A CENTURY OF CHEMISTRY. 115 



called thermochemistry. Thermochemistry is thus 

 the science of the thermal processes conditioned by 

 chemical processes. The quantities of heat evolved or 

 absorbed measure the decrease or increase of chemi- 

 cal energy, in so far as other energy is not involved 

 in the processes." * 



Among the important steps in thermochemistry 

 the following may be noted : 



The extension of the law of Dulong and Petit by 

 Neumann and later by Regnault (1839) ; the ex- 

 periments of Thomas Andrews (1841) on the heat 

 produced during the combination of acid and bases 

 in aqueous solution; Herman Hess's experimental 

 verification (1840) of the conclusion that "the sev- 

 eral amounts of heat evolved during the successive 

 stages of a process are the same in whatever orderthey 

 follow one another " a conclusion subsequently re- 

 inforced by Berthelot; Julius Thomsen's vast accu- 

 mulation of data (from 1853 onwards) as to heats 

 of formation and all kinds of chemical change; and 

 Berthelot's equally voluminous researches. 



We need not, for our purpose, pursue the history 

 further. It is enough to indicate that the aim of 

 discovering the dynamical laws relating to chemical 

 processes is one which has not been lost sight of. At 

 the same time, we have to note the conclusion of an 

 expert like Tilden, that "notwithstanding the labours 

 of half a century, thermochemistry remains for the 

 most part a mass of experimental results, which still 

 await interpretation." 



The doctrine of the conservation of energy is the 

 foundation of chemical dynamics. Every change in 

 the arrangement of particles is accompanied by a 



* Ostwald, Outlines of General Chemistry, trans. 1890, pp. 

 208-209. 



