Ve Immer Bekannten Chemischen Chemie Klassischen Periode Alphabetische Verzeichnis Der Chemischen

8/14/2019 'Ve Immer Bekannten Chemischen Chemie Klassischen Periode Alphabetische Verzeichnis Der Chemischen

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've immer bekannten chemischen Chemie klassischen Periode alphabetische Verzeichnis derchemischen

Harold C. Urey (1893-1981), FG Brickwedde (1903-1989), GM & Murphy (1903-1968)Ein Wasserstoff-Isotope der Masse 2Physical Review 39, 164-165 (1932).

Copyright 1932 von der American Physical Society, reproduziert mit Erlaubnis.Das Proton-Elektron-Grundstck von bekannten Atomkerne zeigt einige ziemlich starkeRegelmigkeiten zwischen Atomen niedriger Ordnungszahl. [1] Bis zu einem einfachen Schritt O16-weise Figur erscheint, in die die nukleare Arten H2, H3 und He4 knnte sehr gut ausgestattet. Birgeund Menzel [2] haben gezeigt, dass die Diskrepanz zwischen der chemischen Atomgewicht desWasserstoffs und Aston Wert durch die Masse Spektrographen durch die Annahme einer Wasserstoff-Isotop der Masse 2 zeigen das Ausma der 1 Teil in 4500 Teile werden knnte entfielen Wasserstoff derMasse 1.Es ist mglich, mit Zuversicht die Berechnung der Dampfdrcke der reinen Stoffe H1H1, H1H2,H1H3, im Gleichgewicht mit dem reinen festen Phasen. Es ist nur notwendig, anzunehmen, dass in derDebye-Theorie der Festkrper-, umgekehrt proportional zur Quadratwurzel der Massen dieserMolekle und die Rotations-und Schwingungs-Energie der Molekle nicht in den Prozess derVerdampfung zu ndern. Diese Annahmen werden in bereinstimmung mit etablierten experimentellenBeweis. Wir finden, dass die Dampfdrcke fr diese Molekle im Gleichgewicht mit den Feststoffen indas Verhltnis von p11: p12: p13 = 1:0.37:0.29 werden sollte. Die Theorie der flssigen Zustand istnicht so verstanden vell, aber es scheint vernnftig zu glauben, dass die Unterschiede in derDampfdruck dieser Molekle im Gleichgewicht mit ihren whould Flssigkeiten recht gro zu sein undsollte es ermglichen, eine rasche Konzentration der schwereren Isotopen, wenn sie vorhanden sind imRckstand aus der einfachen Verdampfung von flssigem Wasserstoff in der Nhe der Tripelpunkt.Dementsprechend sind zwei Proben wurden durch Verdampfen von Wasserstoff groe Mengen vonflssigem Wasserstoff und sammeln das Gas, das aus dem letzten Teil der letzten Kubikzentimetereingedampft vorbereitet. Die erste Probe wurde aus dem Endabschnitt von sechs Liter Flssigkeit beiAtmosphrendruck verdampft gesammelt, und die zweite Probe aus vier Liter bei einem Druckverdampft nur wenige Millimeter ber dem Tripelpunkt. Der Prozess der Verflssigung hatwahrscheinlich keine Wirkung auf die Vernderung der Konzentration der Isotope, da keine merklichenderung wurde in der Probe verdampft beobachtet bei atmosphrischem Druck.Diese Proben wurden fr die Atom-Spektren von H2 und H3 in einer Wasserstoff-Entladungsrohr inHolz so genannten "Black Stage" mit dem zweiten Ordnung eines 21 Fu-Gitter mit einer Dispersionvon 1,31 pro mm ausgefhrt werden untersucht. Mit der Probe verdampft bei Siedetemperatur keineKonzentration so hoch geschtzt worden war festgestellt worden war. Dann stiegen die Forderungen, sodass das Verhltnis der Zeit der Exposition gegenber dem Mindestma, um die H1-Linien auf unserenTellern dich ber 4500:1 war. Unter diesen Bedingungen fanden wir in diesem Beispiel als auch innormalen Wasserstoff schwache Linien auf der berechneten Positionen fr die Linien H2 begleitendenH, H, H. Diese Zeilen mssen allerdings nicht in der Wellenlnge mit einer molekularen Linienstimmen in der Literatur berichtet. [3] Allerdings waren sie so schwach, dass es schwierig war, sichersein, dass sie nicht die Geister der stark berbelichtet Atomlinien.Die Probe von Wasserstoff in der Nhe der Tripelpunkt verdampft zeigt diese Linien stark verbessert,bezogen auf die Linien des H1, sowohl ber die gewhnlicher Wasserstoff-und der ersten Probe. Dierelativen Intensitten kann durch die Anzahl und Intensitt der symmetrischen Geister auf die Plattengerichtet werden. Die Wellenlngen der H2-Linien, die auf diesen Platten leicht innerhalb von ca. 0,02A gemessen werden. Die folgende Tabelle gibt den Mittelwert der beobachteten Verschiebungen dieserLinien von denen der H1 und der berechneten Verschiebungen:Line


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HHHH

calc.

1,7931,3261,1851,119

obs.

Ordentliche Wasserstoff--1,3461,2061,145

1. Stichprobe--1,3301,1191,103

2. Probe1,8201,3151,176--

Die H2-Linien sind breit, wie es zu einer engen ungelste Dubletten zu erwarten, aber sie sind nicht sobreit und diffus wie die H1 Linien wahrscheinlich aufgrund der geringeren Dopplerverbreiterung.Obwohl ihre relativen Intensitten, um die Geister der jeweiligen H1 Linien erscheinen nahezukonstant fr eine Probe von Wasserstoff, sind sie nicht fr ihre Geister Intensitten im Vergleich zu denbekannten Geister fr ihre Intensitten sind nicht dasselbe im Falle des gewhnlichen Wasserstoffs unddes 1. Stichprobe, wie sie im Falle der zweiten Probe. Sie sind nicht molekularen Linien fr sie nichtauf einer Platte mit dem Entladungsrohr in der "weien Stadium" mit den molekularen Spektrumerweitert (H2 getroffen wurde, als eine leichte Unregelmigkeit auf einem Mikrophotometer Kurvevon dieser Platte zu finden). Schlielich die H2 Zeile ist in ein Wams mit einem Abstand von etwa0,16 im Einvernehmen mit den beobachteten Trennung der H1 Linie gelst.Die relative Hufigkeit in gewhnlichem Wasserstoff zu urteilen aus der relativen Minimierung derExposition betrgt etwa 1:4000, oder weniger, im Einvernehmen mit Birge und Menzel schtzt. Einehnliche Schtzung der Zahl in der zweiten Probe angegeben einer Konzentration von etwa 1 in 800.So eine sprbare Fraktionierung wurde gesichert, als von der Theorie erwartet. [4] keine Beweise frH3 gesichert ist, aber die Linien wrden die Gebiete von unseren Tellern fallen, wenn der Lichthof istschlecht.Die Destillation wurde auf dem Bureau of Standards durchgefhrt von einem von uns (FGB), der dieFortsetzung der Fraktionierung mehr hochkonzentrierten Proben zu sichern. Die spektroskopische


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Arbeit wurde an der Columbia University durchgefhrt von den beiden anderen (HCU und GMM), diearbeiten an der molekularen Spektrum.Harold C. UreyF. G. BrickweddeG. M. MurphyColumbia University,

New York, N. Y.Bureau of Standards,Washington, D. C.

5. Dezember 1931.

[1] Urey, J. Am. Chem. Soc. 53, 2872 (1931), Johnston, ibid., 53, 2866 (1931).


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[2] Birge und Menzel, Phys. Rev. 37, 1669 (1931).

[3] Gale, Monk und Lee, Astrophys. J. 57, 89 (1928); Finkelnburg, Z. Physik 52, 57 (1928); Connelly,Proc. Phys. Soc. 42, 28 (1929).[4] Keesom und van Dijk, Proc. Acad. Sci. Amsterdam 34, 52 (1931).

Zurck zur Liste der ausgewhlten historischen Papieren.Back to the top of Classic Chemie.


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SIS SUS


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batang


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Frederick Soddy (1877-1956)Intra-Atomladung.

Nature 92, 399-400 (4. Dezember 1913)

Dass die Intra-Atomladung eines Elements wird durch seinen Platz im Periodensystem bestimmt nichtdurch seine Atommasse, wie von A. van der Broek (Nature, 27. November, S. 372) geschlossen, iststark von der bisherigen Untersttzung Verallgemeinerungen in Bezug auf die Radio-Elemente und dieperiodische Gesetz. Die aufeinander folgenden Vertreibung ein -und zwei Teilchen in drei radio-aktiven nderungen in beliebiger Reihenfolge bringt die Intra-Atomladung des Elements zurck zuihrem ursprnglichen Wert, und das Element wieder in seinen ursprnglichen Platz in der Tabelle,obwohl seine Atommasse reduziert wird um vier Einheiten. Wir haben vor kurzem erhalten so etwas


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wie einen direkten Beweis der Ansicht van der Broek, dass die intra-atomare Ladung des Kerns einesAtoms ist nicht eine rein positive Ladung, wie auf vorlufige Theorie Rutherford. aber ist derUnterschied zwischen einer positiven und einer negativen Ladung kleiner.

