L) I S S C)CIATIOS PK E SSV R E R E FO R E H. S-11S TECL -\I R E I>E I7I L L E: BY P. D C H E J I
I owe to the kindness of 11. G. Rrunel, Dean of the FnricIf? S C ~ C Iof~ C Bordeaux, CS in? acquaintance with a little pamphlet' published in 1837 under tlie title : DC l ' i ~ z f j l i ~ i zde c ~In~ P J , C S . ~ ~ O I / .ir/i* I P S nc.iioris c/ziuiiqzce.s ; this pamphlet is the thesis of Georges -1 i 1116 . Rorn at Netz on the 27th of Januar?., T ~ I O ,Georges Alimi. entered the &ole Sorillale in 1831~ in the same class as ,Ahria and Edouard Desains. T h e possessor of a precocious and fertile intelligence and gifted with a n iiiqiiiring spirit which \vas attracted by a wide range of subjects, at tlie age of twenty-three he began a series of piiblicatioiis OII organic and inorganic chemistry, geology, physics, and physiology. Hetween I 833 and I 837 lie published a _\btc szii' IeJiioi;3 a m i * iiiz proc3n'r ~ o i ~ rl' n i m r ~ z i n i i o ~ar ,_\iltr ~ siir In :,ii-ioiz,4 a note Szir Its pi-odiiits ciysfalIises qzic pr*PscizfcIn ~ i n f z i i * ~a ,note j entitled De la I I Z ( I S S C die Picide b I ~ c t i - i q z iresearches ~,~ Szii, I'l~ctioizde I'nIcooI nitriqzic. S U J - I' 2bdc P f S I I I - IPbl'6lJiC, rf de I' lij~rr5,nfede iizef/zj~lC:?ie uifriqiie w r IC nrC:mc> C O J $ . P S . ~and researches .Sur /'ether chIoi-nc~~niiiqiie t,f / C C?L/OJ*OCJ'ff I2 0 f t' d C 112 bfhJ'l?JZC. ~ C S
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TliGse de Cliiniie, prisent6e et soutenne A la Facult6 des Sciences (le Paris le 9 novernhre 1 S j 7 par 11. (Georges) -1ini6 de JIetz, \ dkp. d e la Moselle), clocteur 6 s Sciences. De 1' influence d e la pression sur 1es actions chimiques. in 4 O . 16 pp. Paris. E J. Baillg, itnprirneur. 1837. ? .11in. Chirn. Pliys. ( 2 ) 5 5 , 443 IS;^). Ihitl. 57, 4 . p (1834). ' Ihitl. 56, IOS IS;^). ' Bull. SOC. philornathique. 6, 305 (1S34-j~). -inxi Chirn. Pliys. ( 2 ) 6 2 , 419 (1Sj6). ' Ibic1. 1 2 ) 64, 2 I j (1Sj:). * Ihid. ( 2 ) 64, 220 (~Sjj).
:
Conipte; renilus. g, ,409 rS39,.
- Ihitl. 9, 703 ' 1S39 I .
' Compte-; rentliis. -I
Ilii(1.
IO. 11
9, ;jj (1S,j9 ' ,
(18joi.
