the generally accepted notion that the temperature of the sun decreases with distance

from the centre.

5. A new eŒect

Lockyer’s hypothesis seemed to be corroborated by other facts, apart from star

temperature and comparisons between the chromosphere and Fraunhofer spectra.

One of these stemmed from research conducted in the 1890s by a number of American

spectroscopists into the eŒects of varying pressure on arc spectra.

In 1883 H. W. Vogel53 published what Lockyer called a `friendly criticism ’ of the

dissociation theory referring to èvidence, then beginning to accumulate, that under

certain circumstances the wave-lengths of lines are changed ’.54 And this (in the period

1883± 4) was when the size of the shifts in red and violet spectral lines began to be

compatible with the sensitivity of the instruments available. A major improvement in

the accuracy of measurements came with the concave gratings built by Henry A.

Rowland (Johns Hopkins University). Like most academics, Rowlands was sceptical

about the shifts of spectral lines. But his scepticism was not shared by one of his

students, Lewis E. Jewell, who began in 1890 to examine the whole question of shifts

and carried out laboratory experiments leading, six years on, to a lively debate in the

pages of the prestigious Astrophysical Journal.55

Thanks to the high dispersive powers provided by the new instruments, Jewell

discovered that certain, but not all, metallic lines showed a shift towards the violet in

comparison with the corresponding solar lines : `there was a distinct diŒerence in the

displacements, not only for the lines of diŒerent elements but also for the lines of

diŒerent character belonging to the same element ’ .54

Jewell’s research stimulated another two scientists, William J. Humphreys and

John F. Mohler (pupils of Rowlands), to study the èŒects of pressure on arc-spectra

of the elements and especially to note the eŒect, if any, on the wave-length ’ .56

Humphreys and Mohler observed that in the literature of spectroscopy a number

of authors had noted the phenomenon of the widening of the spectral lines due to the

increase in the quantity of matter. On the other hand, there seemed to have been no

`theoretical considerations or experiment that the wave-frequency itself may change

and thus lead to a shift of the lines as a whole ’ .

Using an electric arc enclosed in a cast iron cylinder enabling them to vary the

pressure up to 14 atmospheres,57 they discovered there was an apparent link between

pressure change and the shift observed. The two American researchers also managed

to exclude the possibility that such shifts might in some way be physically correlated

to temperature change rather than pressure change.58 Humphreys and Mohler

showed that these shifts `varied greatly for diŒerent elements, but in the case of any

53 Norman Lockyer, `The shifting of spectral lines ’ , Nature, 53 (1896), 415± 17.54 Lewis Jewell, `The coincidence of solar and metallic lines. A study of the appearance of lines in the

spectra of the electric arc and the sun ’ , Astrophysic s Journal, 3 (1896), 89± 113.55 William Humphreys and John Mohler, ÈŒect of pressure on the wavelengths of lines in the arc-

spectra of certain elements ’ , Astrophysics Journal, 3 (1896), 114± 37.56 Norman Lockyer, `The shifting of spectral lines ’ (note 53).57 On this, Humphreys and Mohler cite Wilson’s experiments on the temperatures of electric arcs under

pressure. The possible role of temperature was excluded on the grounds that no changes were observed inthe position of the lines produced by the arc near the positive and negative poles, in spite of the fact thatWilson and Grey’s research showed that the temperature of the negative pole was below that of the positivepole.

58 George Hale, `Note on the application of Messrs. Jewell, Humphreys and Mohler’s results to certainproblems of astrophysics ’ , Astrophysic s Journal, 3 (1896), 156± 61.

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Figure 3. Spectral shifts versus pressure according to Humphreys and Mohler. (From NormanLockyer, `The shifting of spectral lines ’ , Nature 53 (1896), 415.)

one, with a single exception... it was approximately proportional to the wave-length...

[and] to the excess of pressure above one atmosphere ’ [emphasis added]. The

exception was calcium.

The lines H and K, among others, shift only about half as much as g and the

group at wave-length 5600. That g should diŒer in this respect from H and K

is not very surprising, since it is known to diŒer greatly from them in many

respects. According to Lockyer it is the `longest line ’ of the calcium spectrum;

that is, it occurs at the greatest range of temperature, even at that of an ordinary

¯ ame, while the H and K do not appear at temperatures much below that of an

electric arc. (Figure 3)

The two researchers later also found a similar diversity in the proportional coe� cients

between diŒerent groups of lines in the spectrum of another alkaline earth,

magnesium. George E. Hale, founder of the Astrophysical Journal, commented that

`the diŒerence in the behaviour of H and K and the blue line of calcium discovered

by Jewell, Humphreys and Mohler seems to support Lockyer’ s views as to the

dissociation of calcium in the arc and sun ’.59

Research into the phenomenon of the shifting of spectrum lines due to pressure

continued through the 1920s and 1930s, until an explanation was at last provided by

quantum mechanics.60

What is interesting for us here is the fact that in 1896 research projects performed

by Humphreys et al. into the eŒects of pressure on calcium spectra unequivocally

demonstrated the existence of diŒerent groups of lines. Calcium, for example, showed

a diŒerent behaviour of the H and K lines compared with the g. This ànomaly ’ was

exactly reproduced in Lockyer’s experiments on enhanced lines, the diŒerence being

59 Cf., for example, Heinrich Kuhn, `Pressure shift and broadening of spectral lines ’ , PhilosophicalMagazine , 18 (1934), 987± 1003.