Fajans, sich in seiner Abhandlung ber die periodischen Verallgemeinerung (Physikal. Zeitschr., 1913,vol. Xiv., S. 131), lenkte die Aufmerksamkeit auf die Tatsache, dass die Vernderungen der chemischen

Natur auf die Ausweisung von -und Teilchen daraus genau der gleichen Art wie in der gewhnlichenelektrochemischen Vernderungen der Wertigkeit. Er zog daraus den Schluss, dass radio-aktiven

nderungen mssen in der gleichen Region der atomaren Struktur wie gewhnliche chemische

Vernderungen, anstatt mit einem ausgeprgten inneren Bereich des Bauwerks oder "Kern auftreten",wie bisher angenommen. In meinem Vortrag auf der gleichen Verallgemeinerung, dass unmittelbar nachder Fajans verffentlicht (Chem. News, 28. Februar), legte ich Wert auf die absolute Identitt deschemischen Eigenschaften der verschiedenen Elemente besetzen den gleichen Platz im


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Periodensystem.

Eine einfache Folgerung aus dieser Sicht versorgte mich mit einem Mittel zur Kontrolle der Richtigkeitder Schlussfolgerung, dass Fajans Radio-Vernderungen und chemischen Vernderungen sind mit dergleichen Region der atomaren Struktur betroffen. Nach meiner Meinung seiner Schlussfolgerung wrenichts anderes, als dass zum Beispiel, Uran in seiner vierwertigen uranous Verbindungen beteiligt sein

mssen chemisch identisch sein mit und nicht trennbar von Thorium-Verbindungen. Bei Uranium-X,aus Uran ich durch Ausweisung eines Teilchen gebildet, ist chemisch identisch mit Thorium, wie diesauch Ionium in der gleichen Weise vom Uran II gebildet. Uran X verliert zwei Teilchen und gehtzurck in Uran II, chemisch identisch mit Uran. Uranous Salze verlieren auch zwei Elektronen und indie mehr sechswertiges Uranylverbindungen weiterzugeben. Wenn diese Elektronen aus der gleichenRegion des Atoms uranous Salze kommen sollte chemisch nicht trennbar von Thoriumsalze. Aber siesind es nicht.

Es besteht eine starke hnlichkeit in der chemischen Charakter zwischen uranous und Thoriumsalze,und ich fragte Herr Fleck zu prfen, ob sie auf chemischem Wege trennen knnte, wenn gemischtwerden, wobei das Uran unverndert bleiben berall in der uranous oder vierwertigen Zustand. HerrFleck wird die Experimente gesondert bekannt geben, und ich bin ihm zu Dank verpflichtet fr dasErgebnis, dass die zwei Klassen von Verbindungen knnen leicht durch Fraktionierung getrenntwerden.

Dies, so denke ich, beluft sich auf ein Beweis dafr, dass die Elektronen als -Strahlen aus einemKern nicht liefern kann Elektronen oder zurckzukaufen, um sie aus dem Ring kommen vertrieben,obwohl dieser Ring ist zu gewinnen oder zu verlieren Elektronen von auen whrend der gewhnlichenelektrochemischen Vernderungen fhig der Wertigkeit.

Ich halte Ansicht van der Broek, dass die Zahl, die die positive Netto-Ladung des Kerns der Zahl der

der Stelle, die das Element befindet sich im Periodensystem, wenn alle mglichen Orte, vonWasserstoff bis Uran in der Reihenfolge angeordnet sind, als praktisch erwiesen, so ist Was denrelativen Wert der Ladung fr die Mitglieder des Ende der Sequenz, von Thallium, Uran, betroffen ist.Wir sind im Unklaren gelassen, um den absoluten Wert der Ladung, wegen der Zweifel ber die genaueZahl der "Seltenen Erden", die es gibt. Wenn wir davon ausgehen, dass all diese bekannt sind, den Wertfr die positive Ladung des Kerns des Uran-Atoms ist etwa 90. Der Erwgung, dass, wenn wir die eherzweifelhaft Annahme, dass die periodischen fhrt regelmig zu machen, hinsichtlich der Zahl derOrte, durch die Selten-Erd-Gruppe, und dass zwischen Barium und Radium, zum Beispiel, zweikomplette lange Zeit vorhanden ist, wird die Zahl 96. In jedem Fall ist es deutlich weniger als 120, die


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Anzahl der Gebhr in Hhe eines halben Atomgewicht, wie es wre, wenn der Kern der -Partikel nurausgefertigt. Sechs nukleare Elektronen sind dafr bekannt, in der Uran-Atom, das in seinerVernderungen sechs -Strahlen vertreibt existieren. Sind der Kern, der sich aus Teilchen muss esdreiig oder vierundzwanzig nuklearen bzw. Elektronen, verglichen mit sechsundneunzig oder 102bzw. in den Ring. Wenn, wie vorgeschlagen wurde, ist Wasserstoff eine zweite Komponente deratomaren Struktur, es muss mehr als diese. Aber es kann kein Zweifel, dass es einige werden muss, und

dass die zentrale Ladung des Atoms auf die Theorie Rutherford kann nicht eine reine positive Ladung,sondern mssen Elektronen enthalten, wie van der Broek abschliet.

Soweit ich persnlich betroffen bin, hat dies in einer groen Klrung meiner Ideen gefhrt, und es kannhilfreich sein, andere, obwohl kein Zweifel daran gibt es wenig Originalitt in ihm. Das gleichealgebraische Summe der positiven und negativen Ladungen im Kern, als die arithmetische Summeunterschiedlich ist, gibt, was ich "Isotope" oder "Isotopen-Elemente" nennen, weil sie den gleichenPlatz im Periodensystem zu besetzen. Sie sind chemisch identisch, und speichern Sie nur im Hinblickauf die relativ wenigen physikalischen Eigenschaften, die auf Atommasse hngen direkt, auchkrperlich identisch. Unit Vernderungen dieser Kernladung, so algebraisch rechnen, geben dienachfolgenden Pltze im Periodensystem. Fr ein "Ort", oder ein Kernladung, mehr als eine Anzahlvon Elektronen in der ueren Ring-System bestehen knnen und in einem solchen Fall das Elementweist variable Wertigkeit. Aber solche Vernderungen der Anzahl oder der Wertigkeit, beziehen sich

nur auf den Ring und externen Umfeld. Es gibt keinen In-und Out-Gehen von Elektronen zwischenRing und Kern.

Frederick SoddyLaboratorium fr Physikalische Chemie,University of Glasgow.


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-------------------------------------------------- ------------------------------Zurck zur Liste der ausgewhlten historischen Papieren.Back to the top of Classic Chemistry.The Streuung von -und Teilchen durch Materie und derStruktur des AtomsE. Rutherford, F.R.S. *Philosophical Magazine

Series 6, vol. 21Mai 1911, S. 669-688

-------------------------------------------------- ------------------------------

669

1. Es ist bekannt, dass die -und die Teilchen Ablenkungen leiden unter ihrer geradlinigen Pfadendurch Begegnungen mit den Atomen der Materie. Diese Streuung ist viel strker ausgeprgt als fr die fr die Teilchen wegen der viel kleiner Impuls und Energie der ehemaligen Teilchen. Es scheintkeinen Zweifel daran, dass diese sich schnell bewegenden Teilchen durchlaufen die Atome in den Weg,und dass die Ablenkungen beobachtet werden durch das starke elektrische Feld im atomaren Systemdurchlaufen. Es wurde allgemein angenommen, dass die Streuung der ein Bschel von oder -Strahlen beim Durchgang durch eine dnne Platte der Materie das Ergebnis einer Vielzahl von kleinenStreuungen von den Atomen der Materie durchzogen ist. Die Beobachtungen zeigen jedoch, der Geigerund Marsden ** ber die Streuung von -Strahlen, dass einige der Teilchen, etwa 1 in 20.000 biseinem mittleren Winkel von 90 Grad wurden im Vorbeigehen sich aber eine Schicht aus Gold-Folieber 0,00004 cm . dick, das entspricht in Anhalten-Macht der Partikel bis 1,6 Millimeter Luft war.Geiger *** zeigte spter, dass die wahrscheinlichste Winkel der Ablenkung fr ein Bschel von Teilchen abgelenkt ber 90 Grad ist verschwindend klein. Darber hinaus wird es spter sehen, da dieVerteilung der Teilchen fr verschiedene Winkel der groen Ablenkung nicht folgen dieWahrscheinlichkeit Recht zu erwarten, wenn so groe Ablenkung von einer groen Anzahl von kleinenAbweichungen nach oben vorgenommen werden. Es scheint vernnftig, anzunehmen, dass dieAblenkung durch einen groen Winkel zu einer einzelnen atomaren Begegnung zurckzufhren ist, frdie Chance auf eine zweite Begegnung der Art, dass eine groe Ablenkung produzieren, mssen in denmeisten Fllen sehr gering sein. Eine einfache Rechnung zeigt, dass das Atom muss ein Sitz einesstarken elektrischen Feldes, um eine so groe Ablenkung in einer einzigen Begegnung zu produzieren.

