REVIEW OF BERTHELOT’d “ MkCANIQUE CHIMIQUE.”
161
XIX.-REVIEWOF BERTHELOT’S “ MECANIQUE CHIMIQUE.* ” BY CH. DE MARIGNAC.t Translated for the JOURNAL or THE AMERICAN CHEMICAL SOCIETY, by P. CASAMAJOR.
On witnessing the calori6c effects which accompany the act of chemical combination, chemists have at all times sought to determine the relations which exist between these two classes of facts ; on the one side, affinity, considered as the cause of all chemical combination; and, on the other side, the heat produced when this combination takes place. A century ago, Lavoisier explained this relation, by assuming that simple bodics are only known t o us in a state of union with a certain quantity of caloric, which is set a t liberty when these bodies enter into chemical combinations. Having his attention more exclusively attracted t o the phenomena of combustion, he attributed this too exclusively to oxygen. Later, i t was admitted for all the elements. T h e brilliant discoveries of Davy, on the chemical decompositions effected b y the galvanic current, gave rise t o the electro-chemical theory, proposed by this wientist, and afterwards adopted and modified by Ampere and Berzelius, which caused heat and affinity t o be looked upon in an entirely new light. According t o this theory, affinity is merely the reciprocal attraction of the opposite electricities, which exist in either a free, or a polarized state, in the elements which tend to combine, and the heat observed in chemical combinations results from the mutual neutralization of these electricities. W h e n chemists abandoned the electro-chemical theory, they returned t o the system previously adopted, admitting, a t the Rame time, the pre-existence of caloric in all the elements. But, about thia time, an important revolution took place in the physical sciences ; heat and electricity ceased t o be considered as material flnids, combined more or less intimately with ponderable bodies, and came t o be regarded as modes of motion. T h e discoveries of Mayer, of Golding, of Joule, had demonstrated the mechanical equivalent of heat. This new doctrine necessarily modified the ideas entertained b y chemists concerning the origin of the heat and the electricity evolved in chemical reactions, and led them t o consider these mani. *Essai de M6canique Chimique, Fondbe Bur la Thermo-cliemie, par M. Berthelot ; Dunod, Editeur: Paris, 1879. t Published in the Archives des Sciences Physiques et Naturelles, and republished in the Moniteur Scientiflque, of February, 1880.
t 62
REVIEW OF BERTHELOT'B
" Mk!ANIQLlfi:
CHIMIQt'E."
festations as coniplementary to one anotlier, and as resulting Irotii t h e loss of molecular impetus attending the act of combination. Vaat erudition, and long bibliographical researches, would be necessary, to follow this change of docti.ine, arid to apportion to each fioientist the share wliich he had in it. W e may, liuweier, cite tilt. memoir of 31 IIenri Sainte C'ai1.e Deville (1860), ant1 tllose of Babinet (1866). A Danish chemist, MI,. Thornyen, whose labors have greatly cotltributed to the progrew of thermo-cheniistr-j-, published i n 1 8 5 3 a series of meninks,* in which he c.xplained the principler of this new science. Without deriving tliern expiicitlp from the mechanical theory of heat, he expresses the basin on which they rest in tcrrnq which recall this theory. H e founds them on t h e t w o following principles : 1st. T h e intensity of chemical energy in a given body i q constant wheri the temperature i8 invariable. 2d. T h e totality of t h e heat giveii out in a chemical reaction, I R t h e measure of the chemical energy brought t o bear in this reaction. l'he chemical energy of a body, or, what is identical, its capacity of developing heat, is what Mr. Thomsen calle its thermo-dyntrn?ir e2 u i II t r lent. blr. Thomsen has since tried t o show that all the laws of thcrrnochemihti*y subsequently establiahed by Berthelot, derived from the second of these principles. I t is true that they may be conbidered as consequences of it, but only by including conditioiiR and restrictions which are not found in the statement of this principle, 'l'he best proof of this is that fifteen years later,+ when Irlr. Thornsen attempted to establish an agreement between his principler and t h e manner in which bases and acids are divided in a mixture of fialts, he had t o introduce a n e w force, ccvidity, which is an inherent property of these bodies, and in which it is difficult t o find anything different from chemical aflinity tinder another form, iiidependent of t h a t which is measured by the heat giben out in combinations. I t is in reality to M. Bertlielot that the honor belongs, of having stated with perfect precision the fundamental principles of thermochemistry, and, above all, of having shown, by a prolonged and n u ineroufi series of experiments, tliat theqe priiicipleq are sufficient t o accoiint for :rll chemical reactions.