60 Norman Lockyer, `The shifting of spectral lines ’ (note 53).

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that in this case the key parameter was temperature : the H and K lines seemed only

to appear at higher temperatures. The following year Lockyer noted this coincidence,

used it to support his own second theory of dissociation and made predictions that

increased the empirical content.61 On Humphreys and Mohler’s work Lockyer wrote

in 1896 that ìt would be interesting to verify whether the strontium line at 4607 A/

behaves like the g line of calcium in relation to the lines at 4077 and 4215 A/ ,representing the H and K lines ’ . As he observed the following year, this

predictionÐ based on the dissociation hypothesis Ð was subsequently con® rmed by

Humphreys.62 The American researcher provided a table of the shifts he measured in

the above-mentioned strontium lines. On changing the pressure from 6 to 12

atmospheres the shift in the 4077 line was always about half that of the 4607 line.

Shortly afterwards, Lockyer’s hypothesis was also used to predict the ultraviolet

extension of stellar spectra on the basis of the existence of certain enhanced lines. So

at this stage Lockyer’s research programme was at least partially `progressive ’ (in

Lakatos’s meaning). Data from observations on the eŒects of temperature and

pressure seemed to point in the same direction and led Lockyer to a further

conjecture. What was producing the H and K lines was diŒerent from what produced

the g lineÐ a dissociated version of calcium.63

6. Objections

Despite these successes, Lockyer’s dissociation hypothesis was not without its

problems. One of the ® rst criticisms of Lockyer’ s ideas came from Arthur Schuster64

when he proposed a `crucial experiment ’ to establish, one way or the other, the

validity of the theory. Schuster wrote:

Amongst the heavier metals, tellurium, antimony and mercury are not

represented in the sun but they are in Aldebaran. To be consistent, we must, if

we adopt the theory of dissociation, assert that these metals are decomposed in

the sun. But if I understand Professor Lockyer right, he believes that with our

strongest sparks we can exceed the state of dissociation which exists in the

reversing layer of the sun. Take such a strong spark then from a pole of

mercury, do you get lines of helium, or of calcium, or of hydrogen? This seems

to me to be almost a crucial experiment. Possibly, of course, we should get high-

temperature lines not hitherto looked for, but present in the sun. If so, the

objection would fall to the ground, but if this is not the case, and if mercury at

a high temperature refuses to be dissociated into simpler elements, a most

serious objection to the theory would have to be answered.

The objection stood for several years (at least until Saha’s work), owing to a false

assumption in the òbservational ’ theories of Lockyer and his contemporaries, i.e.

that spectroscopic data provide correct information on the chemical composition of

stars irrespective of the physical parameters involved. This assumption obviously

made it possible for chemical analysis to check and confute the dissociation

hypothesis.65

61 Norman Lockyer, Òn the chemistry of the hottest stars ’ (note 30).62 William Humphreys, `The eŒect of pressure on the wavelength of lines in the spectra of certain

elements’ , Astrophysics Journal, 4 (1896), 249.63 We now know, of course, that the H and K lines correspond to ionized calcium.64 Arthur Schuster, Òn the chemical constitution of the stars ’ (note 12).65 Not a new problem and, in fact, Lockyer had already tried to produce experimental proof of the

dissociation of various elements into hydrogen: Norman Lockyer, `Report on dissociation ’ , ChemicalNews, 40 (1879), 101.

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Schuster was more careful than most at the time not to confuse the deeper

meaning of Lockyer’ s experimental results with his dissociation theory. In fact he

expressed his appreciation ìn the general system of classi® cation adopted in the

paper. No classi® cation is likely to prove successful which does not constantly appeal

to laboratory experiments.’

Another objection raised against the dissociation hypothesis centred on the

enhanced lines criterion. This came from spectroscopist William Huggins (1824±

1910), who had anticipated certain aspects of Lockyer’s methodology. He was one of

the ® rst to try correlating diŒerent star spectra and the Fraunhofer spectrum on the

basis of the presence of certain lines.66 In 1863 Huggins67 had already tried using

calcium lines as a spectroscopic benchmark for his star observations and later

discovered that when the quantity of calcium salt on the electrodes became very small,

the H and K lines continued to be intense even though the other calcium lines had

almost completely disappeared. He therefore concluded that this behaviour could

provide useful clues about the phenomena occurring close to the rim of the sun.