with the care of providing the necessary instruments for the magnetic obser\-ations to be carried on in Algeria in accordance with the wish expressed by the Royal Society of London, proposes to ask of the Minister of U'ar a leave of absence of several weeks for 11. .lime i n order that, during his stay in Paris, he may compare the new instruments to he entrusted to his care with those of the Observatory." T h i s appointment hut added a new stirnulus to the scientific zeal of Georges A i d ; for he published numerou> memoirs on meteorology and on the physics of the earth, with an occasional note on pure physics or cheniistr).. In 1840, he published a AfPm%>e s a y les i arinfiotis dz~ZZ'L cnz~ de lo ?JzeY dnns l e p o r f d' A{qer;' i n 1841 a ,Vote S U I * les grrz d&g-ngPsp n r les plrrztrs I I Z ~ ~ ~ Z and I Z Ca S ~paper in German entitled ; ,Cfiftlrre Tenipt.rnfii~*2'12 Algiw ; 3 in I 842, Rccher-chrs expPrinirntnles SUI' lc i i i o i m ~ m c i i fd ~ smgz~es.4 At the meeting of January gth, 1843,'' 11. Xrago submits to the ,lcadernyj numer011s observations on meteorology and on the physics of the earth that 31, Aim6 has made during his stay in Algeria." In the ~ ~ i ~ ~ ~ same year 1843,Georges Aim6 published a A W 6 ~SUI'~ zi?i )lZilJ'CJt 7 1 0 1 1 i EfZlL d& J O l Z d P l * 6 (0 1126'7')' a ,lf&?JiOz'J-€ SZLY COIIZpYES,?ox des l i p i d e s , i and he began the publication of a iW&moz~-c ) Z { I - IC Xngrri.tismc f c r i ~ s f m ;in 1844 appeared a paper on the 1 ki-intiorzs du iz?'i~nii dc la .2fiditcrmtz&e ;9 in I 845,a 1W67r1nirc~ , 1 1 1 , les coii~~niiis de In Llfbditcri*n~iPc,'a and a .WPnioiw SZIP [es ~ E I I Z ,&+n.tures dc la AMPdifei-?wn&r and, i n 1846, a n I m f i ~ i ~ c f z oS Ui iI I' ri~lplozdt, [' n?z&nionicfi-c$nip ~b$t?x~o~i.'~But on the 9th of ___* h n C h i n ~ Phys. ( 2 ) 7 3 , 416 (15400). 3 I 2, j3j ( I S ~ I I. ' Pogg .\VI1 54, 44s ( 1841). .Inn Chiin Phys. ( 3 5 , 417 (1S42). Coniptes rendus, 16,93 (1843j . t Ann C h n i Phys ( 3 ) 7 , 19; ( 1843) Compte, rendus, 16, 1165. .hn. Chitn Phys (3' 8, 2j7 (1S43). ' Compte. rendus, 17, 1031 1S43) , -Inn. Chim. Phys. ( 3 ) IO, 105 ( ~ S l j ) , I
' Ibid
'7, I99 (1846 ' Coniptei reiicius, 18,2 1 9 , Ann. Chim Phys. ( 3 ) 12,2j1 (1844). .Inn. C h i i n Phis ( 3 ) 13, 460 (ISjj). 1 Ihid. ;I 1 55 ~ (184j J Ihiil. : 17, 49s (1846)
September, 1836. death stopped Georges ,lime in the midst of his scientific actil-ity ; he was not thirtj--seven years old. Among the writings of Georges .4in16 there is one which n.ould have secured his fame if the ideas which it contains had been iinderstood when they were published ; we refer to his reFearches on the influence of pressure upon chemical reactions. T h e thesis of Georges Aim6 indeed shows plainly that he had arrived at the notion of dissociation preswre thirt). years before Debray, b?. his experiments, established its existence. Properly to appreciate the meaning and the bearing of tlie neiy views of Georges Aim&, it is necessary to define the part assigned by chemists to pressure, and, for that piirpose, to go back to the phj.sico-cliemica1 theories which were current at the end of the last and a t the beginning of the present century, to the theories which formed the basis of the work of Lavoisier and Laplace. According to these physicists, every body contains ponderable matter and caloric ; the molecules of ponderable matter exert upon one another actions of affinity, which are attractive and which would bring the molecules into contact if they were not prevented bjr the actions of the caloric ; these, on the contrary, tend to separate the niolecules indefinitely. T h e various states of equilibriim which natural bodies attain cannot be explained if the nintaal affinity of the molecules and the actions of the caloric are alone taken into account. T o these