i k
*Pogg. Annal., 88, 549 : 9 0 , 261. t Pogg. Annal., 136,65 ; also, Archives, 1869, 36, 301
REVIEW OF BEBTHELOT’S
‘‘ Y ~ C A N I Q G ’ ECHIMIQUE.”
163
Gifted with an eminently philosophical spirit j deeply versed in mathematics and in the phyRica1 sciences; trained t o the most exteiided researches by his labors on chemical synthesis, this scientist wae better able than any one else t o accomplish the task to which he has devoted himself, which is t o show the application of the principles of mechanics t o the formation of organic compounds, and even to chemical reactions generally. For this, i t was not sufficient merely t o determine the heat given out in chemical reactions. A not less important portion of the labors of M. Rerthelot has been the study of the conditions under which such reactions take place, and more specially the study of the chemical equilibrium which results from two contrary reactions, which are reciprocally limited. This happens in the formation of ethers, in decompoeitions and inverse re-combinations produced by electricity, etc. Through the study of these subjects, the author was led t o new and important results in a branch of chemical mechanics, already intimated a t the beginning of this century b y Berthollet, under the name of chemical statics. He has shown that the theory of Berthollet, by which we are enabled t o foretell ohemical phenomena from the mere knowledge of the physical conditions of insolubility and volatility, has no foundation, except when there already exists a previous equilibrilrim, which constantly tends t o renew itself. But the permanency of this equilibrium, and even its existence, are dependent on a still more general condition, which had never before been pointed out, and which has been defined in the principle of mazcin~umwork laid down by M. Berthelot. These researches have taken sixteen yeara-since 1864. T h e results have been given in numerous memoirs, published in the Annaks de Chiniie et de Phyeiyue. &I. Berthelot acknowledges the great help he has had from the labors of his predecessors : Hem, Graham, Andrewa, Favre and Silbermann, and in the numerous series of numerical determinations of the quantities of heat given out in combinations, made by Mr. Thomsen, which are generally rcmarkable f o r their accuracy. This wibject is not exhausted, and will not be for nianp years, but M. Berthelot thinks that the time has come for him t o state tbe laws and general principles, the search of which has led him t o this long series of experiments, and to show in wh’at manner the ideas recently acquired on the theory of heat, allow us to briug the whole of chemistry, that is, the formation and reactions of organic as well
164
REVIEW O B
BERTHELOr’S
“
MhCAN1QL.E CHIMIQUE.”
as of mineral substances, under t h e same principles of mechiinks which govern other branches of physics; whence the title of “ E s s a y Chemical illechuieics~~uiided oii TfLerino-che?iiistry,” given by the author to the work which we are reviewing. In his introduction, 11. Berthelot states the three fundariiental principles of thermo-chemistry and chemical mechanics, viz : I. PRINCII’LE oF J I O L E C U I A R 2tcTio~.--/’he quantity o f heat giveit out it^ u?iy reactioic. i s the measure q f the s u m of the physical avd chemical ?aork accomplished i n this retrctioii This principle gives the nteasiire of cliemical affinities.
11.
