Berthelot's Thermo- Chemistry. 369 



in the obviously physical condition which the process involves. 

 For example, in the formation of hydrogen gas from diluted 

 sulphuric acid and zinc, the passing of a solid into a liquid, on 

 the one hand, and the development of a gas from a liquid, on 

 the other, involve physical changes which very largely control 

 the amount of heat developed in the process, and therefore 

 also, according to the third principle, control the process. 

 Indeed, as is well known, all chemical action ceases as soon as 

 the water becomes saturated with zinc sulphate, although a 

 large excess, both of sulphuric acid and zinc, may be present. 

 But, after making all allowances for the potency of physical 

 conditions, it would undoubtedly appear, from the present 

 standpoint of Chemistry, that there must be certain differences 

 of qualities inherent in the atoms which correspond to differ- 

 ences of chemical affinity, and which are important factors in 

 determining chemical changes ; and it is certainly legitimate 

 to seek to measure what we may call the relative potential of 

 the atoms when in a state of indefinite expansion. It is ob- 

 vious, however, from Berthelot's discussion of the subject, 

 that we are as yet far from realizing such a result. In fact, 

 the only case in which he claims that we measure directly the 

 heat of chemical action independently of physical changes is 

 in the well-known reaction H 2 + Cl 2 = 2 HC1, which is attended 

 with the evolution of 22 units of heat for every 36*5 grams of 

 hydrochloric acid gas formed. Since in this case the volume 

 of the aeriform compound is equal to the sum of the volumes 

 of the two elementary gases from which the compound has 

 been formed, and since, moreover, there has been no essential 

 change in the specific heat, we may reasonably infer that the 

 heat evolved results from chemical action only. But, accord- 

 ing to the theory which is accepted by the great majority of 

 chemists, this chemical action is by no means so simple as the 

 direct union of two gas-volumes would seem to indicate ; for, 

 as our symbols show, the process implies the parting of the 

 similar atoms which are united in the molecules, both of hy- 

 drogen and of chlorine gases ; and, unless we misinterpret a 

 very large number of facts, this separation implies the expen- 

 diture of a not inconsiderable amount of mechanical work, and 

 may imply a change of physical condition as well. Berthelot, 

 in common with a school of French chemists, rejects the mo- 

 dern theory based on the assumption of the equal molecular 

 volumes of all substances when in the state of gas, and uses 

 throughout his work the chemical equivalents of the older 

 chemistry in place of the atomic weights of the new. At the 

 same time he accepts fully the mechanical theory of heat, and 

 the conceptions of molecular work which this theory implies. 