two kinds of forces, both of which are internal, it is necessary to add a n external force : the pressure exerted by tlie atmosphere or b\- the surrounding bodies. Moreover, here, as in every problem of tnechanics, while the magnitude of this pressure plays a n important part in the determination of the resulting phenomena, the origin of the pressure is of no importance. Suppose we ask, for example, if a given liquid a t a gix-en temperature will change or not into \.apor? T o answer this question we must know the iiiagnititde of the pressure borne by the liquid, but it is a matter of
indifference whether the pressure be exerted by the \-apor itself, 11~.tlie atmosphere, or b!. any other body. IYe find these ideas very defiiiitelj- expressed in a passage of the Trnifi. ? / ? o i ~ ~ i t ~ z($6.i r ci X i u i i ( > I of Lavoisier : ” TYe have seen that tlie same body takes tlie solid, liquid, or gaseous forin according to the quaiitit>- of caloric nliicli it contains, or iiiore esactl!., according as the repiilsive force of tlie caloric is eqnal to, g-reater than. or less than tlie attraction of the iiiolecules. However, if tliire were oiily these tn-o forces, a bod!- could exist in the liyuid form at hut a single teiiiperatiire. and it woiil(1 cliange suddenly froiii the solid to the gaseous state. Thus, for esaiiiple, Ivater, a t the iiioiiient \vheii it ceases to exist as ice, \vould begin to boil : it \voultl take the g-aseous foriii aiid it> iiiolecules \vould separate iiidefiiiitel\.. T h a t tliis is iiot tlie case is due to a third force, tlie pressure of tlie atiiio.spliere, lvliicli opposes this separation ; aiid accorcliiiglj- \vater reiiiaiiis in tlie fluid state froiii zero to So degrees of the French tlieriiioiiieter. T h e quantity of caloric n.liicli it ahsorbs i i i tliat inter\-a1 is insufficient to o\.ercoine the action due to the pressure of the atniospliere. TYe tlius see that \vitlioiit tlie pressure of the atiiiospliere we shoulcl iiot 11al.e an>. perniaiieiit liquid ; n e could o11sen.e Iicdies in the liynid itate onlj. at the iiicmeiit when the>-melt, and the slightest additioii of lieat ncjuld then separate and di.5perse their parts.” These ideas, del-eloped by J,a\xi4er with reference to the traiisformation of a liquid into vapor. \Yere applied 11~.Huttoiito cheniical decomposition. Carhoiiate of calciuin is deconiposetl hy the heat of a liiiie-kiln iiito calciuin oxide and carboiiic acid ; how then does it happrii that \ye finti iiiarlile aiid crJ.stals of calcspar in igneous rocks, n-hose teiiiperature, at certain geological epochs, iiiust greatly ha\-e exceeded the temperature of a liiiie“
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h v o i s i e r . Trait6 61C.inentaire de Chililie. I’renniGre I’artie. Chapitre I James Hutton. 0 1 1 the pliilosophy o f light, heat an(1 fire, Trails. Roy. SOC.F;dinl). 4 ( H i s t . ; , 7 I 1;98). I
kiln ? Alccordiiigto Hutton, it is because calciuiii carbonate heated in a lime-kiln bears onl!. the pressure of the atniospliere, \vliicli is not sufficient t o clelaj- i-er!. loiig tlie separation of the iiiolecules of calciiiiii oxide from the iiioleciiles of carbonic acid ; oil tlie other hand, in tlie i p e o u s rocks an eiioriiioiis pressure must have hindered tliis decoiiiposition, and ha\-e allon.ed the carboilate of calcium to melt, and suhsequeiitlj-, on cooling, to crj.stal 1ize. Tiiis theor!. of Hutton's suggested to James Hall' the faiiiiiiis iisperimeiit which he coiniiiiinicatecl in 1804 to tlie R o ~ x l Society of Ediiiburqli : under siifficieiit pressure, aiiiorphoiis calciuni carbonate melts without undergoing decoiiiposition, aiid changes into crystalline marble. T h e ideas developed by Lavoisier concerning tlie resistance n-hicli pressure offers to the 1-aporization of liqiiids, exteiicled h!. Hiitton to chemical decomposition, found i i i Dalton' an opponent. I n stud!.ing the \.aporization of a liquid iii a closed 1-esse1 containing a gas, Daltoii shows thst, a t a gi\.en temperature, the same aiiiount of vapor is formed as wlien the gas is abseiit, the oiil!. 6ifferetice heiiig that the vapor is formed more slowl!