Krzlich JJ Thomson **** hat einen Theorie

* Mitgeteilt durch den Autor. Ein kurzer Bericht ber dieses Papier wurde auf der Manchester Literaryand Philosophical Society im Februar mitgeteilt, 1911.** Proc. Roy. Soc. LXXXII, S. 495 (1909)*** Proc. Roy. Soc. LXXXIII, S. 492 (1910)**** Camb. Lit. & Phil Soc. xv pt. 5 (1910)

670

erklren, die Streuung von geladenen Teilchen beim Durchgang durch geringen Dicken der Materie.Das Atom soll der eine Anzahl N von negativ geladenen Blutkrperchen, begleitet von einer gleichenMenge von positiver Elektrizitt aus gleichmig ber eine Kugel verteilt. Die Ablenkung eines negativgeladenen Teilchen beim Durchgang durch das Atom ist auf zwei Ursachen zurckzufhren - (1) dieAbstoung der Blutkrperchen durch das Atom verteilt, und (2) die Anziehungskraft der positiven


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Elektrizitt im Atom. Die Ablenkung der Teilchen beim Durchgang durch das Atom soll klein sein,whrend die durchschnittliche Ablenkung, nachdem eine groe Zahl m der Begegnung als bernahm[die Quadratwurzel] m , wenn ist der durchschnittliche Ablenkung durch einen einzigen Atom. Eskonnte gezeigt werden, dass die Anzahl N der Elektronen im Atom aus der Beobachtung der Streuungableiten konnte, war experimentell untersucht Crowther * in einer spteren Arbeit werden. SeineErgebnisse offenbar besttigt, der die wichtigsten Schlussfolgerungen der Theorie, und er leitete, auf

der Annahme, da die positive Elektrizitt Zeitpunkt unterbrochen war, dass die Zahl der Elektronen ineinem Atom etwa dreimal sein Atomgewicht wurde.

Die Theorie von JJ Thomson ist auf der Annahme, dass die Streuung durch eine einzige atomareBegegnung ist klein, und die besondere Struktur fr das Atom nicht fr eine sehr groe Ablenkung derDurchmesser der Kugel von positiver Elektrizitt zugeben, ist winzig im Vergleich mit demDurchmesser des Einflussbereichs des Atoms.

Da die -und Teilchen durchqueren das Atom, sollte es mglich sein, von einer engen Studie ber dieArt der Ablenkung, um eine Vorstellung von der Konstitution des Atoms, die Effekte zu produzierenForm beobachtet. In der Tat, die Streuung von High-Speed-geladene Teilchen durch die Atome derMaterie ist eine der vielversprechendsten Methoden des Angriffs dieses Problems. Die Entwicklung derSzintillationsmethode zhlen einzigen Teilchen bietet auergewhnliche Vorteile der Untersuchung,und die Untersuchungen von H. Geiger mit dieser Methode haben schon viel zu unserem Wissen berdie Streuung von -Strahlen von der Materie hat.

2. Wir werden zunchst theoretisch untersucht die einzelnen Begegnungen ** mit einem Atom voneinfacher Struktur, die in der Lage ist,

* Crowther, Proc. Roy. Soc. LXXXIV. S. 226 (1910)** Die Abweichung eines Teilchens in einem betrchtlichen Winkel von einer Begegnung mit einemeinzigen Atom wird in diesem Papier die Bezeichnung "Single"-Streuung. Die Abweichung einesTeilchens, die sich aus einer Vielzahl von kleinen Abweichungen wird man als "Verbindung" Streuung.

671

produzieren groe Verformungen eines Teilchen, und vergleichen Sie dann die Folgerungen aus derTheorie mit den experimentellen Daten zur Verfgung.

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Harold C. Urey (1893-1981), F. G. Brickwedde (1903-1989), & G.

M. Murphy (1903-1968)

A Hydrogen Isotope of Mass 2

Physical Review39, 164-165 (1932).Copyright 1932 by the American Physical Society; reproduced with permission.

The proton-electron plot of known atomic nuclei shows some rather marked regularities among atomsof lower atomic number.[1] Up to O16 a simple step-wise figure appears into which the nuclear speciesH2, H3 and He4 could be fitted very nicely. Birge and Menzel[2] have shown that the discrepancybetween the chemical atomic weight of hydrogen and Aston's value by the mass spectrograph could beaccounted for by the assumption of a hydrogen isotope of mass 2 present to the extent of 1 part in 4500parts of hydrogen of mass 1.

It is possible to calculate with confidence the vapor pressures of the pure substances H 1H1, H1H2,H1H3, in equilibrium with the pure solid phases. It is only necessary to assume that in the Debye theory

of the solid state, is inversely proportional to the square root of the masses of these molecules andthat the rotational and vibrational energies of the molecules do not change in the process ofvaporization. These assumptions are in accord with well-established experimental evidence. We findthat the vapor pressures for these molecules in equilibrium with their solids should be in the ratio ofp11:p12:p13 = 1:0.37:0.29. The theory of the liquid state is not so vell understood but it seems

reasonable to believe that the differences in vapor pressure of these molecules in equilibrium with theirliquids whould be rather large and should make possible a rapid concentration of the heavier isotopes,if they exist, in the residue from the simple evaporation of liquid hydrogen near its triple point.

Accordingly two samples of hydrogen were prepared by evaporating large quantities of liquidhydrogen and collecting the gas which evaporated from the last fraction of the last cubic centimeter.The first sample was collected from the end portion of six liters of liquid evaporated at atmospheric

pressure, and the second sample from four liters evaporated at a pressure only a few millimeters abovethe triple point. The process of liquefaction has probably no effect in changing the concentration of theisotopes since no appreciable change was observed in the sample evaporated at atmospheric pressure.

These samples were investigated for the atomic spectra of H2 and H3 in a hydrogen discharge tube runin Wood's so-called "black stage" by using the second order of a 21 foot grating with a dispersion of1.31 per mm. With the sample evaporated at the boiling point no concentration so high as had beenestimated was detected. We then increased the exposures so that the ratio of the time of exposure to theminimum required to get the H1 lines on our plates was about 4500:1. Under these conditions we foundin this sample as well as in ordinary hydrogen faint lines at the calculated positions for the lines of H2

accompanying H, H, H. These lines do not agree in wavelength with any molecular lines reported in

the literature.[3] However they were so weak that it was difficult to be sure that they were not ghosts ofthe strongly overexposed atomic lines.

The sample of hydrogen evaporated near the triple point shows these lines greatly enhanced, relative tothe lines of H1, over both those of ordinary hydrogen and of the first sample. The relative intensitiescan be judged by the number and intensity of the symmetrical ghosts on the plates. The wave-lengths ofthe H2 lines appearing on these plates could be easily measured within about 0.02 . The followingtable gives the mean of the observed displacements of these lines from those of H1 and the calculateddisplacements:


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Line H H H H

calc. 1.793 1.326 1.185 1.119

obs.

Ordinary hydrogen -- 1.346 1.206 1.145

1st sample -- 1.330 1.119 1.103

2nd sample 1.820 1.315 1.176 --The H2 lines are broad, as is to be expected for close unresolved doublets, but they are not as broad anddiffuse as the H1 lines probably due to the smaller Dppler broadening. Although their intensitiesrelative to the ghosts of the respective H1 lines appear nearly constant for any one sample of hydrogen,they are not ghosts for their intensities relative to the known ghosts for their intensities are not the samein the case of ordinary hydrogen and of the 1st sample as they are in the case of the second sample.They are not molecular lines for they do not appear on a plate taken with the discharge tube in the"white stage" with the molecular spectrum enhanced (H2 was found as a slight irregularity on a

microphotometer curve of this plate). Finally the H2 line is resolved into a doublet with a separation of

about 0.16 in agreement with the observed separation of the H1 line.

The relative abundance in ordinary hydrogen, judging from relative minimum exposure time is about1:4000, or less, in agreement with Birge and Menzel's estimate. A similar estimate of the abundance inthe second sample indicated a concentration of about 1 in 800. Thus an appreciable fractionation hasbeen secured as expected from theory.[4] No evidence for H3 has been secured, but its lines would fallon regions of our plates where the halation is bad.

The distillation was carried out at the Bureau of Standards by one of us (F.G.B.), who is continuing thefractionation to secure more highly concentrated samples. The spectroscopic work was done atColumbia University by the other two (H.C.U. and G.M.M.) who are working on the molecularspectrum.

Harold C. UreyF. G. BrickweddeG. M. Murphy

Columbia University,New York, N. Y.

Bureau of Standards,Washington, D. C.

December 5, 1931.

[1]Urey, J. Am. Chem. Soc. 53, 2872 (1931); Johnston, ibid., 53, 2866 (1931).

