I. THEORY OF SOLUTIONSCHAIRMAN: HOMER W. SMITH, Sc.D.

"A Knowledge of the Laws of Solutions...'By HoMER W. SMITH, Se.D.

KNOWLEDGE of the laws of solu-tions,' it has been said, "is inmpor-

taut because almost all the chemical processes

which occur in nature. whether in animal or

vegetable organisms, or in the nonliving suir-face of the earth, anid also those which are

carried out in the laboratory, take place be-tween substances in solution. For examiiple, a

sound judgment regarding physiological proc-

esses . is impossible without this knowledge;and this holds true for the greater numuber ofthe scientifically and technicallv imeportantreactions. Solutions are mIlore importantthain gases, for the latter seldom react to-gether at ordinary temperatures, whereas so-

lutions present the best coliditioiis for theoccurrenee of all chemnieal processes."The discovery of the laws of solutiolns is

full of significance for the advanlce of phvs-ical chemistry.... The colligative laws whiehapply to gases and dilute solutions alwaysmaintain their character because the mole-cules of gases and of Fvery] dilute solutionsare so far removed fromn olie another thatnieither their m-utual interaction mior theirspecial nature, but only their iiunmbers, come

into play. But the individual characters ofthe molecules become more importaant whenigases are compressed, or solutions are coneein-trated, and deviations froni the colligativelaws dominate more amid more, unltil at last,in pure liquids produced bv the coiitinued

From the Departmenit of Physiology, Neew York

Ijaiversity College of AMedicine, New York, N. Y.

808

compressioni of gases below their critical tem-peratures, and in the solid state, produced bythe removal of all solvent, [intermolecularforces completely dominate]."The hope may be expressed that the pos-

sibility of representing and studying all theseintermediate states, which cani be accom-plished easily aild fully in the examiniation ofsolutions, will be of considerable help in mak-ing easier the study of the laws of pure liq-uids." The foregoing is quoted, with slightparaphrase, from the second edition of Wil-helm Ostwald 's Lehrbuch der allgemneinevChnerie of 1891.1*Ostwald 's generalization stemmed largely

from 2 papers, then onlv a few years old, onebv the Dutchman, Jacobus Heldricus van 'tHoff (1852-1911), the other by the Swede,Svaante August Arrhenius (1859-1927), whichcontributed the major founLdations of our coIn-temporary physical chemistry, of some of ourcontemporary physiology, and, in no smallmeasure, of that sprightly if as yet ill-definedcontemporary discipline, the proponents of

*Ostwald 's Lehrbach had a rather checkered history.The first edition in 2 volumes was published at Rigain 1885-87, but rapid progress in physical chemistryled to a second edition of 2 volumes, published atLeipzig in 1891-93. A revised, second printing of thissecond edition was published at Leipzig in 1896-190a,"wx ith the second volume in 2 parts, followed by a frag-ment of a tlhird part. The first printing of the secondedition is Inot available to the writer, and he has fol-lowed the seconid priniting. Vol. 1, Book 4 (Solutions)of the first priniting lhas beein tranislated by Muir.1

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which choose to call themselves " biophysi-cists. ' Current studies of the nature of so-lutions are still in the stage of revising andimproving the pioneer ideas of van 't Hoff andArrhenius.

Van 't Hoff had thought to be a chenmist, butas an undergraduate student at the Universityof Leyden he yielded to the lure of mathe-niatics, a subject wholly foreign to the prosaicsynthetic-analytic chemistry of the 1870's. InBonnl he worked with Friedrich Kekule, dis-coverer of the tetravalent nature of carbonand the ring structure of benzene; and inParis with Charles Wiirtz, who proudly saidthat chemistry was "a French science'"-without fulminating either hiluself or hismentors. Fromn Paris he went to Utrechtwhere, in 1874, he won his Ph.D. with a con-ventional and wholly safe thesis on eyaiiaceticand malonic acids 2 wisely refraining fromndrawing the attentioni of his doctoral exam-iners to an heretical 11-page pamphlet which,shortly before his examination, he had pub-lished privately. The Dutch title of thispamphlet is beyond mny lingual ability, andeven in English it is too long to read.3*Twenty-four years elapsed before this pam-phlet reached the English language, where-upon the title was reduced to the simplephrase, The Arrangement of Atoms in Space(1898) .6 Thereafter this work won for van'tHoff an international reputation as thefounder of stereochemistry, which deals withthe arrangements of atoms aiid molecules ini3 dimensions instead of 2. Through the in-stigation of Johannes Wislicenus the pani-phlet was translated into German ini 1877 7

whereupon it was deilounced by HermannKolbe of Leipzig, one of the m-ost eminent or-ganic chemists of the time but one who could

*The Dutch pamphlet was translated by van 't Hoffinto French in 1875,' a translation made necessaryby the fact that Joseph Achille Le Bel (1847-1930)of Paris had expressed very similar views in Novem-ber of 1874. Le Bel and van 't Hoff had come tothe theory of the asymmetry of the carbon atom in-dependently, but because of van 't Hoff 's priorityand more thorough treatment of the problem ofoptical activity he is generally given the major credit.