Huggins observed that the H and K lines were the only lines in the calcium

spectrum to be present in the higher solar regions, which it would be reasonable to

assume were of far lower density than the lower reversing layer. The real explanation

of the changes in the calcium spectrum, according to Huggins, was therefore in the

diŒerence in density.68

Huggins then carried out an experiment in which a spark was passed through

calcium vapour at diŒerent densities. His expectations were con® rmed. At the

maximum density of the calcium vapour the blue line showed the same intensity and

the same diŒuse character as the H and K lines. As the density decreased, the blue line

became less intense with respect to lines H and K till only H and K were left. Huggins

then, without varying temperature but only the density (and thus pressure too),

obtained the same result as Lockyer on switching from arc to spark spectra. This

made Huggins suggest that changes to the calcium spectrum did not bear out the idea

of a `true dissociation of calcium, that is, of its resolution into chemically diŒerent

kinds of matter ’ and that `[This dissociation], though a notion familiar to chemists

since Prout’s time, and regarded as theoretically possible, is, as yet, unknown as a

matter of fact ’ .

It was not till the work of Saha (1920) and the development of his theory of

ionization that it became evident that Huggins’s observations were not in contrast

with Lockyer’ s but complementary to them. Not only temperature but pressure too

aŒects spectra.

7. Lockyer’s proto-elements

In March 1897 Lockyer explicitly formulated his belief that the chemical elements

could be classi® ed according to their resistance to the eŒect of temperature. This eŒect

would lead to a `celestial dissociation ’ , in the sense of a `molecular simpli® cation ’ of

the elements : from iron to manganese, to calcium, to hydrogen, to helium, and ® nally

to gas X. The reciprocal position of H and He, which in the dissociation model

proposed by Lockyer left space for ambiguity (as the order of elements on the basis

66 For a brief explanation, cf. Norman Lockyer, `The sun and stars ’ , Nature, 34 (1886), 227.67 William Huggins, Philosophical Transactions, 154 (1864), 139. Huggins was well known for having

revolutionized observational astronomy by applying the spectroscopic method to the determination of thechemical constitution of stars as well as for his use of photography in stellar spectroscopy.

68 William Huggins and Lady Huggins, Òn the relative behaviour of the H and K lines of the spectrumof calcium’, Astrophysics Journal, 5 (1987), 77± 86.

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Figure 4. Diagram of wavelengths versus stellar temperature. (From Norman Lockyer, Òn theorder of appearance of chemical substances at diŒerent stellar temperatures ’ , Proceedingsof the Royal Society (London), 64 (1899), 396± 401.)

of atomic mass was apparently inverted), was explained in terms of ionization

potential with Saha’s theory of 1920 (cf. Epilogue).

Lockyer observed:

The ultimate molecules of Ca, Mg and Fe thus indicated may be considered as

representing the vibration of a proto-calcium, proto-magnesium and proto-

ferrum in stars of medium temperature.69

But it was in 1899 that Lockyer expressed in its most complete form what was already

clear two years earlier. The data seemed to indicate the role played by mass as the

distinguishing parameter between metallic spectra (arc and spark spectra) and gas

spectra (helium and hydrogen).70 The astrophysicist listed the temperature ranges of

the spark and arc spectra of various metals in the rising and falling series of his

thermal arc. The combined results of his star and laboratory spectroscopic

observations are illustrated in Figure 4.

Lockyer wrote : Ì have retained the pre® x `̀ proto ’ ’ to that condition of each

metallic vapour which gives us the enhanced lines alone ¼ ’

So magnesium arc lines will be produced by magnesium, while the magnesium

enhanced lines will be produced by a mutant form of it : `proto-magnesium ’. And the

enhanced lines of calcium will be produced by `proto-calcium ’, those of iron by

`proto-iron ’, and so on.

Looking at ® gure 4, we note there is one chemical substance, common to only

the hottest stars, that we have not addressed so far, `proto-hydrogen ’. For a number

of good reasons this substance needs to be dealt with separately from all the other

proto-elements. The main reason is simple : the way in which proto-hydrogen was

69 Norman Lockyer, Òn the chemistry of the hottest stars ’ (note 30).70 Norman Lockyer, Òn the order of appearance of chemical substances at diŒerent stellar

temperatures ’ , Proceedings of the Royal Society (London), 64 (1899), 396± 401.

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`discovered ’ had nothing to do with the discovery of the other proto-metals, which

provided a means to interpret the enhanced metallic lines. Proto-hydrogen, on the

other hand, arose in a completely diŒerent context, linked to the question of the

spectral series originated in the work of Pickering,71 Kayser,72 and Rydberg73 on what

seemed to be a new spectral series of hydrogen discovered in a number of star

spectra.74

Without going into detail, it will su� ce to point out that for Lockyer the absence

of such lines from laboratory spectra was explained by the inadequate temperature

conditions obtainable in this context (an opinion shared by Kayser). Lockyer himself

commented that :

As this new hydrogen series seems to bear the same relation to the well-known

one as the proto-metallic lines bear to the metallic, I call the gas which produces

it proto-hydrogen for the sake of clearness.75

And in light of the fact that such lines only appear in the spectra of very hot stars,

`there can be little doubt that... the new series of hydrogen lines represents one among

the last stages of chemical simpli® cation so far within our ken ’ .