P R I S C I P 1 . E O F TIIE
TRANSFORJIATIONS,
CALORIFIC
or, in other words,
EQLTIVALE
OF
CHEMICAL
PRINCIPLE OF T H E ISITIAL ASI)
STATE.-^ a system o f simple or compou~idbodies, tcckeu i i ~ deteriiiiiiute conditions, sufers physicwl or chemiccrl changes capwble of bringiiig it to a tiem state, without giving rise to u mechanical efect exterior to the system, the puantity o f heat giaeic. out or absorbed by the efect o f these changes, d e p e n d s solely orb the iieitial a i d ~ L a l states qf the system ; it is ulways the suine, independently o f the izatiire a i d sequence of intermediate states. FINAL
H I . P R I N C I P L E OF M A X I M U M w o w i , - I h e r y chenzical change, accoiiiplished withovt the irLterceiLtioiL of (8 foreign ereergy, teiids toward the production qf the body, or systeni qf hodies, which gives Q i d the greatest p u i i t i t y of hecit. T h e preiision of chemical phenomena is, by this last principle, brought down t o the purely physical and mechaiiical idea of t h e maximuni labor acconiplished by molecular action. The following principle, deduced from the preceding, is applicable to a multitude of phenomena : Every che,itictal reaction, which m a y he accomplished without the help c!f a prelii)iiii~ryaction, and apart f r o m t h e intervention o f arty eiiergy forciljre to thc~bodies preseiit i i i the system, will TLecessarily tuke p l w ~ if i t be atteirdeed with euolution o f heat. The two first principles, above given, had not heretofore been stated i n such precise terms, but, it may be said that they had been implicitly admitted by the previous investigators in thermo-chemistry. It was, houever, 31. Herthelot, who first called attention t o the principle of maxininm work, which is the basis of chemical statics. In the statements made above, we may note t h e particular care taken h y the author i n distinguishing, in the calculations of re-
REVIEW OF BERTIlELOT’R
“ MkCANIQUE
CtlIMIQUE.”
165
actioos, the heat given out b y chemical reactions, properly SO called, from that due t o change of state, and which is derived more Rpecially from physical energies, such as the liquefaction of gases, the solidification of liquids, changes of volume and of specific heats of gases, of liquids and solids ; changes of tension in vapors, and of fluidity in liquids, of crystallization and of crystalline form in solids ; also, various modifications of the amorphous state. It R i indeed only on the condition of keeping account of all these circumstances, and of eliminating the share of all these physical causes, that we are able to ascertain that the heat given out i n a reaction, is the exact measure of the affinities brought into play. The first volume of M. Berthelot is specially dedicated to chenticcd calorimetry, which is the study of the quantities of heat given out in chemical reactions. I t forms the exposition and confirmation of the first two principles stated above. These principles cannot be demonstrated a priori. They are founded on a hypothesis, in itself very probable, that the imperceptible molecular motions developed in chemical reactions, obey the same laws as the motions of mechanical motors, which may be seen and measured. The accuracy of this hypothesis can only be established by long series of experiments, and by the constant conformity of the results obtained with those foreseen by theory. The first volume is divided into three books. The first book develops the rules and general methods of chemical calorimetry. The author reviews the various reactions, conibinatioiis of elements, formation of salts, &c. H e gives the calculations for each case, also gives rules for taking into account all the physical actions which intervene in these reactions, and shows, by numerous examples, the agreement of experience with the fundamental principles established by him. One chapter is devoted to the quantities of heat which come into play in the reactions which take place in living beings. Another chapter treats of the variation of the heat of combination as influenced by temperature. The second book contains descriptions of experimental processes and of calorimetric apparatus, for the determination of the quantities of heat given out in chemical reactions, of specific heats, and of the heat due to changes of state. T h e great experience of the author gives a special interest t o this descriptian of the methods and apparatus used by him, in which he