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not think in 3 dimensions, as "fanciful non-selnse [which] carefully avoids aniy basis offact, and is quite unintelligible to the calmninvestigator. . . ." As for Wislicenus, Kolbesaid that "he has gone over from the camp ofthe true investigators to that of the specula-tive philosophers of ominous memory, who areseparated by olnly a thini medium from spirit-nalisnz. " 8, pp. 86-87

Vani't Hoff 's capacity for abstractioni hadearly led to an interest in chemnical kinetics.and his thoughts now leaped from 3 to 4 di-mensions. ''Good nmusie,'' he once said,"makes it very pleasant to think of otherthings"-other things, perhaps, being his sec-ond great work, his A'tudes de DynamiqteChimique of 1884.9 This work was translatedilnto Geriman'0 and English'1 12 years afterits publicatioil in French; it is frequentlymnentioned but rarely read because in alny lan-guage it is very searce. In the etudes van'tHoff incorporated all that had hitherto beenkniown and added so imuch that was new that,so it has beeni said, chemical dynamics wasseareely improved in the next 30 years.8, P. 343Though the Table of Contents of the Etudessuggests a veritable encyclopedia (which in asense it is), vani't Hoff 's writing is marked bybrevity and clarity, and by careful definitioinof all mathematical terms; where a mathe-matical relation represelnts a broad generali-zation, this generalization is re-expressed inwords. He did not coneur in the view of Wil-lard Gibbs, who once asserted that "mathe-imuaties is a language'" anid who generallychose to speak no other.*The Itutdes did not evoke the ridicule that

had greeted Atoms in Space; rather it wasreceived in silence because there seemed to be11o one who could comprehend its dynamicapproach-until in a Norwegian review12 forMarch, 1885, there appeared an exhaustivecritique giving it its proper valuation as one

*"It is said that, during his long membershipin the Yale faculty, Willard Gibbs made but onespeech, and that of the shortest. After a prolongeddiscussion of the relative mnerits of language andlmathematics as elementary disciplines, he rose toremark, 'Mathematies is a language.'I )15, p. 26

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of the classics of science. This critique was

written by a relatively unknown chemiiist atthe University of Upsala, one who was des-tined shortly to become as famous as van'tHoff himself, Svante August Arrhenius.

It was in the last section of the Atutdes,which deals with the topic of chemuieal affinity,that van't Hoff first propounded his gaseous

theory of solutionl. He had been thinkinogto determine the affinity of Glauber 's salt(Na2SO4 1OH20) for its water of erystalliza-tionl by measuring the vapor pressure of waterin equilibrium with the dry solid. Then itcamne about that one day in 1883, probablyjust before the Etudes went to the printer, on

leaving his laboratory in Amsterdam van'tHoff and his wife met the botanist Hugo deVries (1848-1935) (of " mutationi" fame, andwho was Professor of Botany at the Univer-sity), and de Vries talked to himi of the plas-molysis of plant cells, of isotonic solutions ofsucrose and salts, and of the remarkable quami-

titative studies in osmotic pressure carried outby the botanist Wilhelmn Friedrich PhilipPfeffer (1845-1920) and summarized in 1877in Pfeffer 's monumental mnonograph, Osmo-tische Untersiuchungen."3

The discovery of osmotic phenomenia hadbeen made in 1748 by the Abbe Jean-AntoineNollet14 who, in seeking the source of thebubbles that form in liquids when thev are

boiled, had covered a vial of boiled spiritswith a nmembrane-presunmably of pig's blad-der-and submerged the covered vial underwater. He noticed that in a short time watermnoved into the vial, causing the meimbraneto bulge outward with great pressure. Theparadox, that water should move into a sealedvial against pressure, he eorrectly explained,after numerous well-controlled experiments,by the supposition that the mnembrane is niore

permeable to water than to alcohol. (TheAbbe 's paper was published half a century be-fore Dalton (1803) formulated the atomictheory, a full century before Cannizzaro(1858) resolved the long controversy betweenthose who went along with Avogadro andthose who went along with Berthollet.)

There followed a century anid more of un-obtrusive experimeentation on osmosis, begin-ning with Parrot, who in 1815 studied themixing of liquids by diffusion. Some 13 yearslater the physiologist Dutrochet introducedthe first, if crude, manometer into this prob-lem and in 1827 Dutrochet16 coined the words"endosmose" and "exosmose," meaning topush or impel. He later showed that osmosiscan occur through a thini sheet of marble, andthat organic membranes are generally slightlypermeable to salts. The history of our prob-lem in the next few decades bears mnany fa-iniliar names: Fischer, Gustav Magnus, Jeri-chau, Briicke, Matteuci and Cima, Liebig,Jolly, Vierordt, Eckard, and-not the leastamong physiologists-Carl LIudwig, who basedhis filtration-reabsorption theory of renalfunction on his own osmotic experiments anidthose of others. Important among chemistswas Thomas Graham, who in 1826 formulatedthe basic laws relating the diffusion of gasesto their density, applied these laws to the dif-fusion of a solute through membranes in 1854,and in 1861 divided solutes into crystalloidsand colloids. It was Graham who translatedDutrochet 's osmose into the English "os-motic. "* The generalized law of diffusionsformulated in 1855 by A. Fick, is so wellknown as to require its mention only.

