LECTURE XV.] HISTORY OF CHEMISTRY. 323 



is consumed again during decomposition or return to the 

 original state, and vice versa. 153 



This principle was enunciated by Hess, in 1840, in another 

 form which is very important and strictly accurate for practical 

 thermo-chemistry : The evolution of heat corresponding to any 

 chemical process is the same whether the process is accom- 

 plished in different stages or all at once. 154 



Hess established this principle empirically, and he employed 

 it extensively in order to determine quantities of heat which 

 were incapable of direct measurement proceeding, therefore, 

 in exactly the same manner as is done at the present day. 



The comprehensive researches of Favre and Silbermann 155 

 are of great value. These consist, in part, of very exact thermal 

 determinations, especially of heats of combustion, and, up till 

 thirty-five years ago, they constituted the empirical basis of 

 thermo-chemistry. They have now been superseded, however, 

 by the excellent experiments of J. Thomsen and of Berthelot, 

 which embrace almost the whole domain of chemistry. 



Thomsen 15C recognises that the principle of Hess is a 

 deduction from the first law of the dynamical theory of heat, 

 which he adopts as the basis of his theoretical considerations. 

 He then advances a second principle, according to which every 

 simple or complex action, of a purely chemical nature, is 

 accompanied by the evolution of heat ; and this he endeavours 

 to establish both theoretically and empirically. The principle 

 in many cases agrees with the results of experiment, but still 

 exceptions are known. These, however, can perhaps be other- . 

 wise explained. It may therefore be asserted that chemical 

 forces, when acting independently, always tend to bring about 



153 Lavoisier, Oeuvres. 2, 287. 154 Pogg. Ann. 50, 385 ; 52, 97. 

 155 Ann. Chim. [3] 34, 357 ; 36, I ; 37, 406. 156 Pogg. Ann. 88, 349 ; 

 90, 261 ; 91, 83 ; 92, 34 ; 138, 65. See also Berichte. 2, 482, 701 ; 3, 

 187, 496, 716. 927 ; 4, 308, 586, 591, 597, 941 ; 5, 170, 181, 508, 614, 

 1014; 6. 233, 423, 697, 710, 1330, 1434; 7, 31, 379, 452, 996, 1002; 

 9, 162, 268, 307 ; 10, 1017, etc. ; Thermochemische Untersuchungen, 

 Four Vols. Leipzig, 1882-86. 