-. The presence of the atmosphere ", concludes Dalton, ('is an ohstacle, iiot to tlie forination, hut to the diffusion of tlie vapor, which diffusion \\-oiild take place iiiimediatel!-, as in a vaciiiiiii, if the iiiolecules o f the air did not prevent it by their inertias. T h i s obstacle is ('
I James Hall. Esperitnents o n wliinstone and lava I 179s I . Sicholsoii's Juiirnal, 4, S. j 6 I S O I ): Trans. Roy. Soc. Etlinb. 5, . \ I I I I ~ ~ I I 7, , 2 S j 1 I S O I ) : Hoff's Magaziiie, 11. 15'5 ( 1801 ). elfects of heat riioc~ifieti1 ) ~ -conipression. Sicliolson's Journal. 9, 98 : Jour, (le I%>-s.60, 19; : Bull. Soc. pliilotnathique, 3, 249 (rSoj). .\cconiit of a series of t:speritiients. sho\r-iiig tlie effects of compression i n modifying the action of heat. Sicholson's Jourual, 1 3 , 3 2 8 , $31 (1806) : Trans. Roy. Sor. Edi111) 6, 7 1 ( 1 8 1 2 ; : Jour. tles Mines. 24, 23 ( 1 8 0 8 ) Jour. de I'hys. 64, 461 [ 1807). '' Ilalton. Esperiniental essays on the constitutioii o f rnixetl gases ; on the force of steam and rapour iron1 water a n d other liquids at different ternIieratiires, both in a Torricellian vacuum anti in air ; on evaporation : and oil the expansion of gases by heat. Mein. Phil. Soc. Xlaiichester, 5, j2j j1So2) ; tilb u t ' s ,\tinalen, 12, 3i0, j S j ; 13, 43s ; 15, I , I Z I : ~ S o j ;) Sicholson's Journ:iI, 3, 267: 5 , 241 [ ~ S o z ) . 8,
the more easily thrust aside as the absoliite force of tlie vapor increases; it is not due to tlie pressure or to the weight of the atmosphere, as has been believed hitherto, for, if such mere the case, no vapor could form above the boiling temperature ; hut i t is a n obstacle analogous to that which a streatn of water encoiiiiters i n runniiig through gravel Dalton protests vigorously against Lavoisier’s assertion that the pressure of the atmosphere alone keeps water, at ordinary temperatures, in tlie liquid state. If he says, ( ( t h eair in the alinospliere were to disappear suddenly. leaving only the water vapor, the latter would increase in qiiaiitity but slightly, for the atmosphere already contains very nearly the masiinum of vapor which the temperature can produce and maintain ; the suppression of the obstacle, however, would hasten the evaporation without sensibly increasing its amount. ‘‘ T h e idea that pressure prevents the evaporation of liquids, a n idea which is accepted as an axiom by modern physicists, is perhaps responsible for more mistakes and confusion in science than any other equally erroneoils opinion.” Rerthollet, ail advocate of the ideas of Lavoisier aiid of Laplace, does not let pass without protest this sweeping criticisin. Dalton’s remark ”, lie says,‘ ‘(does not appear to me to be correct, aiid as a coiiseqiieiice of his opinion it seeins to me that he is led to an incorrect estimate of the quantity of vapor which would be foriiied by the suppression of the atmosphere, and of the amount which can be absorbed b\- the atmosphere.” Nevertheless the clear aiid logical body of doctrine in which Dalton coordinated the phenoinena of vaporization was soon universally accepted. In his T h i t h de $hysiqzic e-~;3ci,inzciz/tr/r. .dfhL:wiqzie, Riot, who so scrupulously keeps the ideas of Lavoisier and of 1,aplace on affinity and on the caloric, completely abandons the theory of vaporization developed h!. Lavoisier ; he no longer even mentions tlie obstacle wliicli pressure by its tnagnititde alone was supposed to offer to vaporization, while on the other )).