For Lockyer the proto-metals were diŒerent, simpli® ed versions of the metals that

give rise to the arc lines. In hypothesizing these simpli® ed forms of matter (proto-

elements) Lockyer based himself on Crookes’s (1832± 1919)76 research performed from

1886 into the so-called `metaelements ’ .77

For Crookes the ultimate constituent of the elements was what he called the

elementary protyle.78 This `contains within itself the potentiality of every possible

combining proportion or atomic weight ’ . Elements were thus formed in a `process

akin to cooling, probably internal, [that] reduces the temperature of the cosmic

protyle to a point at which the ® rst step of granulation takes place ’ , leading to `the

[element] most nearly allied to the protyle in simplicity... Hydrogen ’ and thence to the

heavier elements. As Crookes clari® ed:

Thus we approach nearer and nearer either to a regular gradation in the

molecules or to the recognition of those intermediate links, which I have named

`meta-elements ’ or elementoids. A suggestion here occurs that it may be to the

presence of these meta-elements that so many of the chemical elements, whilst

approaching closely in their atomic weights the values required by Prout’s law,

deviate from it by a small but measurable amount. We can scarcely regard their

approximation as purely accidental.

71 Edward Pickering, `Stars having peculiar spectra. New variable stars in Crux and Cygnus ’ ,Astrophysic s Journal, 4 (1896), 369± 70 ; `The spectrum of Zeta Puppis ’ , Astrophysics Journal, 5 (1897),92± 4.

72 E. Kayser, Òn the spectrum of Zeta Puppis ’ , Astrophysic s Journal, 5 (1897), 95± 6 ; Òn the spectrumof hydrogen ’ , Astrophysic s Journal, 5 (1897), 243± 6.

73 J. R. Rydberg, `The new series in the spectrum of hydrogen ’ , Astrophysics Journal, 6 (1897), 233± 8.74 For an account of the discovery of this series, see Nadia Robotti, `The spectrum of Zeta Puppis and

the historical evolution of empirical data ’ (note 51). See also William McGucken (note 3).75 Norman Lockyer, Inorganic Evolution (note 50), 60.76 William Crookes, `Reports to the Annual General MeetingÐ Elements and meta± elements ’ , Journal

of the Chemical Society (1888), 487± 505. Crookes was the discoverer of the element thallium and thefounder of the magazine Chemical News (which very often published news about Lockyer’s searches).Noteworthy were Crookes’s spectroscopic observation and researches on the electrical discharges througha rare® ed gas. These studies led him to the observation of the dark space which bears his name and to thedevelopment of the idea of `radiant matter ’ .

77 Norman Lockyer, Inorganic Evolution (note 50), 57.78 William Crookes, Òn the nature and origin of the so-called elements ’ (note 11).

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Lockyer called the `regular gradation ’ process, introduced by Crookes, ìnorganic

evolution ’ , with an unmistakable nod towards Darwin.79 He believed the proto-

elements he `discovered ’ by spectroscopy to be diŒerent, simpler forms of those same

elements (and thus closer to Crookes’s elementary protyle) obtained by proceeding in

the opposite direction from that which brought about the aggregation of the

elements, i.e. by increasing the temperature to that of the hottest stars. It was here,

then, that the most elementary components of matter, the proto-elements, were to be

found.

The fact, stressed by Lockyer, that the enhanced lines disappear when helium and

gas X lines appear (as the temperature rises) or metallic arc lines appear (as the

temperature falls), seemed to the British astrophysicist an excellent argument in

support of dissociation as a macroscopic reduction of mass.

It should be remembered that Lockyer and Crookes were not alone in using a

simpli® ed atomic model (dissociation into components with a mass comparable to

that of the atom) to explain the spectral variations due to changes in working

conditions. Humphreys and Mohler suggested something similar when they measured

the eŒects of pressure on spectral wavelengths. They discovered a correlation between

atomic weights and the predicted shifts, a link that did not seem to hold in the case

of the lines of certain elements, such as yttrium, osmium, platinum, hallium, and

calcium (H and K), unless it was assumed `that these substances dissociate into eight

equal parts ¼ in which case the observed shifts and those calculated using an eighth

of the atomic weight fall within the range of experimental error, except in the case of

yttrium ’.80

The existence in diŒerent types of atoms of similar and spectroscopically

detectable subatomic particles was also suggested by Thomas Preston, in 1899.

Noting the diŒering eŒect of a magnetic ® eld on various lines in the same spectrum

(anomalous Zeeman eŒect), Preston conjectured that `not only is the atom a complex

composed of an association of diŒerent ions, or at least from ions which possess the

same e/m, and that the diŒerences which exist in the materials thus constituted arise

more from the manner of association of the ions in the atom than from diŒerences

in the fundamental character of the ions which build up the atoms;... Important

spectroscopic information pointing in this same direction has been gleaned through

a long series of observations by Sir Norman Lockyer on the spectra of ® xed stars, and

on the diŒerent spectra yielded by the same substance at diŒerent temperatures.’ 81

Lastly, Arthur Schuster and Gustav Hemsalech reported in 1899 that the

phenomena they had observed while studying the details of the spark spectra of a

number of elements àre not easily explained, except by assuming the presence of

diŒerent kinds of molecules having diŒerent masses, the lighter ones diŒusing more

quickly. We have thus established a method which is likely to prove of extreme value

in separating the eŒects of diŒerent molecules.’82

79 Lockyer was not the ® rst to suggest the idea of inorganic evolution. This idea had been around sincebefore Darwin. For an example of such position see Robert Chambers, Vestiges of the Natural History ofCreation, 1844 (published anonymously). In this work Chambers argued that the organic world iscontrolled by the law of development, just as the inorganic is controlled by gravitation. According toChambers the solar system had developed from à universal Fire Mist ’ to its present con® guration.

