THE CONSERVATION OF ENERGY IN THE ANIMAL BODY. 259 
dead matter, and we have no scientific right to assume in advance 
of the evidence that no special forces are operative in the former. 
In brief, whatever may’be the probabilities in the case the applica- 
bility of the law to living beings as logically requires experimental 
demonstration as did its applicability in physics or chemistry, and 
no little labor has been within the past few years devoted to this 
problem. 
Nature oF EvipENcr.—Before proceeding to a consideration 
of the experiments bearing upon this question it will be well to 
make clear the nature of the evidence required, 
If the law of the conservation of energy applies to the animal, 
the following are necessary consequences of it: 
1. In an animal doing no work on its surroundings and neither 
gaining nor losing body substance, the potential energy (heat of 
combustion) of the food will be equal to the potential energy of the 
excreta plus the kinetic energy given off in the form of heat plus 
the energy expended in producing physical and chemical changes in 
the body.* 
2. In an animal doing work on its surroundings, but neither 
gaining nor losing body substance, the potential energy of the food 
will be equal to the potential energy of the excreta plus the energy 
of the heat given off plus the energy of the work done plus the 
energy expended in producing physical and chemical changes in 
the body. 
3. In an animal doing no work on its surroundings, but gaining 
or losing body substance, the potential energy of the food will equal 
the potential energy of the excreta plus the energy of the heat given 
off plus the potential energy of the gain by the body (a loss by 
the body being regarded as a negative gain) plus the energy ex- 
pended in producing physical and chemical changes in the body. 
4. In an animal doing work on its surroundings and gaining or 
losing body substance the potential energy of the food will equal 
the potential energy of the excreta plus the energy of the heat given 
off plus the energy of the work done plus the potential energy of the 
gain by the body (a loss by the body being regarded as a negative 
* Such as changes of temperature or aggregation, cleavages, syntheses, 
etc. In case these resulted in an evolution of energy, this term of the equa- 
tion would, of course, have a negative sign. 