[2]Birge and Menzel, Phys. Rev. 37, 1669 (1931).

[3]Gale, Monk and Lee, Astrophys. J. 57, 89 (1928); Finkelnburg, Z. Physik52, 57 (1928); Connelly,Proc. Phys. Soc. 42, 28 (1929).

[4]Keesom and van Dijk, Proc. Acad. Sci. Amsterdam 34, 52 (1931).

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Back to the top ofClassic Chemistry.Frederick Soddy (1877-1956)

Intra-atomic Charge.

Nature 92, 399-400 (December 4, 1913)

That the intra-atomic charge of an element is determined by its place in the periodic table rather than by

its atomic weight, as concluded by A. van der Broek (NATURE, November 27, p. 372), is stronglysupported by the recent generalisation as to the radio-elements and the periodic law. The successiveexpulsion of one and two particles in three radio-active changes in any order brings the intra-atomiccharge of the element back to its initial value, and the element back to its original place in the table,though its atomic mass is reduced by four units. We have recently obtained something like a directproof of van der Broek's view that the intra-atomic charge of the nucleus of an atom is not a purelypositive charge, as on Rutherford's tentative theory. but is the difference between a positive and asmaller negative charge.

Fajans, in his paper on the periodic law generalisation (Physikal. Zeitsch., 1913, vol. xiv., p. 131),

directed attention to the fact that the changes of chemical nature consequent upon the expulsion of and particles are precisely of the same kind as in ordinary electrochemical changes of valency. Hedrew from this the conclusion that radio-active changes must occur in the same region of atomicstructure as ordinary chemical changes, rather than with a distinct inner region of structure or"nucleus," as hitherto supposed. In my paper on the same generalisation, published immediately afterthat of Fajans (Chem. News, February 28), I laid stress on the absolute identity of chemical propertiesof different elements occupying the same place in the periodic table.

A simple deduction from this view supplied me with a means of testing the correctness of Fajans'sconclusion that radio-changes and chemical changes are concerned with the same region of atomicstructure. On my view his conclusion would involve nothing else than that, for example, uranium in itstetravalent uranous compounds must be chemically identical with and non-separable from thoriumcompounds. For uranium X, formed from uranium I by expulsion of an particle, is chemicallyidentical with thorium, as also is ionium formed in the same way from uranium II. Uranium X losestwo particles and passes back into uranium II, chemically identical with uranium. Uranous salts alsolose two electrons and pass into the more hexavalent uranyl compounds. If these electrons come fromthe same region of the atom uranous salts should be chemically non-separable from thorium salts. Butthey are not.

There is a strong resemblance in chemical character between uranous and thorium salts, and I askedMr. Fleck to examine whether they could be separated by chemical methods when mixed, the uranium

being kept unchanged throughout in the uranous or tetravalent condition. Mr. Fleck will publish theexperiments separately, and I am indebted to him for the result that the two classes of compounds canreadily be separated by fractionation methods.

This, I think, amounts to a proof that the electrons expelled as rays come from a nucleus not capableof supplying electrons to or withdrawing them from the ring, though this ring is capable of gaining orlosing electrons from the exterior during ordinary electrochemical changes of valency.
http://index.html/http://index.html/


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I regard van der Broek's view, that the number representing the net positive charge of the nucleus is thenumber of the place which the element occupies in the periodic table when all the possible places fromhydrogen to uranium are arranged in sequence, as practically proved so far as the relative value of thecharge for the members of the end of the sequence, from thallium to uranium, is concerned. We are leftuncertain as to the absolute value of the charge, because of the doubt regarding the exact number of

rare-earth elements that exist. If we assume that all of these are known, the value for the positive chargeof the nucleus of the uranium atom is about 90. Whereas if we make the more doubtful assumption thatthe periodic table runs regularly, as regards numbers of places, through the rare-earth group, and thatbetween barium and radium, for example, two complete long periods exist, the number is 96. In eithercase it is appreciably less than 120, the number were the charge equal to one-half the atomic weight, asit would be if the nucleus were made out of particles only. Six nuclear electrons are known to exist inthe uranium atom, which expels in its changes six rays. Were the nucleus made up of particles theremust be thirty or twenty-four respectively nuclear electrons, compared with ninety-six or 102respectively in the ring. If, as has been suggested, hydrogen is a second component of atomic structure,there must be more than this. But there can be no doubt that there must be some, and that the centralcharge of the atom on Rutherford's theory cannot be a pure positive charge, but must contain electrons,

as van der Broek concludes.

So far as I personally am concerned, this has resulted in a great clarification of my ideas, and it may behelpful to others, though no doubt there is little originality in it. The same algebraic sum of the positiveand negative charges in the nucleus, when the arithmetical sum is different, gives what I call "isotopes"or "isotopic elements," because they occupy the same place in the periodic table. They are chemicallyidentical, and save only as regards the relatively few physical properties which depend on atomic massdirectly, physically identical also. Unit changes of this nuclear charge, so reckoned algebraically, givethe successive places in the periodic table. For any one "place," or any one nuclear charge, more thanone number of electrons in the outer-ring system may exist, and in such a case the element exhibits

variable valency. But such changes of number, or of valency, concern only the ring and its externalenvironment. There is no in- and out-going of electrons between ring and nucleus.

FREDERICK SODDY

Physical Chemistry Laboratory,

University of Glasgow.

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Back to the list of selected historical papers.

Back to the top of Classic Chemistry.The Scattering of and Particles by Matter and the Structure ofthe Atom

E. Rutherford, F.R.S.*

Philosophical Magazine


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Series 6, vol. 21

May 1911, p. 669-688

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1. It is well known that the and the particles suffer deflexions from their rectilinear paths byencounters with atoms of matter. This scattering is far more marked for the than for the particle onaccount of the much smaller momentum and energy of the former particle. There seems to be no doubtthat such swiftly moving particles pass through the atoms in their path, and that the deflexions observedare due to the strong electric field traversed within the atomic system. It has generally been supposedthat the scattering of a pencil of or rays in passing through a thin plate of matter is the result of amultitude of small scatterings by the atoms of matter traversed. The observations, however, of Geigerand Marsden** on the scattering of rays indicate that some of the particles, about 1 in 20,000 wereturned through an average angle of 90 degrees in passing though a layer of gold-foil about 0.00004 cm.thick, which was equivalent in stopping-power of the particle to 1.6 millimetres of air. Geiger***showed later that the most probable angle of deflexion for a pencil of particles being deflectedthrough 90 degrees is vanishingly small. In addition, it will be seen later that the distribution of the particles for various angles of large deflexion does not follow the probability law to be expected if suchlarge deflexion are made up of a large number of small deviations. It seems reasonable to suppose thatthe deflexion through a large angle is due to a single atomic encounter, for the chance of a secondencounter of a kind to produce a large deflexion must in most cases be exceedingly small. A simplecalculation shows that the atom must be a seat of an intense electric field in order to produce such alarge deflexion at a single encounter.

Recently Sir J. J. Thomson**** has put forward a theory to

* Communicated by the Author. A brief account of this paper was communicated to the ManchesterLiterary and Philosophical Society in February, 1911.

** Proc. Roy. Soc. lxxxii, p. 495 (1909)

*** Proc. Roy. Soc. lxxxiii, p. 492 (1910)

**** Camb. Lit. & Phil Soc. xv pt. 5 (1910)

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explain the scattering of electrified particles in passing through small thicknesses of matter. The atom issupposed to consist of a number N of negatively charged corpuscles, accompanied by an equal quantityof positive electricity uniformly distributed throughout a sphere. The deflexion of a negativelyelectrified particle in passing through the atom is ascribed to two causes -- (1) the repulsion of thecorpuscles distributed through the atom, and (2) the attraction of the positive electricity in the atom.


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The deflexion of the particle in passing through the atom is supposed to be small, while the averagedeflexion after a large number m of encounters was taken as [the square root of] m , where is theaverage deflexion due to a single atom. It was shown that the number N of the electrons within theatom could be deduced from observations of the scattering was examined experimentally by Crowther*in a later paper. His results apparently confirmed the main conclusions of the theory, and he deduced,on the assumption that the positive electricity was continuous, that the number of electrons in an atom

was about three times its atomic weight.

The theory of Sir J. J. Thomson is based on the assumption that the scattering due to a single atomicencounter is small, and the particular structure assumed for the atom does not admit of a very largedeflexion of diameter of the sphere of positive electricity is minute compared with the diameter of thesphere of influence of the atom.

Since the and particles traverse the atom, it should be possible from a close study of the nature ofthe deflexion to form some idea of the constitution of the atom to produce the effects observed. In fact,the scattering of high-speed charged particles by the atoms of matter is one of the most promising

methods of attack of this problem. The development of the scintillation method of counting single particles affords unusual advantages of investigation, and the researches of H. Geiger by this methodhave already added much to our knowledge of the scattering of rays by matter.

