MARCELIN BERTHELOT—MATIGNON. 675 
ratus accessory contrivances either to eliminate or to measure the 
different causes of error. Berthelot, however, secured accuracy by 
more simple methods. The experimental technique which he worked 
out from beginning to end for measuring different calorific factors 
is an admirable accomplishment, which would suffice alone to make a 
physicist illustrious. Although I have had occasion to initiate a 
large number of French and other scholars into the calorimetric 
methods of Berthelot, I have never once done it without noting after a 
first experiment their astonishment and their admiration for methods 
so simple and accurate. These methods were afterwards to attain 
perfection in the use of the calorimetric bomb. 
Altogether, Berthelot’s accomplishments in thermochemistry are 
marvelous. Their consequences extend into all domains of science. 
Engineers, experimenters, and theorists are continually using his 
calorimetric data. 
In theory, Berthelot shows that the amount of heat is the prin- 
cipal factor upon which depend the conditions of composition or 
decomposition of substances; but the mass heat of reaction is con- 
nected with these conditions by an extremely complicated relation- 
ship. Berthelot tried to disengege from this mass heat all the calo- 
ries connected with reversible phenomena and obtained a quantity, 
“chemical heat,” which approaches the heat not compensated for 
in the reaction. From 1865 Berthelot worked without interruption 
to establish and render exact the different terms for expressing chemi- 
cal heat. 
This chemical heat, especially in solutions, is not always easily 
calculated, and so in the secondary schools they have let stand the 
old rule of maximum work, which in many cases can give an exact 
idea of the process of reaction. 
The study of electrical piles, which forms, with the working out 
of reactions in advance, one and the same problem, took part of Ber- 
thelot’s time. As I said above, he studied very thoroughly all the 
secondary phenomena which occurred in connection with it in such 
a way as to separate from the chemical mass energy all these second- 
ary forms of energy and to try to give, if possible, an experimental 
interpretation of the differences between the voltaic and chemical 
energies. M. Berthelot has frequently called attention to the im- 
portance of these secondary reactions often neglected by the theorists. 
For this reason the pupils of Helmholtz could verify the accuracy 
of the relation between voltaic and chemical energy only by measur- 
ing the chemical energy directly on the calorimeter, as Jahn did, 
and not, like Brauner, by calculating it from the fundamental 
chemical reaction occurring in the pile. 
Moreover, in a general way, the study of the thermo-chemistry of 
reactions forced Berthelot to go into their slightest details, and, with 