80 William Humphreys and John Mohler, ÈŒect of pressure on the wavelengths of lines in the arcspectra of certain elements ’ , Astrophysics Journal, 3 (1896), 114± 37.

81 Thomas Preston, `Magnetic perturbations of the spectral lines ’ , Nature, 60 (1899), 175± 80.82 Arthur Schuster and Gustav Hemsalech, Òn the constitution of the electric spark ’ , Philosophical

Transactions, 193 (1901), 189± 213. A summary of this paper can be found in Proceedings of the RoyalSociety (London ), 64 (1898), 331± 335.

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Lockyer’s dissociation hypothesis would explain the classi® cation of the atomic

weights of the elements in the star spectra according to temperature. But this

hypothesis Ð in its original formÐ came up against a number of problems.

First of all there were discrepancies between star observations and the `periodic

table ’ hypothesis. The apparent sequence of magnesium and calcium was inverted

and there was no explanation of the fact that, on reducing temperature, the ® rst lines

to appear in hot star spectra were those attributable to magnesium and calciumÐ i.e.

proto-magnesium and proto-calcium (which have an atomic weight of 24 and 40

respectively)Ð rather than oxygen (atomic weight 16).

Lockyer countered this objection by saying that if, for example, proto-magnesium

is the product of the dissociation of magnesium:

¼ the atomic weight of the particle, molecule or mass, call it what you will,

which produces the restricted number of linesÐ the enhanced linesÐ must be

less than that of the magnesium by the breaking up of which it is brought into

a separate existence ¼ Seeing that the smaller masses which produce the

enhanced lines have not yet been isolated, their àtomic ’ weights and their

chemical characteristics have not been determined, and so of course their places

in the periodic table cannot be indicated as it at present exists.83

He then attempted an ad hoc explanation to reconcile the empirical evidence with the

periodic table. He assumed that proto-magnesium was formed by a single process of

`depolimerization ’ , whereas proto-calcium was formed by two similar processes. The

atomic weight of proto-magnesium would thus be 24/2 ¯ 12, while that of proto-

calcium would be 40/4 ¯ 10. In this way star observations and atomic weights could

be reconciled:

Hydrogen 1

Proto-calcium 10

Proto-magnesium 12

Oxygen 16

In brief, Lockyer believed that the proto-elements, observable by means of the

enhanced line, were produced by the dissociation of atoms into two or more

components of equal or at least comparable mass, and in any case of the same order

of size as the original atomic masses. In Lockyer’s conceptual framework, however,

the minuteness of a hypothetical elementary mass is not important in itself, but

merely the last link in an evolutionary chain that starts with the heavier elements and

passes through the lighter ones (proto-metals, hydrogen, helium, gas X). Further,

given Lockyer’s molecular idea of spectrum production and that mass is the only

important parameter, this is a completely reasonable hypothesis. Radical changes in

mass give rise to the enhanced lines.

In the case of hydrogen, Lockyer went into greater detail. His point of departure

was the sodium spectrum, which provided the simplest example of the series

phenomenon with its spectrum changes at low temperatures. The line spectra are made

83 J. D. Liveing and J. Dewar, Òn the ultra-violet spectra of the elements ’ , Philosophical Transactions,174A (1883), pp. 187± 222. See also J. R. Rydberg, `Recherches sur la constitution des spectres des elementschimiques ’ , Kungliga Svenska Vetenskapsakademien Handlingar, 23 (1889), translated in W. R.Hindermarsh, Atomic Spectra (Oxford, 1967), 108± 16 ; Òn the structure of the line-spectra of the chemicalelements’ , Philosophical Magazine, 29 (1890), 331± 7. For a historical reconstruction see Nadia Robotti,`The spectrum of Zeta Puppis and the historical evolution of empirical data ’ (note 51).

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up of three series, a principal and two subordinates (sharp and diŒuse) distinguished

by their intensity, which is greatest in the principal series, intermediate in the diŒused

series, and least in the sharp series (the spectra nomenclature was Dewar’s and

Liveing’ s84): `the former contains the orange line D, constantly seen at all tem-

peratures, the ® rst subordinate the red line, the second subordinate the green line,

representatives of two series of lines which are best seen both in the ¯ ame and arc ’

(i.e. two low-temperature light sources). The fact that with sodium, as with the other

elements examined and unlike the principal series, the subordinate series ends at

practically the same wavelength, ìndicates a link between the two subordinates and

a diŒerence between them and the principal ’ . Lockyer associated each series with a

diŒerent `mass’85 : the principal represented the ultimate state of simpli® cation

produced by temperature, while the other two series indicated lower temperatures.