Interest in osmosis spread from physiologyto botany when Pringsheim in 1854 and Nae-geli in 1855 showed that the protoplasmic coni-tents of plant cells contract in strong sugaror salt solutions, indicating that the surfaceof the protoplasm constitutes a membranepermeable to water but not to many solutes.Moritz Traube is hard to classify: a pupil ofLiebig 's, he was primarily engaged in thewine business, but worked on ehemical andbiological problems, fermentation, and celltheory on the side. He had discovered in 1867that osmosis could be studied by means ofmembranes precipitated from colloidal orother solutions over the open ends of glasstubes; and, seeking a better membrane, he had

*J. Hogg used the nominative form "osmosis" in1867, but the verbal ''osmose'' remained in useuntil the end of the century.

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come upon the film of copper ferrocyanide.Pfeffer, who, like Pringsheim and Naegeli,had been studying plasmolysis (and whoseemed to have been the first to have spokenof the bounding surface of the plant cell asthe "plasma membrane"), saw the potentiali-ties of Traube's artificial membrane and in1877 reported the most important single ad-vance in the study of osmosis: he had pre-cipitated the copper ferrocyanide film in theporous wall of an unglazed earthen pot, thuspreparing the first truly rigid osmometer bywhich osmotic pressure could be measuredaccurately at widely different concentrationsand temperatures. It was about this date thatHamburger began his studies on the hemolvsisof red blood cells, and that de Vries himselfhad begun his studies using (as he called it)the "plasmolytic" method. In the ensuing 8years de Vries had established the "isotonic"concentrations (another word of his) for nu-inerous electrolytes and nonelectrolytes, usinga variety of plant cells.

Hence, it seems that the stars of sciencewere in some sort of favorable conjunctionover Amsterdam when de Vries, the isotonic-ally minded botanist who was well informedon Pfeffer's plasma membrane and copperferrocyanide manoineter, met van't Hoff, thedisillusioned organic chemist who thought interms of 4 dimensions and whose E'tudes hadpractically given birth to the science of chem-ical dynamics.And so it was that in the last, hurriedly

written chapter of the Ytudes of 1884, van'tHoff's attention turns from the chemical affin-ity of Glauber's salt for its water of crystal-lization, to the attraction which an aqueoussolution has for pure water, as revealed byeither de Vries' plasmolytic method or Pfef-fer 's osmometer. Here van 't Hoff coins theword "semipermeable" to describe Pfeffer 'scopper-ferroeyanided pot because it permitsthe passage of water but niot of sucrose. Ourerstwhile chemist proceeds with prescient in-spiration to show that the osmotic pressure ofPfeffer's solutions is related to the (naturallogarithm of the) ratio of the vapor pressureCirculation, Volume XXI, May 1960

of pure water divided by the vapor pressureof the solution; and to this end he calculatesthe needed vapor pressures from the relation,established in 1870 by Guldberg (of mass-lawfame), between freezing-point lowering andvapor-pressure reduction,'7 using data on thefreezing point of sucrose solutions publishedby Raoult in 1883.'8

Then, it seems as though haste to get themanuscript of the Etudes off to the printerlands him in an anticlimax: using the re-corded vapor pressure data, he calculates that-if 2 molecules of water are removed fromCuSO4 5H20, an osmotic pressure of 1,300atmospheres would be produced. He adds,"The experiment could not of course be car-ried out in the usual way [i.e., in Pfeffer'spot] since we are here dealinig with a solidsubstanee."

* *d

That is all. Van 't Hoff had the rational ap-proach to the study of solutions in his iktudesof 1884, and he abandoned it. Within the yearhe had turned one of those narrow cornersthat make of life a tangled skein. On October14, 1885, he purportedly "read" a "paper"in Stockholm before the Royal Swedish Acad-emy of Sciences, entitled "The lIaw of Chem-ical Equilibrium in the Dilute State, Gaseousor Dissolved." This paper was published in 3parts in the Transactions of the Academy of1886.19*Now, there is one thing of which I am cer-

tain, and that is that van't Hoff never "read"this paper anywhere. As published, it runs tosome 57 large pages replete with tabular data,diagranms, and differential equations. He un-doubtedly said one thing and published an-other-but this is no sin because we all havecommitted it. The spoken and written wordare different dialects within the mothertongue. What he did say to the academicianswe will probably never know.

