Berthelot's Thermo- Chemistry. 371 



be supplemented by the principle of atomic work ; and it is 

 certain that neither clearness of conception nor definiteness of 

 statement has been gained by the obvious attempt to avoid the 

 recognition of the modern theory of chemistry. 



We readily accept Berthelot's second fundamental principle 

 of thermo-chemistry when enunciated as above, because it so 

 obviously falls under the general law of the conservation of 

 energy; but it is obvious that this principle could not have been 

 assumed prior to its experimental verification, any more than 

 could the principle of the conservation of mass, prior to the 

 experiments of Lavoisier ; and as Lavoisier worked out this 

 last great principle with the balance, so Berthelot and Thom- 

 sen have demonstrated with the calorimeter the corresponding 

 fundamental principle of thermo-chemistry, which must be 

 regarded as a generalization from the results of their work. 

 Moreover, although in cases of simple direct combination the 

 principle under discussion is almost self-evident, and has been 

 long admitted, yet, before the investigations of Berthelot and 

 Thomsen, no chemist conceived of its application in the very 

 complex and indirect reactions by which the greater part of 

 the thermo-chemical data have been obtained. It must be 

 remembered that very few processes of direct chemical combi- 

 nation fulfil the conditions which an accurate measure of the 

 accompanying thermal change involves ; and a vast amount of 

 chemical knowledge and ingenuity has been shown in devising 

 indirect methods by which the results could be reached. The 

 general theory of these indirect methods may be stated thus: — 

 We arrange two systems of reactions, both of which begin 

 with the same factors in the same conditions, and end with the 

 same product in the same conditions. In one of these series 

 of reactions there must be no process whose thermal result, if 

 not already known, cannot be measured with the calorimeter. 

 In the other series the chemical combination or decomposition, 

 whose thermal effect we are investigating, enters as an un- 

 known term, the effect of the other chemical changes involved 

 being known or capable of measurement, as in the first series. 

 It follows now, from the principle we are discussing, that if 

 we subtract the sum of the quantities measured in the second 

 series from the sum of those measured in the first series, we 

 shall have the value of the unknown quantity. An example 

 will make the method more intelligible. 



It is required to determine the heat evolved when aluminium 

 combines with bromine to form Al 2 Br 6 ; and in the following 

 scheme we assume, as is usual in this subject, that the che- 

 mical symbols stand for a number of grams corresponding to 

 the atomic weight, and that the amount of heat is expressed 