2. We shall first examine theoretically the single encounters** with an atom of simple structure,which is able to

* Crowther, Proc. Roy. Soc. lxxxiv. p. 226 (1910)

** The deviation of a particle throughout a considerable angle from an encounter with a single atomwill in this paper be called 'single' scattering. The deviation of a particle resulting from a multitude ofsmall deviations will be termed 'compound' scattering.

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produce large deflections of an particle, and then compare the deductions from the theory with theexperimental data available.

Consider an atom which contains a charge Ne at its centre surrounded by a sphere of electrificationcontaining a charge Ne [N.B. in the original publication, the second plus/minus sign is inverted to be aminus/plus sign] supposed uniformly distributed throughout a sphere of radius R. e is the fundamentalunit of charge, which in this paper is taken as 4.65 x 1010 E.S. unit. We shall suppose that fordistances less than 1012 cm. the central charge and also the charge on the alpha particle may besupposed to be concentrated at a point. It will be shown that the main deductions from the theory areindependent of whether the central charge is supposed to be positive or negative. For convenience, thesign will be assumed to be positive. The question of the stability of the atom proposed need not beconsidered at this stage, for this will obviously depend upon the minute structure of the atom, and on


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the motion of the constituent charged parts.

In order to form some idea of the forces required to deflect an alpha particle through a large angle,consider an atom containing a positive charge Ne at its centre, and surrounded by a distribution ofnegative electricity Ne uniformly distributed within a sphere of radius R. The electric force X and thepotential V at a distance r from the centre of an atom for a point inside the atom, are given by

Suppose an particle of mass m and velocity u and charge E shot directly towards the centre of theatom. It will be brought to rest at a distance b from the centre given by

It will be seen that b is an important quantity in later calculations. Assuming that the central charge is100 e, it can be calculated that the value of b for an particle of velocity 2.09 x 109 cms. per second isabout 3.4 x 1012 cm. In this calculation b is supposed to be very small compared with R. Since R issupposed to be of the order of the radius of the atom, viz. 108 cm., it is obvious that the particlebefore being turned back penetrates so close to

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the central charge, that the field due to the uniform distribution of negative electricity may beneglected. In general, a simple calculation shows that for all deflexions greater than a degree, we maywithout sensible error suppose the deflexion due to the field of the central charge alone. Possible singledeviations due to the negative electricity, if distributed in the form of corpuscles, are not taken intoaccount at this stage of the theory. It will be shown later that its effect is in general small comparedwith that due to the central field.

Consider the passage of a positive electrified particle close to the centre of an atom. Supposing that thevelocity of the particle is not appreciably changed by its passage through the atom, the path of theparticle under the influence of a repulsive force varying inversely as the square of the distance will bean hyperbola with the centre of the atom S as the external focus. Suppose the particle to enter the atomin the direction PO (fig. 1), and that the direction of motion

on escaping the atom is OP'. OP and OP' make equal angles with the line SA, where A is the apse of thehyperbola. p = SN = perpendicular distance from centre on direction of initial motion of particle.


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f . . . . . . . 5.7 11.4 28 53 90 127 152

3. Probability of single deflexion through any angle

Suppose a pencil of electrified particles to fall normally on a thin screen of matter of thickness t. Withthe exception of the few particles which are scattered through a large angle, the particles are supposedto pass nearly normally through the plate with only a small change of velocity. Let n = number of atomsin unit volume of material. Then the number of collisions of the particle with the atom of radius R isR2nt in the thickness t.

* A simple consideration shows that the deflexion is unaltered if the forces are attractive instead ofrepulsive.

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The probability m of entering an atom within a distance p of its center is given by

m = p2nt.

Chance dm of striking within radii p and p + dp is given by

dm = 2pnt . dp = ( / 4)ntb2 cot f/2 cosec2 f/2 df . . . . (2)

since

cot f/2 = 2p / b

The value of dm gives the fraction of the total number of particles which are deviated between theangles f and f + df.

The fraction p of the total number of particles which are deflected through an angle greater than f isgiven by

p = ( / 4)ntb2 cot2 f/2 . . . . . . (3)

The fraction p which is deflected between the angles f1 and f2 is given by


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p = ( / 4)ntb2 (cot2 f1/2 - cot2 f2/2) . . . . . . . . . . . . . (4)

It is convenient to express the equation (2) in another form for comparison with experiment. In the caseof the rays, the number of scintillations appearing on the constant area of the zinc sulphide screen are

counted for different angles with the direction of incidence of the particles. Let r = distance from pointof incidence of rays on scattering material, then if Q be the total number of particles falling on thescattering material, the number y of particles falling on unit area which are deflected through anangle f is given by

y = Qdm / 2r2 sin f . df = (ntb2 . Q . cosec4 f/2) / 16r2 . . . . . . . (5)

Since b = 2NeE / mu2, we see from this equation that the number of particles (scintillations) per unitarea of zinc sulphide screen at a given distance r from the point of

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Incidence of the rays is proportional to

(1) cosec4 f/2 or 1/f4 if f be small;

(2) thickness of scattering material t provided this is small;

(3) magnitude of central charge Ne;

(4) and is inversely proportional to (mu2)2, or to the fourth power of the velocity if m be constant.

In these calculations, it is assumed that the particles scattered through a large angle suffer only onelarge deflexion. For this to hold, it is essential that the thickness of the scattering material should be sosmall that the chance of a second encounter involving another large deflexion is very small. If, forexample, the probability of a single deflexion f in passing through a thickness t is 1/1000, theprobability of two successive deflexions each of value f is 1/106 , and is negligibly small.

The angular distribution of the particles scattered from a thin metal sheet affords one of the simplestmethods of testing the general correctness of this theory of single scattering. This has been done

recently for rays by Dr. Geiger,* who found that the distribution for particles deflected between 30and 150 from a thin gold-foil was in substantial agreement with the theory. A more detailed account ofthese and other experiments to test the validity of the theory will be published later.

4. Alteration of velocity in an atomic encounter

It has so far been assumed that an or particle does not suffer an appreciable change of velocity asthe result of a single atomic encounter resulting in a large deflexion of the particle. The effect of such


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an encounter in altering the velocity of the particle can be calculated on certain assumptions. It issupposed that only two systems are involved, viz., the swiftly moving particle and the atom which ittraverses supposed initially at rest. It is supposed that the principle of conservation of momentum andof energy applies, and that there is no appreciable loss of energy or momentum by radiation.

* Manch. Lit. & Phil. Soc. 1910.

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Let m be mass of the particle,

1 = velocity of approach,

2 = velocity of recession,

M= mass of atom,V = velocity communicated to atom as result of encounter.

Let OA (fig. 2) represent in magnitude and direction the momentum m1 of the entering particle, andOB the momentum of the receding particle which has been turned through an angle AOB = f. Then BArepresents in magnitude and direction the momentum MV of the recoiling atom.

(MV)2 = (m1)2 + (m2)2 - 2m212 cos f . . . (1)

By conservation of energy

MV2 = m12 - m22 . . . . .(2)

Suppose M/m = K and 2 = p1, where p < 1.

From (1) and (2),

Consider the case of an particle of atomic weight 4, deflected through an angle of 90 by anencounter with an atom of gold of atomic weight 197.

Since K= 49 nearly,


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or the velocity of the particle is reduced only about 2 per cent. by the encounter.

In the case of aluminium K=27/4 and for f = 90 p = 0.86.

It is seen that the reduction of velocity of the particle becomes marked on this theory for encounterswith the lighter atoms. Since the range of an particle in air or other matter is approximatelyproportional to the cube of the velocity, it follows that an particle of range 7 cms. has its rangereduced to 4.5 cms. after incurring a single

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deviation of 90 in traversing an aluminium atom. This is of a magnitude to be easily detectedexperimentally. Since the value of K is very large for an encounter of a particle with an atom, thereduction of velocity on this formula is very small.

Some very interesting cases of the theory arise in considering the changes of velocity and thedistribution of scattered particles when the particle encounters a light atom, for example a hydrogenor helium atom. A discussion of these and similar cases is reserved until the question has beenexamined experimentally.

5. Comparison of single and compound scattering

Before comparing the results of theory with experiment, it is desirable to consider the relativeimportance of single and compound scattering in determining the distribution of the scattered particles.Since the atom is supposed to consist of a central charge surrounded by a uniform distribution of theopposite sign through a sphere of radius R, the chance of encounters with the atom involving small


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deflexions is very great compared with the change of a single large deflexion.

This question of compound scattering has been examined by Sir J. J. Thomson in the paper previouslydiscussed (1). In the notation of this paper, the average deflexion f1 due to the field of the sphere ofpositive electricity of radius R and quantity Ne was found by him to be

The average deflexion f2 due to the N negative corpuscles supposed distributed uniformly throughoutthe sphere was found to be

The mean deflexion due to both positive and negative electricity was taken as

(f12 + f22)1/2


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In a similar way, it is not difficult to calculate the average deflexion due to the atom with a centralcharge discussed in this paper.