In the printed paper, however, he draws theanalogy between de Vries' plant cells andPfeffer's copper ferroeyanide osmometer; and,under the principle of conservation of energy

*Part I of this papeir was republished, with minorchainges, in Holland in 1886.20

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and the Carnot-Clausius prinieiple (that heatdoes not pass spontaneously from one body toanother one which has a higher temiperature),he applies a reversible pistoni-compression ey-ele to 2 ideal fluids separated by an ideal semi-permeable membrane. Usinlg Pfeffer 's data onthe osmotic pressure of sucrose at various con-eentrations and temperatures, de Vries' dataoIn the isotonie concentrations of varioussolutes, anid Hamburger 's data onl the hemol-vsis of red blood cells, he deduces the relationisbetweeni 3 of the 4 colligative properties ofsolution-s: osmotic pressure, freezing-pointlowerincg, andvapor-pressure redletion.* Andhle shows that, for very dilute or ideal"' so-lutionis in whiclh inltermloleeular forces can benleglected the osmotic pressure conlformls withthe laws describing dilute or ideal gases, inthe senses that:

1. As stated by Boyl.e for gases, osimloticpressure is proportional to the coneenitrationof solute, the tem:lperature remlainiing constanit.

"Guldberg, wi-e Ihaxve aotel, lha(l dedluce(l the rela-tion between freezinig-point lowverinig aind vapor-pressure reductioni in 1870,'i a relation experimentall-confirmed by Raoult in 1878.-' Van 't Hoff now hadavailable to him Raoult 's demoonstration that: (1)the ieduction of freezinig poinlt of equimiolecularaqueous solutioils is indepenidenit of the niatur e ofthe solute;"8 anid (2) the reduction of freezing poinltat equimolecular concentrations in various solveentsis constant (water being a notable exception).-

The 3 initerrelated properties of a solution osmoticpressure, freezing-point loweriing, anid vapor-pressurereduction were, on the suggestion of Wundts, calledI' colligative'" by Ostwald in 1891.1 P. 30 Ostwvalddistiinguished additive, coinstitutive, aind colligativeproperties: additive properties are those which arethe sums of the properties of the constituents (e.g.,mass) ; constitutive properties depenid on the arrange-nieit of the constituents of a pure substaniee (e.g.,boiling poinit, optical activity, etc. ; colligative prop-erties are those which have equal values for echemicallycomparable (molar) quantities of the mnost differentsubstances (e.g., volume and pr essure relations ofgases, and the aforementioaed properties of solutiolnsin whielh the componen-t miolecules ar-e so Avidely sep-aratedl from oine another that the mnutual interactionsof these molecules are reduced to a minimum).

It was not until 1889 that Beckmann" added boil-ing-point elevation to the other 3 colligative proper-ties, the theoretical relationiships having been sup-plied bly ''myv lhonorled friend'' Arrhenius.

2. As stated by Gay-Lussac for gases, os-motic pressure is proportiolnal to the absolutetemperature, the coneentratioln renmainillgconistamit.

3. As stated bv Avogadro for gases, otherconidition-s remaining constant, osmotic pres-sure is the same for anly 2 solutions containingthe same iiunuber of solute miolecules per unitvolumLe; aiid furthermnore, this pressureainounts to 22.4 atmospheres at O Celsius,the figure 22.4 being all the imiore remearkablewlihen one considers the differenee betweeni thegrain-nolecular weight of gaseous hydrogen.which is 2, and that of dissolved sucrose,wvhich is 342.

Aind so he applies to dilute or ideal solu-tionis the familiar ideal gas law equationi:

PY = nRTwhere P is now osmnotic pressure in-stead ofgas pressure and a is the niumLber of mnolst dis-solved in 1 liter of solution.

In short, lie savs that the osmotic pressureof a dilute solutiomi is equal to the pressurewhich tlie solute would exert if dispersed asa gas at the saime temiperature amid in a voluneequal to that of the solution.

Inl van 7t Hoff 's interpretationi, osmoticpressure depenids oni the inipact of the mole-eules of the solute against the seniipermeablemembramie-for the all too obvious and equallyerronieous reasoni that "the ilmolecules of thesolvent, being presenit upoii both sides of themiiembranie through which thev pass [freely!],

*He inieasis ceintigrade, of eourse, because Celsius'thermiiiomiieter was the cenitigrade thermiionmieter turneidupsi(le (lown.

i has heie beeen added to van 't Hoff 's gas lawi-equation simnply to rouind out his thought. The "kilo-gr amii equivalent'e was apparently introduced intothe gas lanv equatioiu by Horstmann in 1881,[ and(lconvenience led rapidly to the sulbstitutioni of the''gram equivalent' ' oi " gram-l miiolecule. " The lat-teie was first called a Mole by Ostwald in 1902,4priniting 2,9.l12 whlen this writer also coined the phraseMolenibrvch, oir viol fraction, to describe the mnolecular

iixture formiiula, , first used by Raoult inn1 + n

1888 inl his geineral law of vapor pressure reductionof ethereal solutions :', 21

P1-- 1' t1

P lii -t 11'

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do lnot enter into considerationi. ''@24, P. 15t Herecognized, however, that one could equallywell treat osmotic pressure in terms of the at-traction of the solute moleeules for the solven-tmolecules-in dilute solutions the bombard-ment and attraction theories would be mathe-matically indistinguishable. The third inter-pretation, that osmotic pressure reflects thedifference in vapor pressure of the solvent inthe pure state and in the solution, that thisvapor-pressure difference is indeed the vis atergo of osmosis, was germinal in the 'titdesof 1884; but he had turned the narrow cornerbefore he went to Stockholm and theneeforthhe follows the wrong road.Why'?The obvious fact of the 2-way traffic of the

solvent through the membrane can searcelyby itself explain van't Hoff's predilection forthe "bombardment" interpretation becausein the Ettudes he had clearly recognized thatthe great affinity which Na2SO4- 1OH20 ex-hibits for its water of crystallization is re-flected in a proportional reduction in the va-por pressure of water above the salt; and indiscussing Pfeffer's experiments he had uti-