Lockyer considered the known hydrogen spectral series and applied the depoly-

merization process to this element too by means of a statistical and arbitrary

method to evaluate the number of series that ought to be present. Such a number

would be approximately equal to the number of observed spectral lines divided by the

number of known series lines. By equating each series present to each depolymerization

step and hypothesizing that in each step the mass was halved (see Table 3), he

obtained a value for the last dissociation step of a mass equal to 0.0019 times the mass

of the hydrogen atom, or approximately one six-hundredth of the mass of the

hydrogen atom.

Table 3. Spectral series versus elementary masses according to Lockyer’s `depolimerization ’process. (From Norman Lockyer, Inorganic Evolution (London, 1900), 182.)

Spectrum Where existent Series, etc. Mass

Line spectrumCelestial

Terrestial

PrincipalSubordinateSubordinate

0 ± 00190 ± 00390 ± 0078

Fluted spectrum

Set BTerrestrial

Set ATerrestrial

PrincipalSubordinateSubordinatePrincipalSubordinateSubordinate

0 ± 01560 ± 03120 ± 06250 ± 1250 ± 250 ± 5

Continuous spectrumHydrogen weighedin the cold

1

This was the value that J. J. Thomson in 1899 was to reconsider and compare with

his estimate of the mass of the corpuscle.

8. Proto-elements and corpuscles

In April 1897, a month after Lockyer had found the origin of the ènhanced ’ lines

in proto-elements, Thomson (1856± 1940) discovered a ® rst atomic constituent of

negative charge : the electron or corpuscle as it was initially called by Thomson.

In 1897 J. J. Thomson, who nine years later would be awarded with the Nobel

prize in physics for his work on the conduction of electricity through gases, was

84 Norman Lockyer, `The method of inorganic evolution ’ , Nature, 61 (1899), 129± 31.85 Norman Lockyer, Inorganic Evolution (note 50), 179.

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working on this subject and in particular the nature of the so-called `cathode rays ’ .

These studies led him to a new concept of the atom as structured, divisible, and made

of corpuscles. This is how Thomson introduced the `new atom’ :

If, in the very intense electric ® eld in the neighbourhood of the cathode, the

molecules of the gas are dissociated and are split up, not into the ordinary

chemical atoms, but into these primordial atoms, which we shall for brevity call

corpuscles ; and if these corpuscles are charged with electricity and projected

from the cathode by the electric ® eld, they would behave exactly like the

cathode rays. They would evidently give a value of m/e which is independent of

the nature of the gas and its pressure, for the carriers are the same whatever the

gas may be ¼

Thus on this view we have in the cathode rays matter in a new state, a state

in which the subdivision of matter is carried very much further than in the

ordinary gaseous state : a state in which all matterÐ that is, matter derived from

diŒerent sources such as hydrogen, oxygen, etc.Ð is of one and the same kind;

this matter being the substances from which all the chemical elements are built

up.86

Even if up to that moment research projects had had nothing to do with spectroscopy,

Thomson looked with interest on Lockyer’ s spectroscopic research, to such an extent

that he made reference to these research projects in proposing his `new atom ’.

Thomson wrote :

The assumption of a state of matter more ® nely subdivided than the atom of an

element is a somewhat startling one ; but a hypothesis that would involve

somewhat similar consequences, viz. that the so-called elements are compounds

of some primordial elements, has been put forward from time to time by various

chemists. Thus Prout believed that the atoms of elements were built up of atoms

of hydrogen, and Mr Norman Lockyer has advanced weighty arguments,

founded on spectroscopic considerations, in favour of the composite nature of

the elements.

It is very interesting to observe what Schuster thought about Lockyer’s spectroscopy

in March 1897 :

I now pass on to say a few words on Mr Lockyer’ s ® nal conclusions : most of

us are convinced in our innermost hearts that matter is ultimately of one kind,

whatever ideas we may have formed as to the nature of the primordial

86 J. J. Thomson, `Cathode rays ’ , Philosophical Magazine, 44 (7 August 1897), 293± 316 ; 311, 312.Another version was published in Proceedings of the Royal Institution, 15 (30 April 1897), 419± 32 andin The Electrician (21 May 1897), 104± 11. The Philosophical Magazine version was diŒerent from the RoyalInstitution one due to the work carried on by Thomson in the intervening months. In particular, while inthe ® rst are reported two diŒerent methods to estimate the charge/mass ratio of cathode raysÐ i.e. themethod founded on magnetic de¯ ection and on conversion of cathode rays ’ kinetic energy in thermalenergy, and the method based on both magnetic and electrical de¯ ectionÐ in the Royal Institution paperonly the ® rst method is reported. On this subject see Nadia Robotti, `The Discovery of the Electron 1 ’ ,European Journal of Physics, 17 (1997), 133± 8. On J. J. Thomson’s work see also Isabel Falconer,`Corpuscle, Electrons and Cathode Rays : J. J. Thomson and the `̀ Discovery of the Electron ’ ’ ’ , BritishJournal of the History of Science, 20 (1987), 241± 86, 268, Nadia Robotti, `J. J. Thomson at the CavendishLaboratory: the History of an Electrical Charge Measurement’ , Annals of Science, 52 (1995), 265± 84 ; PerF. Dahl, Flash of the Cathode RaysÐ A History of J. J. Thomson’s Electron (Bristol & Philadelphia, 1997) ;E. A. Davis and I. J. Falconer, J. J. Thomson and the Discovery of the Electron (London, 1997).

