Berthelot's Thermo -Chemistry. 373 



amount of heat evolved in the formation of the definite hy- 

 drates, although it would be very difficult, if not impossible, 

 to form these substances with a definite composition in the 

 calorimeter. Thus, to determine the heat evolved in the reac- 

 tion SO3 + H2 = H 2 SO4, we have only to dissolve in one ex- 

 periment S0 3 and in another H 2 S0 4 in a comparatively large 

 amount of water, when the difference in the heat evolved in 

 the two cases will be the quantity required. So also the heat 

 of formation of a hydrocarbon may be determined, by com- 

 paring the heat of combustion of the compound with the heat 

 of combustion of the hydrogen and carbon of which it consists. 

 Thus the heat of combustion of acetylene, C 2 H 2 (26 grams), 

 has been directly measured, and is equal to 321,000 units, 

 while the heat of combustion of 2 (24 grams), plus the heat 

 of combustion of H 2 (2 grams), only amounts to 257,000 units. 

 Hence it is evident that, in the formation of 26 grams of ace- 

 tylene, 64,000 units are absorbed. Acetylene, indeed, belongs 

 to a class of compounds whose formation is attended with the 

 absorption of heat. This class of compounds, which have a 

 special interest in thermo-chemistry, are said to be endother- 

 mous: while by far the larger class of compounds, whose for- 

 mation is attended with an evolution of heat, are said to be 

 exothermous. 



Theorem VI. — When a compound gives up one of its ele- 

 ments to another body, the heat evolved in the reaction is the 

 difference betioeen the heat of formation of the first compound and 

 that of the resulting product. 



Thus, when an aqueous solution of chlorine is used as an 

 oxidizing agent, for every 18 grams of water decomposed 

 9600 units of heat are evolved ; and this amount is the differ- 

 ence between the heat of formation of H 2 O and 2 HC1. As 

 can easily be seen, the same theorem applies to the problem 

 presented by explosive agents of various kinds, and simplifies 

 the solution to a remarkable extent. 



These few illustrations will serve to give a general idea of 

 the mode of investigation in this new field of thermo-chemistry; 

 but they are wholly inadequate to show either the extent of 

 the field or the great skill with which it has been cultivated. 

 We must reserve for another number a notice of some very 

 interesting relations which, under his third fundamental prin- 

 ciple, Berthelot discusses in the second volume of his great 

 work. 



