July 29, 1880] 



NA TURE 



287 



study of the conditions which determine chemical changes. 

 This general study divides itself into two branches : 

 chemical decompositions and recompositions — included 

 under the title of " Dynamique Chimique " ; and secondly 

 those final distributions of matter which result from 

 reciprocal actions between simple or compound bodies, 

 grouped together as " Statique Chimique." Would it not 

 have been better to have entitled the general subject 

 "Chemical Dynamics," and the branches "Chemical 

 Kinetics" and " Chemical Statics" respectively? 



It would obviously be impossible to give here even an 

 outline of Berthelot's treatment of this immense field of 

 work ; one or two instances must suffice. 



The two fundamental generalisations of the French 

 chemist have already been mentioned. Let us turn to his 

 treatment of the specific heats of elementary bodies and 

 of chemical equilibrium. 



Berthelot refuses to accept the law of Dulong and Petit 

 as applied to solid elements. He says that the actually- 

 determined specific heats of the elements vary much 

 with temperature, and that the products of these numbers 

 into so-called atomic weights are of very difterent values. 

 He gives a list of 1 1 elements, the specific heats of whose 

 equivalents is about 6 '4; and a list of 31 for which the 

 product of specific heat into equivalent weight is about 

 3'2. 



This result well illustrates what will probably be 

 regarded by most chemists as a fundamental error on the 

 part of the author of the "Essai" ; Berthelot is still to 

 be classed among the staunch supporters of the system of 

 notation founded on equivalents. In this country we have 

 no such phenomenon as a great chemist who writes the 

 formula of nitric acid AzO^. Nevertheless Berthelot's 

 thermal chemistry is founded on a molecular theory. He 

 constantly speaks of molecules and of action between the 

 parts of molecules ; he also speaks of the architecture of 

 atoms, and seems to legard the modern atomic theory as 

 utterly opposed to such an idea. 



"The kinetic energy of the molecule may be regarded 

 as made up of two parts — that of the mass of the molecule 

 supposed to be concentrated at its centre of mass, and 

 that of the motions of the parts relative to the centre of 

 mass. The first part is called the energy of translation, 

 the second that of rotation and vibration. The sum of 

 these is the whole energy of motion of the molecule. The 

 pressure of the gas depends on the energj' of translation 

 alone. The specific heat depends on the rate at which the 

 whole energy, kinetic and potential, increases as the tem- 

 perature rises." (Clerk i\Iaxwell, Cliem. Soc. Journ., 13, 

 502.) 



In the present state of our knowledge of the internal 

 motion of the parts of a molecule it is impossible to 

 determine satisfactorily the ratio of the two parts of the 

 energy of the molecule, and it is extremely difficult to 

 reconcile the observed with the calculated ratios of 

 specific heats. 



Nevertheless, if we adopt the mean numbers found for 

 the specifi,c heats of the solid elements and multiply these 

 into t,he maximum atomic weights as determined by the 

 aid of Avogadro's law, we get a result which is too 

 constant to be merely accidental. Taking Kopp's 

 numbers, calculated from specific heats of compounds, 

 for those elements which have not yet been obtained in 

 the solid form, we find that the product of specific heat 



into atomic weight (iiot egiiiva!e?it nveight) is about 6-4 

 for forty-four elements, about 5 '5 for ten elements, less 

 than 5 for two elements, and is yet unknown for eight 

 elements. Furthermore we find that the specific heats 

 of the elements are fairly constant, provided they be 

 determined for a temperature- interval known to be 

 considerably below the temperature of fusion of the 

 elements. 



We seem, therefore, fully justified in accepting the 

 law of Dulong and Petit as an empirical statement 

 of very considerable value, although not as a final 

 statement of the connection subsisting between the ratio 

 of the two parts of the energy of the elementary mole- 

 cules, and the relative weights of the parts of the same 

 molecules. 



In treating the subject of chemical equihbrium Berthelot 

 first of all examines processes of chemical combinations in 

 general, and contrasts these with processes of decomposi- 

 tion; hethenstudiesthose changeswhich aremadeupof two 

 parts- a direct and reverse — and which are characterised 

 by the attainment of a limit dependent on conditions of 

 temperature, pressure, relative masses, &c. The che- 

 mical equilibrium thus established he divides into two 

 kinds : equilibrium of homogeneous bodies, i.e., when the 

 original and final substances are all liquid or gaseous and 

 capable of complete admixture during the course of the 

 change ; and equilibrium of heterogeneous bodies, i.e., 

 when some of the substances are solid and some liquid, 

 or some liquid and some gaseous, or when all are liquid 

 or gaseous, but are nevertheless incapable of complete 

 admixture. Examples are given of [each kind lof equi- 

 librium, and of the conditioning influence of temperature, 

 pressure, mass of solvent, contact with other substances, 

 relative masses of reacting bodies, chemical functions 

 of reacting bodies, velocity of the change, &c. The 

 phenomena of equilibrium of heterogeneous systems lead 

 to a discussion of dissociation ; this to a consideration of 

 precipitation, and thence to an instructive chapter on 

 the state of salts in solution, and the meaning of the 

 terms "feeble" and "strong" as applied to acids and 

 bases. 



Although, in considering Berthelot's treatment of chemi. 

 cal equilibrium, one'misses the bold and fascinating results 

 obtained by Gibbs in his great paper on the " Equilibrium 

 of Heterogeneous Substances," and the exactitude and 

 simplicity of the beautiful theory of Guldberg and Waage, 

 and although one cannot but much regret that he should 

 not have written his formula: and equations in a language 

 more easily understood by the chemist of to-day, one 

 must nevertheless admire the breadth of view, the felicity 

 of illustration, and the suggestiveness of the work of the 

 French chemist. 



The publication of the "Essai" marks an important 

 point in the advance of modern chemistry : it comes to 

 the chemist with the message, amongst others, that his 

 science demands more than the stereotyped so-called 

 original investigation, ip which are detailed a few proper- 

 ties of a number of new compounds produced by method* 

 long ago marked out and defined : it tells him that hs 

 must revise and advance his methods, that he must frji 

 to explain his facts by appeal to principles, thai be 

 must not be afraid to strike off the beaten path into tfte 

 by-ways of research, and that there is more to be hoijed 