has succeeded i n uniting simplicity with accuracy, and in which h e has tried to eliminate or, at least, t o reduce as much as possible, t.he cwrrections made necessary by the influence of. exterior causes. T h e third book is devoted t o numerical tables. I t includes numbers obtained by experiment, i.pprest?nting the yuantitim of h e a t given out or a t m r b e d during physical or chemical changes of state which these Iiodies undergo in the operatioris of o u r laboratories. I t therefore contains, b a i d e s resii!ts of thermo-ohernid e x p r i m e n t n , all the data relating t o specific heats, and t o changes of state (latwit heats). It forrrrr an extensive arid e x c c d i n g l y iisefiil c.onipe~idium of tlie labors of’ all the scientists n.110 have studied these yneatioiis. \ire also tirid in this portion very intc.resting discussions on nunieiwiis theoretical questions suggested by thc examinatiori of these results. W e may cite more sperially those whit!\ rel:tte t t ) specqifio heat. M. Berthelot states that the specific heat i s t h e u n l ~ - physical property by which simple gases differ froin cornl.)ountl gases. ‘rhis property, however, is sufficient t o cstabliRh between these t w o groups such an important difference, that it is impossible t o s ~ p p u s ethat a n y of tlie gases, now considered as simple, rmults from thc union of several other elements, such as ive admit them a t the present time, or from the condensntion of several tqiiivalcnts of the same element. This union or condensation must be understood t o be comparable t o that which gives rise to compo~indbodies, suc*has are i i o w kriown to l i s . Simple gases differ from all compound gases formed with condensation, by the constancy of their specific. heat, while for these last, the specific heat increases rapidly with the temperature, and always exceeds in a notable manner, a t all temperatures, that of simple gases. They also differ from compound gases, formed without condensxtion of their elements, such as hydrochloric. acid and carhonicb oxide (c )riwponding to foiir volumes, according t o tho system of not‘atiori of the author), whose inoleciilar specitic lieat is equal to that of simple gases, and also remains constant at, all temperatures, by the fact that the molecular volume always corresponds to one eqiiivdlent for these compound gases, while it represents two or four equivalents for all the known elements.* ____~ ~. -
* This difference is not so much in the specific heats themselves. as in the relations of molecular volumes to equivalents. I n this respect, the reasoning of 31. Berthelot does not seem to me unobjectionable. I t wonltl he so if the determinations of equivalenta rested on fixed rules. Hut, as lie Iiimself acknowledges (Introduction, page 21), ’ ‘ equivalents are only determiued within an approximation of a inultiple,” it would only be necessary to change the multiple which has been chosen more or less arbitrarily. for a given hody, to change t l i ~
REVIEW OF BEBTlIEI.oT'S
" M ~ C A N I Q U ECHIMIQUE."
I6i
In the chapter relating t o specific heats of solid bodies; occur very interesting observations on the law of Dulong aitd Petit,* and on the relations which exist between the specific heats of compound bo3ies a n d those of their elements. T h e second volume of the work of M. Berthelot, is specially dedicated to establishing the bases of ohemical mechanics. T h e object at which the anthor aims is the highest t h a t science c a n propose to itself. Instead of confining itself t o the inrfividnal description of the properties, the preparation and transformations o f chemical species, science must endeavor to discover the laws governing these transformations ; the caiises and proximate conditions which determine them. T h e complete solution of this problem of cheniical inechanics is riot yet possihle, and will not be possible for many years. To solve it, we 31iould know, in every reaction, the masses of the particles brought into play, their relative positions, t h e proper motions of each particle, the exact nature, and the laws which govern the forces which react on each other. Most of these data'are wanting, and even if they were known, it is doubtful if any method of calcnlation would allow us t o solve such a complicated problem. However, sonie portions of the problem may be attempted. Since a long time, all that relates to the proportions in which conibinations take place is known by tlie laws of definite proportions, of multiple proportioiis, and of equivalents. As t o the prevision of reactions, we have only hail the laws laid down by Berthollet a t the beginning of this ceiitury. These only apply t o special cases and meet with frequent exceptions. M. Berthelot replaces them by a more general law, by which the prevision of the reciprocal actions between simple and conipound bodies is reduced t o the determination relation between its molecular weight and its equivalent, which the author regards as characteristic of simple and compound gases. As an instance, it is not clear nhy M. Berthelot considers the molecule of carbonic oxide, C,O, (four volumes), 8 s representing one equivalent. In the combinations of this gas with oxygen, chlorine and sulphur, it would appear that this quantity rather corresponds to two equivalents. * Although we acknowledge, with M. Berthelot, that the law of Dulong and Petit, when applied to solids, is not rigorously exact, and that it cannot serve, in an absolute manner, for the determination of atomic weights, still we believe that he exaggerates the exceptions which it suffers,and that it must be taken into account in the case of elements whose physical properties are very similar, as i n the case of metals whose fusion points are not very far apart.
