324 HISTORY OF CHEMISTRY. [LECTURE XV. 



exothermic reactions ; whilst endothermic reactions are re- 

 garded as consequent upon the action of heat. 157 



Thomsen has endeavoured, more recently, to determine 

 from the numbers found empirically the values of the affinities 

 of carbon, expressed in calories ; and from these values, to 

 further deduce the heats of formation of many organic com- 

 pounds. Here likewise a remarkable agreement has been 

 observed, but exceptions have to be noted in respect to this 

 matter also. It has already been pointed out at the proper 

 place 158 that considerations of this kind can be employed in 

 confirming the structure of organic compounds. 



Berthelot 159 advanced three principles, the first of which 

 states that the evolution of heat in chemical processes is a 

 measure of the chemical and physical work done during the 

 actions. The principle is thus an application of the first law of 

 the dynamical theory of heat. According to the second prin- 

 ciple, the evolution of heat in a chemical process in which no 

 external work is done, depends only upon the initial and the 

 final states of the system. This is a more precise statement 

 of the principle of Lavoisier and Laplace (compare p. 322). 

 The third principle states that every chemical transformation 

 which is completed without the aid of any external energy, 

 tends to produce that substance or system of substances in 

 whose formation the maximum evolution of heat takes place. 



This " principle of maximum work " raised a great deal of 

 commotion. Not only was its originality contested, since it was 

 looked upon as a repetition of Thomsen's principle lt;0 (com- 

 pare p. 323), but its accuracy was also attacked. Berthelot 

 defended it on both grounds, but still he was not able to prove 

 the general accuracy of the principle. 161 



157 Compare Horstmann, Theoretische Chemie. 612 et seq. 158 Com- 

 pare p. 278. 159 Ann. Chim. [4] 6, 290 ; 18, 103 ; 29, 94 ; [5] 4, 5, etc. ; 

 Mecanique Chimique fondee sur la Thermochimie, Two Vols. Paris, 1879. 

 160 Thomsen, Berichte. 6, 423 ; Berthelot, Bull. Soc. Chim. [2] 19, 485 ; 

 Ostwald, Allgemeine Chemie. 2, 20. 161 . Compare Rathke, Ueber die 

 Principien der Thermochemie, Halle 1881 ; and especially Helmholtz, Zur 

 Thermodynamik chemischer Vorgange, Berlin. Akad. Ber. 1882, 22, 825, 