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~ _ _ _ _ _~ _ _ Bertliollet. Essai tie Statique cliimique, Tome I , 1). 491 (rEoj). I
hand lie gi\.es a niasterl!. exposition of the theories of Dalton. IThat Dalton had established in opposition to the ideas of Lavoisier concerning vaporization, Georges Aiini. undertook to repeat for the theor!- of Hutton. He undertakes to slion that, when a hod\- is decomposed by heat, it is not tlie presstire of an!- gas or \vapor chosen a t randoin that can stop its decoinposition : it is the gas \vliich arises from the decomposition, whicli alone can act." Georges -Aim&slion-s. in the first place, that the deconiposition of a carbonate b y an acid. in a closed i.essel, stops when the carbonic acid formed reaches a certain pressure, which depends lipon the nature of the carbonate and of the acid used ; \vith carlionate of calciuni and dilute hydrochloric acid he finds this pressure to be eight atiiiosplieres. " \Tit11 sodium carbonate and tartaric acid, the pressure did not exceed se\-eii atniosplieres : \\-it11 acetic acid, the pressure reached six atmospheres : n-ith citric acid the pressure gauge indicated six atmospheres and a half." T h e pressure, which stops the decomposition of a gil-en carbonate b!, a given acid! is fixed. " I t might lie thought that t h e pressures obtaiiied depend upon tlie relati\-e aiiioiiiits of tlie snbstances used or iipoii their indi\-idual masses, considered witli reference to tlie capacitj- of the vessel. T h i s is lion- I decider1 tlie question : having poured a considerable ynantitJ- of acetic acid upon some sodium carbonate, I n-aitetl until tlie presstire gauge became stationary ; then I allowed part of the gas to escape, and the pressure gauge returned to the saiiie reading. " I then \.aried the qnantitJ- of acid and of carbonate ; a11tl \vhen these quantities were such that the gas generated b!. tlieril under atinosplieric pressure ~ ~ o u have l d occupied a \-oluiiie thirt!. or fort!- times that of the closed 1-essel, I found aln-a!-s \'erJ. nearly the same presstire." Rut when, a t a definite pressure of the carbonic acitf generated by the reaction, the reaction stops, to \\--hatshould tlli? lie attributed ? T o the magnitude of tlie pressure, merel!., according to Hntton's idea, or to the fact that this pressure is ' $
exerted b! the carbonic acid? T o decide this dilemma, Georges .\iin4 establishes, with respect to the pressure of the carbonic acid which stops the reaction, a law similar to the law of tlie mixture of gases and vapors discol.ered hj- Dalton: TYisliing to know”, he says, ‘.if the decomposition of the carbonate is stopped onlj- by the pressure of carbonic acid, or bj’ the pressure of any gas whatever, I placed a n acid in a glass bulb teriiiiiiated bj- a capillar? tube of about the same length as the test-tube. T h i s bulb was placed in the test-tube ; I increased the pressure of the air in the tube to three or four atriiospheres : I :hen broke the bulb and the pressnre gauge indicated three or four atmospheres more than in the case of tlie carbonic acid alone. I t is, thus, the pressure of the carbonic acid which alone is able to stop the decomposition of the carbonate.” T h u s tlie action of carbonic acid can counterbalance the action of another acid, such as actic acid, upon a base, such as soda, so that these actions equilibrate ; for snch a n equilibriuin i t is sufficient that the carbonic acid be suitably compressed. “ I t seems thus to follow from these facts, that the affinitj. of carbonic acid, suitably Compressed, can becoine equal to that of a n organic acid chosen a t random, in such u-ise that the base combines indifferently with the carbonic acid or with the organic acid.” Does the pressnre of the carbonic acid which ensures the equilibrium of the system, depend iipoii the temperature to which the sj.steni is raised? George5 Aiiii6 does not put this question to any experimental test and does not eveti mention it : but if i t be noticed that his object is precisely to correct the e s planation, given by Hutton’s theory, of James Hall’s experiment, there can remain no doubt that lie tacitly admitted the variation of this pressure with the temperature. In this brief memoir, he leaves to the reader the application of the ideas which result from his experinients, to the fusion of carbonate of calcium under pressure, obtained by the English phj.sicist ; but this application admits of no aiiibiguitj-, it leads of necessity to the explanation given later bj- H. Sainte-Claire Deville and Debraj.. “