*Ostwald echoes this argument in 1981 when hewrites "'There is no doubt that the cause of [osm-iotie]pressure is to be sought for in the dissolved sub-stance, for water cannot produce any enduring pres-sure, inasmuch as it passes through the separatingmembrane without difficulty.'" How close he camiieto the proper interpretation is revealed by the latercomment that "the [osmotic] cell behaves as if therewere a partial vacuum for water in its interior:water flows in, and, if no opposing pressure is allouvedto develop, produces a continuous movement whichiceases only when the contents of the cell have beeomethe same as those of the space surrounding the cell,i.e., have become pure water. A condition of [os-motic] equilibrium is possible only nhein the pressurewhich prevails within differs sufficiently from thatwhich prevails without. PP. 101 102 The belief thatonly the solute (not the solvenit) in the osmometer isunder pressure was expressed as late as 1928 by-Bancroft and Davis.25

tThis volumiie contains an excerpt of referenice'and all of referenceS22, 26, 27, 29, 32 and 37 translated intoEnglish by Jones. Reference37 is reproduced in partin Leicester, H. M. and Klickstein, H. S.: A SourceBook of Chemtstry: 14o00-900. New York, Torontoand LIondon, McGraw-Hill Book Co., Inc., 1952, pp.483-490.

lized a reversible cycle consisting of a semai-permeable membrane and a vapor phase-"The pressure," he said, "which causes theflow of vapour fromn [one chamber to theother] is therefore equal to the diminution inthe vapour pressure of water which is pro-duced by dissolving the salt in it; . . . " Henceone may suspect that he was also influencedby that charm which is only skin deep (andby which too many of us have at times beenswayed)-namely, the esthetic appeal of histheory. The gaseous theory of solution, themiraculous figure of 22.4, promised the answerto an age-old mystery of why sugar dissolvesin water, an answer both beautiful and simple-it turns into a gas. But nature is neitherbeautiful nor simple; it is damnably puzzlingand complex, and no one yet has turned su-crose into a gas or even explained a semi-permeable membrane.

However, to return to matters of record,from the beginning vanl't Hoff recognized thatonly dilute solutions conform with the gaslaw equation, and then, only dilute solutionsof a few substances-sucrose is the classicalexample. Most substances, such as strongacids, strong bases, and most salts, whichFaraday long ago had called electrolytes* be-eause their aqueous solutions conduct the vol-taic current, behaved anomalously not onlywith respect to osmotic pressure but also withrespect to vapor-pressure lowering and freez-ing-point reduction. For this anomalous be-havior he had no explanation.The Stockholmi paper of 1885 was reworked

in 1887 for publication in volumne 1 of the newZiitschrift fur physikalische Chemie, of whichOstwald and van t Hoff were co-editors.(Nothing elarifies our ideas so much as re-writing a paper once a year, perhaps for sev-eral years.) Van 't Hoff has nlow seeni Raoult 'spaper on " The General Law of the VaporPressure of Solutions, ' '29 showing that forequimolecular concentrationis of nonvolatile

*Faraday had introduced the words ' electrode, "''electrolyte,'" "'electrolysis,'" and "'ion'" in 1839."Ion" he derived from the Greek ion meaning trav-eling or traveler.

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solintes the vapor pressure of various solventsis reduced by the same fractioni: i.e, solutionsthat have the same osmotie pressure have thesame vapor pressure-but for van't Hoff, va-por pressure still remains merely another ofthe "colligative" properties of a solution.

Something new, however, is added in theZeitschrift paper, namely, ani explanation ofthe anomalous behavior of strong acids, bases,and salts.

Nearly 2 years before, on August 4, 1885,van't Hoff had written Arrhenius, thankinghim for his favorable review of the k'tudes,commentiiig on a mnemoir by Arrhenius con-cerning electrolytic conduction, and recapitu-lating some of the arguments in his Stockholmlecture concerning chemical equilibrium. Itwas not until March 30. 1887, however, thatArrhenius replied, having waited until hecould read van't Hoff's paper in the Trans-actions of the Academny. Now he suggests thatthe anomalous behavior of electrolytes inlaqueous solution may be explained by theirdissociation into ions ;30, pP- 219f., 239f. in this let-ter he makes what is perhaps his most explicitstatement on the matter: "What I called inmny paper, 'Sur la Couductibilite' [2nd part],active molecules, are thus the same as disso-ciated molecules. One of the propositionswhich I then put forward would now be writ-ten:-In all probability all electrolytes arecompletely dissociated [into ions] at the mostextreme dilution. ''31, p. 1392; 30, p. 241With this explanationi available to him,

van't Hoff writes, in this, his third paper onosmotic pressure, whiceh appeared in theOctober 21 number of the Zeitschrift,24' 32"It may, theii, have appeared daring to giveAvogadro 's law for solutions [equal num-bers, equal volumes, equal pressures] sueha prominent place [iii the theory of solu-tion], and I should not have done so hadnot Arrhenius pointed out to mne, by let-ter, the probability that salts anid analogoussubstanees, when in solution break down intoions." Daring it may have been, daring in-deed it was, but he had already committedhimself to the gaseous theory, electrolytes not-

withstanding, in the ftudes of 1884 and theStockholm lecture of 1885.