Since the radial electric field X at any distance r from the

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centre is given by

it is not difficult to show that the deflexion (supposed small) of an electrified particle due to this field isgiven by

Where p is the perpendicular from the center on the path of the particles and b has the same value asbefore. It is seen that the value of increases with diminution of p and becomes great for small value off.

Since we have already seen that the deflexions become very large for a particle passing near the centerof the atom, it is obviously not correct to find the average value by assuming is small.

Taking R of the order 10-8 cm., the value of p for a large deflexions is for and particles of the order10-11 cm. Since the chance of an encounter involving a large deflexion is small compared with thechance of small deflexions, a simple consideration shows that the average small deflexion is practicallyunaltered if the large deflexions are omitted. This is equivalent to integrating over that part of the crosssection of the atom where the deflexions are small and neglecting the small central area. It can in thisway be simply shown that the average small deflexion is given by

This value of f1 for the atom with a concentrated central charge is three times the magnitude of theaverage deflexion for the same value of Ne in the type of atom examined by Sir J. J. Thomson.Combining the deflexions due to the electric field and to the corpuscles, the average deflexion is


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It will be seen later that the value of N is nearly proportional to the atomic weight, and is about 100 forgold. The effect due to scattering of the individual corpuscles expressed by the second term of theequation is consequently small for heavy atoms compared with that due to the distributed electric field.

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Neglecting the second term, the average deflexion per atom is 3b / 8R. We are now in a position toconsider the relative effects on the distribution of particles due to single and to compound scattering.Following J. J. Thomson's argument, the average deflexion after passing through a thickness t ofmatter is proportional to the square root of the number of encounters and is given by

where n as before is equal to the number of atoms per unit volume.

The probability p1 for compound scattering that the deflexion of the particle is greater than f is equal toe-f2/ t2.

Consequently

Next suppose that single scattering alone is operative. We have seen (3) that the probability p2 of adeflexion greater than f is given by

p = ( / 4)b2 . n . t (cot2 f / 2) .

By comparing these two equations

p2 log p1= - 0.181f2 cot2 f / 2 ,

f is sufficiently small that

tan f/2 = f/2,

p2 log p1= -0.72


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If we suppose that

p2 = 0.5, then p1 = 0.24

If

p2 = 0.1, then p1 = 0.0004

It is evident from this comparison, that the probability for any given deflexion is always greater forsingle than for compound scattering. The difference is especially marked when only a small fraction ofthe particles are scattered through any given angle. It follows from this result that the distribution ofparticles due to encounters with the atoms is for small thicknesses mainly governed by singlescattering. No doubt compound scattering produces some effect in equalizing the distribution of thescattered particles; but its effect becomes relatively smaller, the smaller the fraction of the particlesscattered through a given angle.

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6. Comparison of Theory with Experiments

On the present theory, the value of the central charge Ne is an important constant, and it is desirable to

determine its value for different atoms. This can be most simply done by determining the small fractionof or particles of known velocity falling on a thin metal screen, which are scattered between f and f+ df where f is the angle of deflexion, The influence of compound scattering should be small when thisfraction is small.

Experiments in these directions are in progress, but it is desirable at this stage to discuss in the light ofthe present theory the data already published on scattering of and particles,

The following points will be discussed: --

(a) The 'diffuse reflexion' of particles, i.e. the scattering of particles through large angles (Geigerand Marsden.)


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(b) The variation of diffuse reflexion with atomic weight of the radiator (Geiger and Marsden.)

(c) The average scattering of a pencil of rays transmitted through a thin metal plate (Geiger.)

(d) The experiments of Crowther on the scattering of rays of different velocities by various metals.


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(a) In the paper of Geiger and Marsden (loc.cit.) on the diffuse reflexion of particles falling onvarious substances it was shown that about 1/8000 of the particles from radium C falling on a thickplate of platinum are scattered back in the direction of the incidence. This fraction is deduced on theassumption that the particles are uniformly scattered in all directions , the observation being made fora deflexion of about 90. The form of experiment is not very suited for accurate calculation, but from


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the data available it can be shown that the scattering observed is about that to be expected on the theoryif the atom of platinum has a central charge of about 100 e.

In their experiments on this subject, Geiger and Marsden gave the relative number of particlesdiffusely reflected from thick layers of different metals, under similar conditions . The numbersobtained by them are given in the table below, where z represents the relative number of scatteredparticles, measured by the of scintillations per minute on a zinc sulphide screen.

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Metal Atomic weight z z / A3/2

Lead 207 62 208

Gold 197 67 242

Platinum 195 63 232

Tin 119 34 226

Silver 108 27 241

Copper 64 14.5 225

Iron 56 10.2 250

Aluminium 27 3.4 243

Average 233


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On the theory of single scattering, the fraction of the total number of particles scattered through anygiven angle in passing through a thickness t is proportional to n . A2t , assuming that the central chargeis proportional to the atomic weight A. In the present case, the thickness of matter from which thescattered particles are able to emerge and affect the zinc sulphide screen depends on the metal. SinceBragg has shown that the stopping power of an atom for an particle is proportional to the square rootof its atomic weight, the value of nt for different elements is proportional to 1 / [square root of] A . In


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this case t represents the greatest depth from which the scattered particles emerge. The number z of particles scattered back from a thick layer is consequently proportional to A3/2 or z / A3/2 should be aconstant.

To compare this deduction with experiment, the relative values of the latter quotient are given in thelast column . Considering the difficulty of the experiments, the agreement between theory andexperiment is reasonably good.*

The single large scattering of particles will obviously affect to some extent the shape of the Braggionization curve for a pencil of rays. This effect of large scattering should be marked when the rayshave traversed screens of metals of high atomic weight, but should be small for atoms of light atomicweight.

(c) Geiger made a careful determination of the scattering of particles passing through thin metal foils,by the scintillation method, and deduced the most probable angle


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* The effect of change of velocity in an atomic encounter is neglected in this calculation.

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through which the particles are deflected in passing through known thickness of different kinds ofmatter.

A narrow pencil of homogeneous rays was used as a source. After passing through the scattering foil ,the total number of particles are deflected through different angles was directly measured. The anglefor which the number of scattered particles was a maximum was taken as the most probable angle. Thevariation of the most probable angle with thickness of matter was determined, but calculation from

these data is somewhat complicated by the variation of velocity of the particles in their passagethrough the scattering material. A consideration of the curve of distribution of the particles given inthe paper (loc.cit. p. 498) shows that the angle through which half the particles are scattered is about 20per cent greater than the most probable angle.

We have already seen that compound scattering may become important when about half the particlesare scattered through a given angle, and it is difficult to disentangle in such cases the relative effectsdue to the two kinds of scattering. An approximate estimate can be made in the following ways: --From (5) the relation between the probabilities p1 and p2 for compound and single scatteringrespectively is given by

p2 log p1= -0.721.

The probability q of the combined effects may as a first approximation be taken as

q = (p12 +p22)1/2.


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If q = 0.5, it follows that

p1 = 0.2 and p2 = 0.46

We have seen that the probability p2 of a single deflexion greater than f is given by

p2 = ( / 4)n . t . b2 (cot2 f / 2) .

Since in the experiments considered f is comparatively small

Geiger found that the most probable angle of scattering of the rays in passing through a thickness ofgold equivalent in stopping power to about 0.76 cm. of air was 1 40'. The angle f through which halfthe particles are tuned thus corresponds to 2 nearly.

t = 0.00017 cm.; n = 6.07 x 1022;

u (average value) = 1.8 x 109.

E/m = 1.5 x 1014 E.S. units; e = 4.65 x 10-10,

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Taking the probability of single scattering = 0.46 and substituting the above value in the formula, thevalue of N for gold comes out to be 97.

For a thickness of gold equivalent in stopping power to 2.12 cms, of air, Geiger found the most

probable angle to be 3 40'. In this case, t = 0.00047, f = 4.4, and average u =1.7 x 109, and N comesout to be 114.

Geiger showed that the most probable angle of deflexion for an atom was nearly proportional to itsatomic weight. It consequently follows that the value for N for different atoms should be nearlyproportional to their atomic weights, at any rate for atomic weights between gold and aluminum.


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Since the atomic weight of platinum is nearly equal to that of gold, it follows from these considerationsthat the magnitude of the diffuse reflexion of particles through more than 90 from gold and themagnitude of the average small angle scattering of a pencil of rays in passing through gold-foil are bothexplained on the hypothesis of single scattering by supposing the atom of gold has a central charge ofabout 100 e.

(d) Experiments of a Crowther on scattering of rays. -- We shall now consider how far theexperimental results of Crowther on scattering of particles of different velocities by various materialscan be explained on the general theory of single scattering. On this theory, the fraction of particles pturned through an angel greater than f is given by

p = ( / 4)n . t . b2 (cot2 f / 2) .

In most of Crowther's experiments f is sufficiently small that tan f/2 may be put equal to f/2 withoutmuch error. Consequently

f2 = 2n . t . b2 if p =1/2

On the theory of compound scattering, we have already seen that the chance p1 that the deflexion of theparticles is greater than f is given by

Since in the experiments of Crowther the thickness t of matter was determined for which p1 = 1/2,

f2 = 0.96 n t b2.