.%ltliougli the thesis iii \vliich Georges A%im6'sobservations are recorded is dated 183;,the ohm-\-ations, as the aiithor hiniself tells us, were comiiiuiiicated to Iluiiias as early as 1834. It is not until 1864 that Heiiri Saiiite-Claire ISe\.ille publislies similar views coiiceriiing the influence of pressure iipoiii cheiiiical reactions, and then precisel!- iii connection n.i tli James Hall's ex peri iiie 11 t . I n tlie course of his earlier researches upon tlie phenomena of dissociation, H. Sainte-Claire De\-ille pays atteiitioii to tlie changes n.liidi chemical reactions iuidergo as a coiiseqiieiice of I-ariations of temperature, the influence of pressure upon t1ies;c reactions do not occnpy liiiii iii the least. Tliii; point of vien.. eiitirel!. different froiii that of Georges -%iiiiC, is slion-ii clearl!, li!. the titles which Sainte-Claire Ileville gives to his first piiblications 011 dissociation : Lk 10 diLysocintioit 011 rli.c.oiiiposiii~)lI sjoiitnir&r dcs ~'oip.isolis 1'ii;fktciic.c' I((' ln chnli iir.' h'c~~hcrL-1ic~~ s i f r lo (?&coiii~o.~i'~ioii ni.s coi$s pirr In rhnlcni* r't lt7 tJi~c,soc-i(rficiir. It is oiil!- after 1S63 that H. Sainte-Claire De\.ille takes into account the influence that pressiire exerts upon the coiiditioiis of chemical equilibriniii and that lie introduces the idea of tlissociatioii pressure. T h i s idea, to n-hich lie is led li!. t!ie desire t o establisli a complete analogy between tlie \-aporizatioii of a liquitl aiid the decoiiipositioii which a gaseous body, sticli as water \-apor, undergoes tliroiigli tlie action of heat, first preseiits itself iii his n-ritings in a somen.liat \-ague atid iiidefiiiite form. 111 his first note ,.Ski- lr ph&iioiii?ii~ rJc In disso(.i(rfioiid ( z l'?(r71\ H . Sainte-Claire Deville expresses Iiiiiiself as f o l l o w : '' T h e decoinposition of a body is a plieiioiiieiion qiiite similar to the boiling of a liqiiid, the chief character of nliicli is tlie fixed teiiiperature a t which it takes place uiic1t.r tlie iiiflneiice of a soiirce of lieat of :in!- intensity n-hatever, pro\.idt.d the pressiire remains constant. T o siiiii t i p , n-ater vapor cannot resist ~
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' Comptes rentlus, 45, Sj; IS50
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RihlioLheyue nnirerselle. *h-cli. tie Geiisre ; nourt!lle pi.rio(lt., g, Coniptes rentlus. 56, 1 9 j 186;).
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the action of a temperature which increases ten-fold its volume at oc ; accordiiigl?., i t is decomposed." Then, further on : '* T h e decoiiipositioii of a body a t a relatiI-ely low temperature, the plienoriielioti of dissociation, corresponds to the 1.aporization of a liquid at a temperature l o n e r than the boiling-point, and the quantity dissociated a t a given teinperature will be proportional to tlie dissociation presslire e s pressed in iiiilliiiieters of iiiercur?-, as the quantity of vapor formed above a liquid a t a given temperature is proportional to the iiiaxi~iiiunpressure of the vapor." In his note entitled : DL, /adism-iofio/z de I' nri& cnr-hiziquc~ dcs dcirsif>.s dc ; V & ~ Z / Y , ~ H. Sainte-Claire Deville x r i t e s : I have compared di'sm-iafion or decomposition below its fixer1 point to the el-aporation of a liqiiid below its boiling-point, a phenomenon which is iiecessarilJ- inconiplete and of which tlie unoniit varies n-ith the temperature and the medium in which i t takes place ; aiid I have called di.isoci'atic)lr jwsszlrc the quantit>-of a body n-hicli is decoiiiposed into its o ~ v i vapor, i compared with the total mass subjected to the action of heat." In his Lrgo/rs szo- /a Di.ss~ic-i~rfio~/~ H. Sainte-Claire Deville defines more elearl!- what he understands by the dissociation pressnre of water vapor : What '0 lie asks,3 is the pressure of the uiicoinbiued gases (that is, what \ye shall call later the dissociation pressure) in the flame of the blow-pipe ? " He finds tliis pressure to be 426 iiiillitneters a t 2 j 0 0 O aiid h e adds : If I coiitinne m y coiiipai-ison between tlie phenomena of coiiihiiiatiori and of condensation, I shall sa\- that the fixed temperature of combination of hydrogen and oxygen, a t a pressure of 426 iiiillimeters of merciirj'. is 2 j0oC,just as the fixed point of condensation or dew poiiit of water vapor is 84.6" a t the same pressure of 426 ~iiilliiiietei-s. TYe kaon-, iiioreo'i-er, that this d e n point is always tlie same, whether the vapor be alone in a closed vessel or mixed with air or'aiiy other gas which does not react with it, (if
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Cornptes reiidns. 56, 729 ', 1863). J , e p i s sur la Dissociatioii profess6es devaiit la S o c i M chixxiique de Paris. le IS mars et le I avril rS64 :I,e$oiis (le la Soci6t6 chimique de Paris). loc. cit. Chapitre IT. I