The question mlay be raised as to whiehman, van 't Hoff or Arrhenius, stood in greaterdebt to the other. Without the dissociationtheory the colligative properties of solutionstreated by van't Hoff could not be rational-ized because "most" compounds did not con-form with the gaseous theory; on the otherhand Arrhenius had not dared go all theway on the dissociationi theory until this the-ory had found support in the observations onosmotic pressure, etc., which had been colli-gated by van't Hoff.*We may but briefly turn back to the history

of Arrhenius' idea. The study of electrolysisaiid electrolytic conduction had had a longhistory which cannot be encompassed here be-yoond mentioning a few outstanding names,such as Grotthus, Faraday, Clausius, William-son, Hittorf and Kohlraush-the last-nalmed,with his studelits, having perfected the use ofalternating current and the Wheatstonebridge for the measurement of electrical con-ductivity of solutions. The necessary datawere available for reinterpretation by 1870,but the ultimate idea of the complete disso-ciation of the solute had been effectivelyblocked by the long-held conviction, basic tothe atomiic theory itself, that the 2 atoms ofNaCl, for example, are held together by pow-erful electrical forces and can- be separatedinto "charged radieles" only bv the applica-tion of a substantial electrical current; andalso by the properties of pure metals such as

*To say that Arrhenius' theory "was a direct out-comiie of vani It Hoff 's osmotic pressure studies,' 8t P. 111is to ignore the long history of electrolytic conductionand other interpretations which had been advancedbefore Arrhenius' time.Vanlt Hoff had read Arrheniusi' 2 papers3 "4 of

1884, but the vague notion of "'active" and "inac-tive" particles in solution served only to supply himnMx itlh a new approach to the mass lawx, electromotiveforce and related matters. The fact that the partialdissociation of an electrolyte, such as NaCl, into 2-electrically active particles would increase the nuim-ber of osmoticallv active particles wxas Arrheniiis '

contribution.

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sodium, and of pure gases such as chlorine,which were irreconcilable with the propertiesof a solution of NaCl.

Arrhenius had begun his studies on the elec-trical conductivity of solutions at Upsala un-

der the organic chemist, Per Theodor Cleve,but his ideas were never popular at Upsalaand his major work was carried out in Stock-holm between 1881 and 1884 under Erik Ed-lung, Professor of Physics to the SwedishAcademy. Beyond his own observations, nu-

merous physical-chemical data were availableto him by 1883, all of which called for some

unifying interpretation, but in his thesis sub-mitted at Upsala in 188333 34 he had handledthe problem of electrical conductivity cau-

tiously by referring (in Clausius' terms) to"active" and "inactive" forms of the solute,only the "active " form serving to conductthe electric current. No explanation of "activ-ity" is given nor is it stated why "activity"increases with dilution, and the word "disso-ciation" is not mentioned. The older inter-pretations were reworked, quantified, andgiven new applications, but without a com-

plete break with tradition. Even so, the thesiswas heterodox enough to bring opprobriumupon its author, who was granted his degree(1884) non sine laude approbateur, the doublenegative indicating in Sweden that he was

just "a little better than passing. ''31

Only after reading van't Hoff's paper, andnoting the anomalous behavior of electrolytesin respect to osmotic pressure, etc., didArrhenius break with tradition and state thatin infinitely dilute solutions electrolytes are

completely dissociated, and that van't Hoff'slaw for osmotic pressure, and indeed all thecolligative laws, apply to all substances, non-

electrolytes and electrolytes, if partial or com-

plete dissociation is taken into account.The idea that some of the molecules of a

solute may, when in solution, undergo com-

plete dissociation for a finite time into ionswhich have virtually complete electrical andchemical independence went considerably be-yond the undefined term "active." It was (ineffect) this idea which, as Arrhenius latersaid, occurred to him " on the night of the

Circulation, Volume XXI, May 1960

17th of May in the year 1883,* and I couldnot sleep -that night until I had workedthrough the whole problem. "8, p. 115

Only after he had read van 't Hoff's Stock-holm lecture in 1886, however, did he ventureto publish 2 papers in Sweden35' 36 and towrite his definitive paper in German,37' 24 thelast appearing shortly after van 't Hoff'spaper in volume 1 of the new Zeitschrift. TheZeitschrift elicited a mixed reaction on thepart of the scientific public-between van 'tHoff and Arrhenius, so it seemed to some, ithad gotten off to a bad start. Though the gas-eous theory of solution was soon accepted, thetheory of electrolytic dissociation was formany years opposed by every seemingly ra-tional, and many irrational arguments, exceptby Ostwald, who championed the new theoryfrom the start. Once when Ostwald visitedUpsala, Cleve asked him ". . . and you arealso a believer in these little sodium atomsswimming around'? "8, p. 117 It was unthinkablethat the bland action of water as a solventcould cause a molecule to separate into itselectrically charged atoms-and one imaginesthat there were those who hesitated to washtheir hands for fear that electric sparks mightjump up to their fingertips.