168
HEVIEW OF BERl'HELOT'8
" MkCANIQUE
CIIIMIQUP."
of the therniic properties of the reacting bodies. W e may, by this new law, foresee t h e reciprocal actions of compounds towards one another and towards simple bodies, when we know the quantities u f heat given out i l l the formation of each cuompound. This new law ia n o other than the third principle of the author, \t hich ha5 already been stated, the law of maximum work Whence the division of this volume in two part5 : T h e first treats of what may be called chemical dynamics. I t is the study of combination and dec*oniposition, and contains definitions of the opposing actions, on one side, of vhcmical forces, and on t h e other sid?, df calorific, luminoiis an 1 ele 'tric. energies, which determine the phenomena. In thib way, the conditions are laid down which determine the existence and the Ytability o f combinations for each separate body. T h e second part is the study of the final state whirh results from reciprocal actions between simple and compound bodies ; this is chemical statics I n this part comes into action the principle of maximum work, which is as remarkable for its simplicity as for its generality. I t brings everything down t o the knowledge of t w o things : the lieat given out by the transformations, and the stability which belongs to each compound. T h e author d~nionstrates the exiatence of this principle, by t h c discussion of the general phenomena of chemistry ; afterwards, he develops the application of it t o the reciprocal actions of t h e principal groups of substances, buch, for instance, as take place between elements and binary coinpounds ; the reciprocal displacement of binary compounds, and specially of hydracids, when opposed t o oiie another or to water ; the reciprocal displacements of acids in salts, arid, finally, d i n e dorible decompositions. I t m;ty be easily understood that it is imposaible t o give, in a review of this kind, a clear idea of a work of such importance a n d of euch vast extent. W e have only tried t o give an idea of t h e subjects treated in it. T h e theories of M. Berthelot are well known, as they have been developed by him for the last sixteeii years in a seriea of papers, which have been read by all chemists. Nevertheless, the present work forms a n important epoch in the history of chemistry, and shows the iieceseity of transforming its methodq. In the future, it will not be sufficient t o state the reactions of whicli a body is susceptible, as if they were mere experimental data. I t will be necessary t o derive them from the general principles, of which these re-
REVIEW OF BERTHELOT’S “ h C A N I Q V E CHIMIQUE.”
169
actions are consequences. Doubtless, the edifice of chemical mechanic#, whose foundations have beeh laid by M. Berthelot, cannot be considered as complete. Sixty years ago, the laws which govern chemical pibportions were established, and nevertheless, the numbers which represent equivalents have frequently to be rectified. W e may expect that th0W which express the quantities of heat of combination, or transformation, will also require change, more especially, as many of them cannot be determined directly by experiment, b u t result from complicated calculation# in which a great number of reactions have t o be taken into account. There are, moreover, many theoretical quentiom of the highest importance, the solution of which is a t prcwnt beyond our reach. If the heat given out in the combination of two bodies, is the iinmediate consequence of the loss of momentum which comes from t h e union of their particles, and if it measures a t the same time their mutual affinity, what then is the first cause of affinity ? Is i t a special force as was once believed ? Or is it the concordance in the vibratory motions of the particles? Those combinations which are attended with evolution of heat, are the only ones which can take place directly; but they do not always take place spontaneously. Many of them must be determined by an exterior cause, such as heat, light, electricity, hut these agents do not give to the elements an energy of which they have already an excess. How then do they a c t ? r, 1 hese questions, and many others which will come up, cannot fail to exercise for a long time to come the sagacity of chemists. A t any rate, an important step has been made towards their solution when it i t has been shown that the physical and chemical energies which come into play in chemical reactions, follow the laws of rational mechanics.