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i)ro\.ided that its pressure be 426 niillitneters. I n the flalue at 33coc.tlie detonating gas is iiiised with water \-apor, its pressiire is 426 milliliieter:j, the total pressure being 760 iiiillinieters. " Oh\.ionsly, tlie notion of dissociation pressure coiitaii:eci i i i this passage is qiiite the one which, a few !.ears later, H. Debra!-,' a faithful interpreter of the ideas of H. Sainte-Claire I)eville, defined in these terms : Certain gaseous or 1-olatile compounds are partiall!. and progressively decornposed as tlie temperature rises. -1mixture is then obtained, formed of the guses resiilting from tlie deconiposition of tlie original hod!., ant1 of tliat portion of the latter which has not beeti decomposed, a iiiixtiire in wliicli tlie pressure of the elements separated b!. heat, constant at a given teiiiperatiire, increases with that temperature." l y e iiow kiiow that this first esaiiiple gi\-eii by SainteClaire Del-ille of a dissociation pressure aiialogoiis to tlie pressiire of a saturated vapor, and, like tlie latter, a fnnction of tlie temperature alone, was taken from a chemical systaii which doe< iiot possess such a dissociation pressure. T h e pressure of the gas arising from the dissociation of water which is raised to a gi\.eii teiiiperature does not depend upon that teinperatiire alone ; i t changes n-itli the total pressiire to which the sj.steiii is subjected ; i t changes if an excess of osj.geii or of li!.drogen is introduced into the s!-stcxii. Of tlie analog\- which he assiiiiies between the dissociatiou pressure and the pressure of saturated vapor, and coiiseqiientl!. between tlie temperature of decoiiiposition of a body under a gi\.eii pressure and the boiling teiiiperature of a substance iinder :i given temperature, Sainte-Claire Deville makes a more fortiiiiate application in showing that i t furnislies an explanation of Jaiiies Hall's experiment. T h e explanation to which lie is led agrees perfectl\- with the one that follows from the principles estahlislied by Georges A4i~ii6. Let lis quote the passage' in which he gi\-es the explanation of that noted esperiinent. b c
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' H . Debray. rb6;
Recherclies sur la Dissociation.
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I
loc. cit.. Cliapitre IV,6'
Comptes rendus, 64, 603
' L T l ~famons e experiment of Jaiiies Hall, the fusion of calciuiii carbonate under pressure. is another phenonienon of the same kind as the best k n o n n plienoiiieiia of cliaiige of state. '' The decoinposition of calcium carbonate takes place, like the subliinatioii of a body, in virtue siiiiplj- of the action of heat, and changes iii two quite distinct waj-s : I '. Its fixed point of decomposition, like tlie boiling-point, rises with the pressure : 2'. I n virtiie of its dissociation pressure, the amoiiiit of decomposition varies wid1 the nature of the gzLswhich, at any iiioiiiei:t, is in iiiiinediate contact with its molecules. T h e decomposition of calcium carhoiiate is like the siibliiriation of arsenic ; but the product of the decoinpositioii is carbonic acicl, n h i l e arsenic vapor is tlie product of the sublimation of solid arsenic. I t is clear that below its hoi/z'uq$ni~zt, if the arsenic is in contact with a layer of its saturated vapor a t the teaiperature of tlie experiiiient, the arsenic will no longer sublime, because of its pressure at that temperature. For the same reason, a t a given temperature and in a closed vessel, tlie decomposition of calciuin carbonate is oiily partial, but it will become total if the carbonic acid formed is swept away! as in the experiment of (;aj.-T,ussac. Under the same conditiolis tlie solid arsenic nil1 change entireljinto vapor. But, if closed vessels be used, the presslire of the carbonic acid or of the arsenic will each raise the point of total decomposition or of total subliinatioii of the two bodies, and each will reach a temperature at nliicli it can exist in the liqiiid state." T h e researches of I>e\.ille left the idea of a dissociatioii pressure, fixed at each temperatiire and analogous to the pressure of a saturated 1-apor, as an hg-potliesis which 110 direct esperiiiient justified. H. Ilehraj. iindertook to transform this hJ.pothesis into a n esperiiiieiita! fact, and. for that piirpose, he turned' to the reaction which, from the time of Jaiiies Hall, had tried the iiigeiiuitj. of phj-sical chemists : the decomposition of calciuiii carbonate by heat. ~~
I j
156;;.