For the probability of 1/2, the theories of single and compound

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scattering are thus identical in general form, but differ by a numerical constant. It is thus clear that themain relations on the theory of compound scattering of Sir J. J. Thomson, which were verifiedexperimentally by Crowther, hold equally well on the theory of single scattering.

For example, it tm be the thickness for which half the particles are scattered through an angle f,Crowther showed that f / [square root of] tm and also mu2 / E times [square root of] tm were constantsfor a given material when f was fixed. These relations hold also on the theory of single scattering.


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Notwithstanding this apparent similarity in form, the two theories are fundamentally different. In onecase, the effects observed are due to cumulative effects of small deflexion, while in the other the largedeflexions are supposed to result from a single encounter. The distribution of scattered particles isentirely different on the two theories when the probability of deflexion greater than f is small.

We have already seen that the distribution of scattered particles at various angles has been found byGeiger to be in substantial agreement with the theory of single scattering, but can not be explained onthe theory of compound scattering alone. Since there is every reason to believe that the laws ofscattering of and particles are very similar, the law of distribution of scattered particles should bethe same as for particles for small thicknesses of matter. Since the value of mu2 / E for particles isin most cases much smaller than the corresponding value for the particles, the chance of large singledeflexions for particles in passing through a given thickness of matter is much greater than for particles. Since on the theory of single scattering the fraction of the number of particles which areundeflected through this angle is proportional to kt, where t is the thickness supposed small and k aconstant, the number of particles which are undeflected through this angle is proportional to 1 - kt.From considerations based on the theory of compound scattering, Sir J.J. Thomson deduced that theprobability of deflexion less than f is proportional to 1 - em / t where m is a constant for any givenvalue of f.

The correctness of this latter formula was tested by Crowther by measuring electrically the fraction I /Io of the scattered particles which passed through a circular opening subtending an angle of 36 withthe scattering material. If

I / Io = 1 - 1 - em / t,

the value of I should decrease very slowly at first with

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increase of t. Crowther, using aluminium as scattering material, states that the variation of I / Io was ingood accord with this theory for small values of t. On the other hand, if single scattering be present, asit undoubtedly is for rays, the curve showing the relation between I / Io and t should be nearly linearin the initial stages. The experiments of Marsden* on scattering of rays, although not made with quiteso small a thickness of aluminium as that used by Crowther, certainly support such a conclusion.

Considering the importance of the point at issue, further experiments on this question are desirable.

From the table given by Crowther of the value f / [square root of] tm for different elements for rays ofvelocity 2.68 x 10-10 cms. per second, the value of the central charge Ne can be calculated on thetheory of single scattering. It is supposed, as in the case of the rays, that for given value of f / [squareroot of] tm the fraction of the particles deflected by single scattering through an angle greater than f is0.46 instead of 0.5


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The value of N calculated from Crowther's data are given below.

Element Atomic weight f / [square root of] tm N

Aluminium 27 4.25 22

Copper 63.2 10.0 42

Silver 108 29 138

Platnium 194 29 138

It will be remembered that the values of N for gold deduced from scattering of the rays were in twocalculations 97 and 114. These numbers are somewhat smaller than the values given above for platinum(viz. 138), whose atomic weight is not very different from gold. Taking into account the uncertaintiesinvolved in the calculation from the experimental data, the agreement is sufficiently close to indicatethat the same general laws of scattering hold for the and particles, notwithstanding the widedifferences in the relative velocity and mass of these particles.

As in case of the rays, the value of N should be most simply determined for any given element bymeasuring

* Phil. Mag. xviii. p. 909 (1909)

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the small fraction of the incident particles scattered through a large angle. In this way, possible errorsdue to small scattering will be avoided.

The scattering data for the rays, as well as for the rays indicate that the central charge in an atom isapproximately proportional to its atomic weight. This falls in with the experimental deductions ofSchmidt.* In his theory of absorption of rays, he supposed that in traversing a thin sheet of matter, asmall fraction of the particles are stopped, and a small fraction are reflected or scattered back in thedirection of incidence. From comparison of the absorption curves of different elements, he deduced thatthe value of the constant for different elements is proportional to nA2 where n is the number of atomsper unit volume and A the atomic weight of the element. This is exactly the relation to be expected on

the theory of single scattering if the central charge on an atom is proportional to its atomic weight.

7. General Considerations

In comparing the theory outlined in this paper with the experimental results, it has been supposed thatthe atom consists of a central charge supposed concentrated at a point, and that the large singledeflexions of the and particles are mainly due to their passage through the strong central field. The


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approximately proportional to their atomic weights, at any rate of atoms heavier than aluminium. It willbe of great interest to examine

688

experimentally whether such a simple relation holds also for the lighter atoms. In cases where the massof the deflecting atom (for example, hydrogen, helium, lithium) is not very different from that of the particle, the general theory of single scattering will require modification, for it is necessary to take intoaccount the movements of the atom itself (see 4).

It is of interest to note that Nagaoka* has mathematically considered the properties of the Saturnianatom which he supposed to consist of a central attracting mass surrounded by rings of rotatingelectrons. He showed that such a system was stable if the attracting force was large. From the point ofview considered in his paper, the chance of large deflexion would practically be unaltered, whether theatom is considered to be disk or a sphere. It may be remarked that the approximate value found for the

central charge of the atom of gold (100 e) is about that to be expected if the atom of gold consisted of49 atoms of helium, each carrying a charge of 2 e. This may be only a coincidence, but it is certainlysuggestive in view of the expulsion of helium atoms carrying two unit charges from radioactive matter.

The deductions from the theory so far considered are independent of the sign of the central charge, andit has not so far been found possible to obtain definite evidence to determine whether it be positive ornegative. It may be possible to settle the question of sign by consideration of the difference of the lawsof absorption of the particles to be expected on the two hypothesis, for the effect of radiation inreducing the velocity of the particle should be far more marked with a positive than with a negativecenter. If the central charge be positive, it is easily seen that a positively charged mass if released from

the center of a heavy atom, would acquire a great velocity in moving through the electric field. It maybe possible in this way to account for the high velocity of expulsion of particles without supposingthat they are initially in rapid motion within the atom.

Further consideration of the application of this theory to these and other questions will be reserved for alater paper, when the main deductions of the theory have been tested experimentally. Experiments inthis direction are already in progress by Geiger and Marsden.

University of Manchester

April 1911

Nagaoka, Phil. Mag. vii. p. 445 (1904). Pierre Curie (1859-1906) and Marie Sklodowska Curie (1867-1934)

On a New Radioactive Substance Contained in Pitchblende[1]

note by M. P. Curie and Mme. S. Curie, presented by M. Becquerel

Comptes Rendus 127, 175-8 (1898) translated and reprinted in Henry A. Boorse and Lloyd Motz, eds.,


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The World of the Atom, Vol. 1 (New York: Basic Books, 1966)

Certain minerals containing uranium and thorium (pitchblende, chalcolite, uranite) are very active fromthe point of view of the emission of Becquerel rays. In a previous paper, one of us has shown that theiractivity is even greater than that of uranium and thorium, and has expressed the opinion that this effectwas attributable to some other very active substance included in small amounts in these minerals.[2]

The study of uranium and thorium compounds has shown in fact that the property of emitting rayswhich make the air conducting and which affect photographic plates, is a specific property of uraniumand thorium that occurs in all compounds of these metals, being weaker in proportion as the activemetal in the compound is diminished. The physical state of the substances appears to have an entirelysecondary importance. Various experiments have shown that the state of mixture of these substancesseems to act only to vary the proportions of the active bodies and the absorption produced by the inertsubstances. Certain causes (such as the presence of impurities) which have so great an effect on thephosphorescence or fluorescence are here entirely without effect. It is therefore very probable that ifcertain minerals are more active than uranium and thorium, it is because they contain a substance more

active than these metals.

We have sought to isolate this substance in pitchblende and experiment has just confirmed thepreceding conjectures.

Our chemical researches have been guided constantly by a check of the radiant activity of the separatedproducts in each operation. Each product was placed on one of the plates of a condenser and theconductivity acquired by the air was measured with the aid of an electrometer and a piezoelectricquartz, as in the work cited above. One has thus not only an indication but a number which gives a

measure of the strength of the product in the active substance.

The pitchblende which we have analysed was approximately two and a half times more active thanuranium in our plate apparatus. We have treated it with acids and have treated the solutions obtainedwith hydrogen sulfide. Uranium and thorium remain in solution. We have verified the following facts:

The precipitated sulphides contain a very active substance together with lead, bismuth, copper, arsenic,and antimony. This substance is completely insoluble in the ammonium sulphide which separates itfrom arsenic and antimony. The sulphides insoluble in ammonium sulphide being dissolved in nitric

acid, the active substance may be partially separated from lead by sulphuric acid. On washing leadsulfate with dilute sulphuric acid, most of the active substance entrained with the lead sulphate isdissolved.