It has been said that, to van't Hoff, hisosmotic theory meant chiefly a new and con-venient way to determine molecular weightsof dissolved substances. I do not believe it.Van't Hoff was less interested in molecularweights than in chemical dynamics, chemicalaffinity, and a rational theory of solution. Inretrospect, I regret that he abandoned the con-sideration of experimentally measured vaporpressure for the circuitous reasoning of theClausius-Carnot cycle because thereby thetheory of solution lost many years. Yet van'tHoff and Arrhenius-we will not attempt tojudge the relative merits of their contribu-tions-between them started us along the roadwe travel now.

*His thesis was presented to the Academy on June6 and must have been completed some time before.

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References1. OSTWALD, W.: Solutions. Book 4 (in vol. 1 of")

with some additions, of the second editioni ofOstwald 's Lehrbuch der allgemeinen Chemie,translated by 2I. Mi. Pattison Muir. Londonand New York, Longmans, Green & Co., 1891.

2. VAN'T HOFF, J. H.: Bijdrage tot de Kennis van

het Cyanazijnzuur en Malonzuur. Disserta-tion. Utrecht, 1874.

3. -: Voorstel tot uitbreiding de tegenwoordig inde scheikunde gebruikte structuurformules inde ruimte, benevens een daarmee samenhan-gende opmerking omtrent bet verband tuss-chen optisch actief vermogen en chemischeConstitutie van organische verbindingen. (Pro-posal to extend the structural space formulasused in present-day chemistry, together with a

related observation concerniing the relationshipbetween optical activity and chemical consti-tution of organic compounds.) Utrecht, J.

Greven, 5. September, 1874.

4. OSTWALD, W.: Lehrbuch der allgemeinen Chemie,ed. 2, vol. 2, part 2. Verwandtschaftslehre.Erster Teil. Leipzig, W. Engelmann, 1896-1903.

5. VAN 'T HOFF, J. H.: La Chimie dans 1 'Espace.Rotterdam, P. M. Bazendijk, 1875.

6. -: The Arrangement of Atoms in Space. Witha preface by Johannes Wislicenus and an ap-

pendix: Stereochenmistry among inorganic sub-stances by Alfred Werner. Tr. and ed. byArnold Eiloart. New York and Bombay,Longmans, Green & Co., 1898.

7. -: Die Lagerung der Atome in Raume. F.Herrmann. Mit einem Vorwort von Johannes

Wislicenus. Braunschweig, Vieweg und Sohn,1877.

8. HARROW, B.: Eminent Chemists of Our Time.Ed. 2-Enl., New York, D. Van Nostrand Co.,1927.

9. VAN'T HOFF, J. H.: rtudes de Dynamique Chimi-

que. Amsterdam, Frederik Muller & Co., 1884.

10. : Studien zur chemisehen Dynamik. Nach J.H. van't Hoff's rtudes de Dynamique Chimi-

que, bearbeitet von Ernst Cohen. Amsterdam,Frederik Muller & Co., Leipzig, Wilhelm Engel-mann, 1896.

11. -: Studies in Chemical Dynamics. Tr. by Thos.Ewan. Amsterdanm, Frederik Muller & Co.,London, Williams and Norgate, 1896.

12. A[RRHENIUS], S.: Book review section, NordiskRevy, 2: columns 364-365, Upsala, March 31,1885.

13. PFEFFER, W. F. P.: Osmiotische Untersuchungen.Leipzig, WV. Engelmann, 1887.

14. NOLLET, L 'ABBn: Recherches sur les Causes duBouillonnement des Liquides. Mem. Acad. Roy.SGi. (Paris), June, 1748.

15. LEWIS, G. N., AND RANDALL, M.: Thermodynamicsand the Free Energy of Chemical Substances.Ed. 1, New York, MeGraw-Hill Book Co., 1923.p. 26.

16. DUTROCHET, R. J. H.: Nouvelles observations sur1 'endosmose et 1 'exosniose, et sur la cause dece double ph6nomene. Ann. chim. et phys., [2],35: 393, 1827.

17. GULDBERG, C. M.: Sur la loi des points de con-g6lation de solutions salines. Coinpt. rend.Acad. Sci. (Paris) 70: 1349, 1870.

18. RAOULT, F.-M.: Loi de congelation des solutionsaqueuses des matieres organiques. Ann. chim.et phys., [5] 28: 133, 1883.

19. VAN'T HOFF, J. H.: (1) Lois de l'quilibrechimique dans 1'itat dilu6, gazeau ou dissous.(2) Uine propriete g6nerale de la matierediluke. (3) Conditioiis 6lectriques de le'equilibrechinmique. K. Svenska vetenskAkad. Handl.21: ilo. 17, 1886.

