‘FORCE AND ENERGY. 229 
the same, no matter whether the starch be burned almost instanta- 
neously in pure oxygen or whether it be subjected to slow oxidation 
in the tissues of a plant buried in the soil; whether carbon dioxide 
and water are the immediate products of the action or whether the 
starch be previously transformed into maltose, glycogen, dextrose, 
lactic acid, etc., etc., as in the body of the animal. We have simply 
to determine the potential energy of the system in its initial and in 
its final state, and the difference is equal to the amount of kinetic 
energy evolved during the change. The truth of this law, as ap- 
plied to chemical processes, has been: fully demonstrated by the 
researches of Berthelot and Thomsen. That the same law applies 
to the processes taking place in the body of the animal is exceed- 
ingly probable, a priort, and has been demonstrated experimentally 
by the researches of Rubner and of Atwater and his associates. 
Heats or Comspustion.—We have no means of determining 
the total amount of potential energy contained in a system, but can 
only measure that portion which is manifested by the change to 
the kinetic or the potential form during some change in the system. 
In other words, we may assume the potential energy of the system 
in some particular state as zero and obtain a numerical expression 
for its potential energy in some other state as compared with this 
standard state. For the latter we shall naturally select that one in 
which no further conversion of potential into kinetic energy can, 
according to our experience, take place. 
In the case of organic substances, such as those entering into 
the metabolism of the animal, the system consists of the substance 
itself and oxygen, and the state of complete oxidation is the one in 
which experience shows that no further evolution of kinetic energy 
is possible by chemical means. Thus, to recur to the example of 
starch, if one gram be oxidized in accordance with the equation 
C,H,,0;+ 60,—6C0,+5H,0, 
the amount of heat evolved will be 4183 cals.,* this being the amount 
of energy converted from the potential to whe kinetic form. From 
the system represented by the second member of the above equa- 
tion we can get no further evolution of heat. We therefore repre- 
* For the units of measurement see the following paragraph. 