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substance. That opinion is not under discussion. The question is not whether we

believe in the unity of matter, but whether a direct proof of it can be derived

from the spectroscopic evidence of stars.87 [emphasis added.]

The credit not attributed to Lockyer here by Schuster was later given, as we have seen,

by Thomson in order to reinforce his own hypothesis of a structured and divisible

atom.

When Thomson introduced the idea of corpuscle he had a value only for the

mass/charge ratio of this new hypothetical particle. However, subsequent studies,

including work on the discharge processes in gases, enabled Thomson in 1899 to

obtain a separate measurement of charge and mass. In this way Thomson was faced

with a mass value of the order of 3 ¬ 10 26 g, so obtaining, as Rutherford wrote on 23

July 1899, `direct proof of the existence of masses only 1/1000 of the hydrogen ion’.88

As we saw earlier, a value of the same order was found just a few months later by

Lockyer for the mass of `proto-hydrogen ’.

When he was informed of Lockyer’s estimate, Thomson interpreted it as a further

con® rmation of his results, this time from the ® eld of spectroscopy in nature. On 15

November 1899 Thomson wrote to Lockyer as follows, before his article on the mass

of the corpuscle (signi® cantly entitled Òn the existence of masses smaller than the

atoms’ ) was published:

Dear Sir Norman,

My paper is coming out in the Philosophical Magazine. I will send you a

copy as soon as it appears.

I was much interested in the paper you sent me, especially in the estimate

you give of the mass of the smallest atom of hydrogen, which is about 1/600 of

that of an ordinary atom. I get for the mass of the particles with which I have

been dealing values which in diŒerent experiments have varied between 1/500

and 1/700 of that of the ordinary atom, so that the two lines of enquiry lead to

very concordant results. I shall be very glad to have an opportunity of talking

the matter with you and discussing the subject at greater length.89

Thomson’ s interest in Lockyer’ s spectroscopic research projects was not, however,

limited to this particular case. In May 1903 Thomson held a series of lessons on the

composition of matter and the nature of electricity at Yale University. In one of them,

entitled `the composition of the atom’ , after reviewing the studies of Prout, Dumas,

Newlands and Mendeleev, he stated :

¼ indeed spectroscopic evidence alone has led Sir Norman Lockyer for a long

time to advocate the view that the elements are really compounds which can be

dissociated when the circumstances are suitable.90

Notwithstanding the backing Thomson found in Lockyer’s ideas, it must be observed

that between the positions there was a basic diŒerence. It was one thing to put

87 Arthur Schuster, Òn the chemical constitution of the stars ’ (note 12).88 J. J. Thomson, letter to E. Rutherford (23 July 1899), E. Rutherford Correspondence, Cambridge

University Library, Cambridge.89 J. J. Thomson, letter to Norman Lockyer (15 November 1899), Lockyer Correspondence, Exeter

University Library, Exeter. We visited the Exeter archives in December 1996, but found nothing useful asregards the second dissociation hypothesis, beyond Thomson’s letter (we saw only a Xerox copy ofThomson’s letter). The text of the letter is also quoted in Arthur J. Meadows, Science and Controversy : ABiography of Sir Norman Lockyer (note 28), 170. For a reproduction of the handwritten letter see RoyMcLeod and Paul Gary Wersky, Ìs it Safe to Look Back? ’ , Nature, 224 (1969), 417± 76.

90 J. J. Thomson, Electricity and Matter (London, 1904), 92.

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forward the hypothesis, as Thomson did, that the atom was a complex structure,

comprising charged and divisible parts. It was quite another to hypothesize, as

Lockyer did, that the chemical elements, without aŒecting in any way their chemical

speci® city, could transform themselves into simpli® ed forms of matter (or proto-

elements) by means of a radical reduction of mass.

Nevertheless, Lockyer’s `dissociative ’ hypothesis, and with it the concept of

`proto-element ’ (even if it was to be upheld by Lockyer for many years to come), was

soon abandoned and eventually replaced by ionization and ìonized atom ’.

9. Epilogue

From the early 1900s other research programmes on enhanced lines ran alongside

Lockyer’ s studies. In this framework the study of enhanced lines is a complex process,

made even more problematic by a set of often confused and contradictory data, as

well as by the lack until 1913 of a quantitative theory suitable to interpret it.