The active substance present in solution with bismuth and copper is precipitated completely byammonia which separates it from copper. Finally the active substance remains with bismuth.

We have not yet found any exact procedure for separating the active substance from bismuth by a wet


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method. We have, however, effected incomplete separations as judged by the following facts:

When the sulphides are dissolved by nitric acid, the least soluble portions are the least active. In theprecipitation of the salts from water the first portions precipitated are by far the most active. We haveobserved that on heating pitchblende one obtains by sublimation some very active products. Thisobservation led us to a separation process based on the difference in volatility between the activesulphide and bismuth sulphide. The sulphides are heated in vacuum to about 700 in a tube ofBohemian glass. The active sulphide is deposited in the form of a black coating in those regions of thetube which are at 250 to 300, while the bismuth sulphide stays in the hotter parts.

More and more active products are obtained by repetition of these different operations. Finally weobtained a substance whose activity is about four hundred times greater than that of uranium. We havesought again among the known substances to determine if this is the most active. We have examinedcompounds of almost all the elementary substances; thanks to the kindness of several chemists we havehad samples of the rarest substances. Uranium and thorium only are naturally active, perhaps tantalummay be very feebly so.

We believe therefore that the substance which we have removed from pitchblende contains a metal notyet reported close to bismuth in its analytical properties. If the existence of this new metal is confirmed,we propose to call it polonium from the name of the country of origin of one of us.

M. Demaray has been kind enough to examine the spectrum of the substance which we studied. Hewas not able to distinguish any characteristic line apart from those ascribable to impurities. This fact isnot favourable to the idea of the existence of a new metal. However, M. Demaray called our attentionto the fact that uranium, thorium, and tantalum exhibit spectra formed of innumerable very fine lines

difficult to resolve.[3,4]

Allow us to note that if the existence of a new element is confirmed, this discovery will be uniquelyattributable to the new method of detection that Becquerel rays provide.

--------------------------------------------------------------------------------

[1]This work was done at the Municipal School of Industrial Physics and Chemistry. We particularlythank M. Bmont, head of chemical operations, for his advice and the assistance he willingly provided.--original note

[2]Mme. P. Curie, Comptes Rendus, vol. 126, p. 1101. --original note

[3]The peculiarity of these three spectra is described in the fine work of M. Demaray, Electric Spectra(1895). --original note


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[4]The excerpt in Boorse and Motz ends here. The remainder of the paper was translated by CarmenGiunta, as were the original footnotes.--CJG

--------------------------------------------------------------------------------

Back to the list of selected historical papers.

Back to the top of Classic Chemistry.Ernest Rutherford (1871-1937) & Frederick Soddy (1877-1956)

The Cause and Nature of Radioactivity

from Philosophical Magazine 4, 370-96 (1902) [as abridged and reprinted in Henry A. Boorse & LloydMotz, The World of the Atom, Vol. 1 (New York: Basic Books, 1966)]

Introduction

The following papers give the results of a detailed investigation of the radioactivity of thoriumcompounds which has thrown light on the questions connected with the source and maintenance of theenergy dissipated by radioactive substances. Radioactivity is shown to be accompanied by chemicalchanges in which new types of matter are being continuously produced. These reaction products are atfirst radioactive, the activity diminishing regularly from the moment of formation. Their continuousproduction maintains the radioactivity of the matter producing them at a definite equilibrium-value.The conclusion is drawn that these chemical changes must be sub-atomic in character.

The present researches had as their starting-point the facts that had come to light with regard to thoriumradioactivity. Besides being radioactive in the same sense as the uranium compounds, the compounds

of thorium continuously emit into the surrounding atmosphere a gas which it has been named, is thesource of rays, which ionize gases and darken the photographic film.[1]

The most striking property of the thorium emanation is its power of exciting radioactivity on allsurfaces with which it comes into contact. A substance after being exposed for some time in thepresence of the emanation behaves as if it were covered with an invisible layer of an intensely activematerial. If the thoria is exposed in a strong electric field, the excited radioactivity is entirely confinedto the negatively charged surface. In this way it is possible to concentrate the excited radioactivity on avery small area. The excited radioactivity can be removed by rubbing or by the action of acids, as, forexample, sulphuric, hydrochloric, and hydrofluoric acids. If the acids be then evaporated, the

radioactivity remains on the dish.

The emanating power of thorium compounds is independent of the surrounding atmosphere, and theexcited activity it produces is independent of the nature of the substance on which it is manifested.These properties made it appear that both phenomena were caused by minute quantities of special kindsof matter in the radioactive state, produced by the thorium compound.


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The next consideration in regard to these examples of radioactivity, is that the activity in each casediminishes regularly with the lapse of time, the intensity of radiation at each instant being proportionalto the amount of energy remaining to be radiated. For the emanation a period of one minute, and for theexcited activity a period of eleven hours, causes the activity to fall to half its value. ... The radioactivityof thorium at any time is the resultant of two opposing processes--

The production of fresh radioactive material at a constant rate by the thorium compound;

The decay of the radiating power of the active material with time.

The normal or constant radioactivity possessed by thorium is an equilibrium value, where the rate ofincrease of radioactivity due to the production of fresh active material is balanced by the rate of decayof radioactivity of that already formed. It is the purpose of the present paper to substantiate anddevelope this hypothesis.

The Rates of Recovery and Decay of Thorium Radioactivity

A quantity of the pure thorium nitrate was separated from ThX ... by several precipitations withammonia. The radioactivity of the hydroxide so obtained was tested at regular intervals to determinethe rate of recovery of its activity. For this purpose the original specimen of .5 gram was leftundisturbed throughout the whole series of measurements on the plate over which it had been sifted,and was compared always with .5 gram of ordinary de-emanated thorium oxide spread similarly on asecond plate and also left undisturbed. The emanation from the hydroxide was prevented frominterfering with the results by a special arrangement for drawing a current of air over it during themeasurements.

The active filtrate from the preparation was concentrated and made up to 100 c.c. volume. One quarterwas evaporated to dryness and the ammonium nitrate expelled by ignition in a platinum dish, and theradioactivity of the residue tested at the same intervals as the hydroxide to determine the rate of decay

of its activity. The comparison in this case was a standard sample of uranium oxide kept undisturbed ona metal plate, which repeated work has shown to be a perfectly constant source of radiation. Theremainder of the filtrate was used for other experiments.

The following table gives an example of one of a numerous series of observations made with differentpreparations at different times. The maximum value obtained by the hydroxide and the original value ofthe ThX are taken as 100: Time in days Activity of Hydroxide Activity of ThX

0 44 100

1 37 117

2 48 1003 54 88

4 62 72

5 68 --

6 71 53

8 78 --


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9 -- 29.5

10 83 25.2

13 -- 15.2

15 -- 11.1

17 96.5 --

21 99 --

28 100 --

[Figure 1] shows the curves obtained by plotting the radioactivities as the ordinates, and the time indays as abscissae. Curve II. illustrates the rate of recovery of the activity of thorium, curve I. the rate ofdecay of the activity of ThX. It will be seen that neither of the curves is regular for the first two days.The activity of the hydroxide at first actually diminished and was at the same value after two days aswhen first prepared. The activity of the ThX, on the other hand, at first increases and does not begin tofall below the original value till after the lapse of two days. ... These results cannot be ascribed to errorsof measurement, for they have been regularly observed whenever similar preparations have been tested.The activity of the residue obtained from thorium oxide by the second method of washing decayed verysimilarly to that of ThX, as shown by the above curve.

If for present purposes the initial periods of the curve are disregarded and the later portions onlyconsidered, it will be seen at once that the time taken for the hydroxide to recover one half of its lostactivity is about equal to the time taken by the ThX to lose half its activity, viz., in each case about 4days, and speaking generally the percentage proportion of the lost activity regained by the hydroxideover any given interval is approximately equal to the percentage proportion of the activity lost by the

ThX during the same interval. If the recovery curves is produced backwards in the normal direction tocut the vertical axis, it will be seen to do so at a minimum of about 25 per cent., and the above resultholds even more accurately if the recovery is assumed to start from this constant minimum, as indeed,it has been shown to do under suitable conditions. ...

This is brought out by [Figure 2], which represents the recovery curve of thorium in which thepercentage amounts of activity recovered, reckoned from this 25 per cent. minimum, are plotted as

ordinates. In the same figure the decay curve after the second day is shown on the same scale.

The activity of ThX decreases very approximately in a geometrical progression with the time, i.e. if I0represent the initial activity and It the activity after time t, (1) It/I0 = e-lt ,

where l is a constant and e the base of natural logarithms.

The experimental curve obtained with the hydroxide for the rate of rise of its activity from a minimum


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to a maximum value will therefore be approximately expressed by the equation (2) It/I0 = 1- e-lt ,

where I0 represents the amount of activity recovered when the maximum is reached, and It the activityrecovered after time t, l being the same constant as before.

Now this last equation has been theoretically developed in other places to express the rise of activity to

a constant maximum of a system consisting of radiating particles in which

The rate of supply of fresh radiating particles is constant.

The act

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