1896.] Relative Efficiency of Animals as Machines. 789 
far as needed, is used again as the motive power in rebuilding 
the disintegrated tissues and other physiological processes. 
Animal heat is, therefore, the result of physiological activi- 
ties that are carried on in accordance with the laws of the 
conservation of energy, without the slightest indication of 
anything analogous to processes of combustion. 
Living substance, as pointed out by Foster, is matter con- 
stantly undergoing change, energy being used and stored up 
in constructive metabolism, and liberated again as heat in the 
correlated and quite as essential processes of destructive meta- 
bolism. Energy doing work, or stored as potential energy in 
the tissues, cannot be detected by the thermometer, and the 
heat liberated from foods in the various processes of disinte- 
gration they undergo, and from the tissues through destruc- 
tive metabolism, is again made latent, as far asit is utilized in 
doing the work required in constructive metabolism, in vapor- 
izing water exhaled by the lungs or thrown off by the skin as 
perspiration, and animal heat is the sensible residue not dis- 
posed of in these physiological processes. 
Our domestic animals may then be looked upon as ma- 
chines for doing work in the repairs and other vital activities 
of the animal machine itself, including muscular exercise, and 
the manufacture of animal products used as food by man. 
The importance of these animal machines as factors in do- 
mestic economy leads us to inquire as to their relative effi- 
ciency in utilizing the potential energy of foods in the special 
work they are fitted to perform. 
Aside from the individual and class peculiarities that re- 
quire attention, their efficiency in utilizing energy must vary 
with the quality and quantity of food consumed and a tenta- 
tive solution of the problems presented in this line of inquiry 
can only be made. The quantitative estimates of the expen- 
ditures of energy in different ways we are able to make must, 
therefore, be interpreted as representing approximately the re- 
sults with the particular animals under the conditions to 
which they were subjected. 
The feeding experiments at Rothamsted, and the composi- 
tion of different animals and their increase, as shown in the 