So far as laboratory physics is concerned, it was discovered that enhanced lines

were produced in the widest range of conditions; other parameters, in addition to

temperature, seemed to have an in¯ uence (pressure, electrical conditions, impurities

present, etc.). Research projects were focused for the most part on the nature and

measurement of this class of spectral lines, usually without any reference to their

origin (see for example the studies by Fowler, King, and others).

While they were still subject to laboratory studies, the enhanced lines were almost

immediately incorporated and used in astrophysics as a classi® cation tool for stellar

spectra.91 Particularly important was the work performed at the Harvard College

Observatory by Pickering, Cannon, and Maury. It is important to recognize how

both these research programmes, while performed in the framework of various

disciplines, did not consider the question of what produced this class of lines.

Running alongside studies of this type in the ® rst decade of this century was

another research programme carried on by a number of spectroscopists (including

Schuster, Schenck, Walter, Milner, etc.) aimed at a detailed examination of the spark

and its spectrum. Independently of Lockyer’s studies, this new line of research led to

the discovery that arc lines and enhanced lines were two distinct groups of lines,

produced by distinct entities.92 Initially this distinction, in line with Lockyer’s

research, was attributed to a diŒerence in the mass parameter. As experiments

proceeded, another parameter was found to be important Ð the chargeÐ and a link

was demonstrated between the production of enhanced lines and the presence of

charged particles.93

What was missing was an adequate theoretical background. This was provided in

1913 by Bohr94 and his atomic theory. As Fowler95 demonstrated the next year, the

91 In 1914 the enhanced lines became a new instrument for the measurement of stellar parallaxes. SeeW. Adams and A. Kohlschu$ tter, `Some spectral criteria for the determination of absolute stellarmagnitude ’ , Astrophysics Journal, 40 (1914), 385± 98.

92 Arthur Schuster and Gustav Hemsalech, Òn the constitution of electric spark ’ (note 83); CharlesSchenck, `Some properties of the electric spark and its spectrum’, Astrophysic s Journal, 14 (1902), 116± 31.

93 Bernhard Walter, Ù$ ber die Bildungsweise und das Spektrum des Metalldampfes im elektrischenFunken’ , Annalen der Physik, 21 (1906), 223± 38 ; S. Roslington Milner, Òn the nature of the streamers inthe electric spark ’ , Philosophical Transactions, 209 (1908), 71± 87 ; T. Royds, `The constitution of theelectric spark ’ , Philosophical Transactions, 208 (1908), 333± 47.

94 Niels Bohr, Òn the constitution of atoms and molecules ’ , Philosophical Magazine, 26 (1913), 1± 25,476± 502, 857± 75.

95 Alfred Fowler, `Bakerian LectureÐ Series lines in spark spectrum’, Philosophical Transactions, 214(1914), 225± 66.

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266 Stellar, Solar and Laboratory Spectra

enhanced lines had a precise place in the framework of this theory. It was a question

of recognizing that they were nothing more than the lines emitted by atoms that had

lost one or more electrons as a result of the strong electrical forces in sparks. And so

Lockyer’ s proto-elements gave way to Bohr’s ìonized atoms ’. It is interesting to note

that the attribution of enhanced lines to charged particlesÐ emerging from the spark

spectrum studies looked at earlierÐ predates Bohr’s atomic theory.

In any case the Danish physicist’ s theory, while on one hand successfully

explaining the spectral laws on which the enhanced lines were based, had little to say

about their intensity and above all their relationship with the various macroscopic

parameters that in the meantime had been discovered in connection with their

appearance.

From this point of view neither astrophysics nor laboratory physics made any

concrete progress until 1920, the year in which Indian physicist Megh Nad Saha

(1894± 1956) formulated his well-known ìonization law’ .96 For the ® rst time this law

enabled a quantitative relationship to be established between the degree of ionization

of each element and the temperature and pressure. In turn this made it possible to

study the enhanced lines to identify the chemical elements in celestial bodies in precise

physical conditions.

Therefore, thanks to Saha’s law, stellar spectroscopy could change from being a

simple classi® cation tool into a formidable technique to analyse the structure and

characteristics of celestial bodies. In Saha’ s own words, this was enabled by Lockyer’s

pioneering work:

To him is due not only the idea, but also extended and elaborate studies of the

enhanced and super-enhanced lines of elements, and their application to the

study of the ordered sequence in stellar spectra.97

Acknowledgements

The authors thank the staŒof Exeter University Library and Professor G. A. Wilkins

for their assistance in the examination of Lockyer Correspondance. They are also

grateful to Drs L. Fenzi, A. Poggio and M. Beghetto of the Biblioteca À. Borsellino ’

(Department of Physics, University of Genoa) for their support in the bibliographical

research.

96 Megh Nad Saha, Ìonization in the solar chromosphere ’ , Philosophical Magazine, 40 (1920), 472± 88.97 Megh Nad Saha, Òn the temperature ionization of elements of the higher groups in the periodic

classi® cation ’ , Philosophical Magazine 44 (1922), 1128± 39.

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Documents

Stellar, Solar and Laboratory Spectra: The History of Lockyer's Proto-elements