152 BIOGKAPHIES OF SCIENTIFIC MEN 



final states of the changing system. (3) "The Law of 

 Maximum Work," or " the theorem of the necessity of 

 reactions " : every chemical change accomplished without 

 the aid of external energy tends to the formation of a 

 body, or system of bodies, the production of which 

 evolves the maximum amount of heat. This law of 

 Berthelot is of considerable importance, as it will often 

 enable the chemist to decide beforehand whether a 

 contemplated reaction is or is not possible by direct 

 means, and if, because accompanied by absorption of 

 heat, not possible in the direct way, it may enable him 

 to bring it about by making it one of a series of reactions, 

 the total effect of which is an evolution of heat. . . . 

 This law is the fundamental principle of Berthelot's 

 thermo-chemistry : " The quantity of heat evolved in a 

 reaction measures the sum of the physical and chemical 

 changes which occur in that reaction " " ce principe 

 fournit la mesure des affinites chimiques." 



Berthelot's agricultural station and laboratory were 

 at Meudon, and here experiments on vegetable soils, the 

 fixation of atmospheric nitrogen in soils by the agency 

 of microbes, the action of electricity on the growth of 

 plants, etc. , were conducted. Berthelot states that twenty- 

 five pounds of nitrogen per annum per acre might be 

 fixed by bacteria. 



Berthelot's work on the explosive wave, his classical 

 experiments on the union of carbon and hydrogen, and 