H . Ikhray.
Kecherches sur la ilissociation.
Cotiiptes retit1u.i. 64, 603
Tlie experimental laws established b!. H. Debra!. are tlie following : I”. Tlie dissociation pressure of calciuiii carbonate is constaiit a t a gix-en teiiiperature. 2 ’ . T h i s pressure increases with tlie teiiiperature. 3”. It is independent of the state of decomposition of the calciuiii carbonate.” These laws lead to ideas concerning tlie influence of pressiire upon chemical reactions, wliicli are in all poiiits similar to those n-liicli (;eorges ,Iiiiii. had formulated thirty !.ears before. Tlie esperiineiits of Debra! gi1-e a direct denionstration of two laws n h i c h George.5 -Iiiiii. n i t h o u t doubt tacit]!. adniitted, but which lie neglected to I.erif\. experiinentallj-, naniely : T h e clissociatioii pressure varies with the teinperature ; IT-lien the pressiire of the carbonic acid exceeds the dissociation pressure, instead of the generation of carbonic acid, the ;ihsorptioti of the gas is observed. Bloreol-er, the experiments of H . Debray dealt directly with t!ie s!-stem wliich James Hall had studied, while tlie experiinents of Georges AIiin6dealt with a sj’stem in which a iiiore complicated chemical reaction occiirs. R:e now know that i f a s!.stem contains carbonic acid gas, solid sodium carbonate, solid sodium acetate. and a liqiiid mixture of these substances in crj.stallizable acetic acid, tlie states of equilibrium of the s!.stern follow the c,aIiie laws as the states of equilibriam of a system which contains only carbonic acid, calciuin carbonate, and calciuin oxide ; i i i the one system as in the other. at a given teinperature, equilibriuin is reached \Theti the carbonic acid exerts a definite 1)ressiire which depends upon the teinperature alone. In both cases this pressure ma\- be called by the same name: dissociation pressure of the carbonate. Rut this assimilation, allowable today, of a system in which a carbonate is decomposed bj. heat rvitli a sjwtem in which acetic acid replaces the carbonic acid of a carbonate, might have seemed particularl!, venturesome at tlie time when Georges -4iiii6 admitted i t without discussion. Finall!., the c0ndit:ion.c under which tlie experiments of Georges Ximi. were carried on are not well known, and it is not ,$
3 78 Dissocin fiou Prfzssirre bl;f;,rc H. S'ninte- L'lnire Dcz'illc easy to decide whether these conditions are reall:. the ones nnder which the lams formulated by Aliin6are applicable. On the other hand, the esperiinents of Georges Alinii.,inore complete in this respect than those of Debra:., show that the dissociation pressure is the same in an inert gas as in a vaciiiiin, and their author very properly attributes great importance to this law, analogous to the law of the mixture of gases and vapor.: discovered by Dalton. H. Sainte-Claire I k v i l l e and H. Debra!. never knew of the discoveries of Georges Ximi. concerning the influence of pressure upon chemical reactions ; if the). had, with their scrupulous honesty, they would not have failed to mention tliein. Their ignorance is in no may siirprising ; when the:- took up the stud!. of dissociation, Xim4's thesis,' the only place in which these ideas had been recorded, had doubtless become very rare, and no voice was raised in support of the claims of their author, long since dead. In exliiiniitig to-day *\im6's work, we do not mean to diminish in any way the evergrowing fame of H. Sainte-Claire Del-ille and of H. Debray, inasmuch as this work, u n k i i o \ ~ nto them, can have been of no service to them. R u t we think it right, hereafter, to place n e s t to their names that of the pliysicist who, the first, clearly formulated the idea of dissociation pressure ; long enough has he reinained buried in oblivion- the too frequent fate of thinkers who are in advance of their time.
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-4t its meetingof April z j , 1S99, the Socii.fi: dcs .Jiie/iCes Physiques rf A\\;rfrii,elks de h'oi-iiennx decided that a reprint of Georges .4im&'s thesis shoul(1 be published iii its M h i z o i i u s . 1