20. -: L 'equilibre chimique dans les systemes ga-zeux ou dissous a l'etat dilue. Arch. Neerl.Sci. Exactes et Naturelles, [1], 20: 239, 1886.(Essentially identical with (1) of"9.)

21. RAOULT, F.-M.: Sur la tension de vapeur et surla point de cong6lation des solutions salines.Compt. renLd. Acad. Sci. (Paris) 87: 167, 1878.

22. -: Loi gen6rale de cong6lation des dissolvants.Ann. chinm. et phys. [6], 2: 66, 1884,

23. BECKMANN, E.: Studien zur Praxis der Bestim-iiung des Molekulargewichts aus Dampfdrue-keriniedrigungen. Ztschr. phys. Chemie 4: 532,1889.

24. JONES, H. C., ed.: The Modern Theory of Solu-tion. Memoirs by Pfeffer, van 't Hoff, Ar-rhenius, and Raoult. New York and Loindon,Harper, 1899.

25. HORSTMANN, A.: Ueber die Aniwendungen deszweiten Hauptsatzes der Warmetheorie aufchenische Erscheinuingen. Berl. Ber. 14: 1242,1881.

26. RAOULT, F.-M.: Sur les tensions de vapeur desdissolutions faites dans 1 '6ther. Ann. chim.et phys. [6], 15: 375, 1888.

27. -: tber die Dampfdrucke iitherischer Lisungen.Ztschr. phys. Chemie 2: 353, 1888.

28. BANCROFT, W. D., AND DAVIS, H. L.: Osmoticpressures of concentrated solutions. J. Phys.Chem. 32: 1, 1928.

29. RAOULT, F.-M.: Loi generale des tenisions devapeur des dissolvants. Compt. rend. Acad.Sci. (Paris) 104: 1430, 1887.

30. COHEN, ERNST: Jacobus Henricus van 't Hoff:Sein Leben unid Wirken. L.eipzig, AkademischeVerlagsgesellschaft m. b. H., 1912.

31. WALKER, SIR JAMIES: Arrhenius MIemorial Lec-ture. J. Chem. Soc. 1: 1380, 1928.

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32. VAN 'T HoFF, J. H.: Die Rolle des osmotisehenDruekes in der Analogie zwischen L6sungenund Gasen. Ztschr. phys. Chemie 1: 481, 1887.

33. ARRHENIUS, S.: La conductibilite galvanique deslectrolytes. Premiere Partie. La conducti-

bilite des solutions aqueuses extremement di-luees determinee au moyen du depolarisateur.Bihang, K. Svenska vetenskAkad. Handl. 8:no. 13, 1884.

34. -: La conductibilite galvanique des electrolytes.Seconde Partie. Th6orie chimique des elec-trolytes. Bihang, K. Svenska vetenskAkad.Handl. 8: no. 14, 1884.

35. -: Fbrsbk att beriikna Dissociationen (Aktivitet-skoefficienten) hos i vatten iosta Kroppar.(Read on June 8, 1887 by E. Edlund.) Of-

versigt, Svenska vetenskAkad. FPrhandl. 44:405, 1887.

36. -: Ueber additive Eigenschaften der VerdiunntenSalzlsungen. (Read on Nov. 9, 1887, by E.Edlund.) Ofversigt, [Svenska] vetenskAkad.Firhandl. 44: 561, 1887.

37. - :tber die Dissociation der in Wasser ge-ibsten Stoffe. Ztschr. phys. Chemie 1: 631,1887.

38. OSTWALD. W.: Lehrbuch der allegemeinen Chemie.Ed. 2, vol. 1: Stochiometrie. Printing 2, Leip-zig, W. Enigelmann, 1891-93.

39. VAN 'T HOFF, J. H.: The funietioni of osmoticpressure in the analogy between solutions andgases. Phil. Mag. [51, 26: 81, 1888. Thisis a translation of" by W. Ramsay.

ProtoplasmProtoplasmn is a systeim of exquisite sensitiveness. In order that it may survive it

must be protected from too great, or too rapid, or too irregular fluctuations in thephysical, physico-chemiiical, and chemical conditions of the environment. Stability maysometimes be afforded by the natural environment, as in sea water. In other cases anintegument may sufficiently temper the external changes. But by far the most interestingprotection is afforded, as in man and higher animals, by the circulating liquids of theorganism, the blood plasma and lymph, or, as Claude Bernard called them, the milieuinterieur. In his opinion, which I see no reason to dispute, the existence and the con-stancy of the physico-chemical properties of these fluids is a necessary condition forthe evolution of free and independent life. This theory of the constancy of the milieuinterieur was an induction from relatively few facts, but the discoveries of the lastfifty years and the introduction of physico-ehemical methods into phvsiology have provedthat it is well founded.-L. J. Henderson. Blood. A Study in General Physiology. NewHaven, Yale University Press, 1928, p. 20.

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HOMER W. SMITH and HOMER W. SMITHI. THEORY OF SOLUTIONS: "A Knowledge of the Laws of Solutions ..."

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