558 The American Naturalist. [July, 
completion of the fall of 772 feet, heat is liberated sufficient 
to raise one pound of water one degree in temperature, or, the 
equivalent of the energy required to raise the weight to the 
height from which it fell. This serves to illustrate what is 
meant by the conservation of energy. 
The transformation of food constituents into animal sub- 
tance involves the performance of work by the animal 
machinery of nutrition, which is carried on at the expense of 
the stored energy of the food consumed. An expenditure of 
energy in work is as necessary to convert corn or grass, into 
animal subtance, as in the hauling of a load on the road, and 
-the term work is as applicable, in the same sense, in the one 
case asin the other. Sheep growing wool, cows giving milk, 
and animals fed for the butcher, should, therefore, be recog- 
nized as working animals, as well as those used in draft, or in 
lighter, more rapid work on the road. 
Internal work must be done in the first place to convert 
vegetable substances into animal substance; and, in the next 
place, an additional amount of work must be done in the 
further conversion of animal substance into the special ani- 
mal products of meat, milk, wool and muscular force, which 
are the real sources of profit in feeding. Moreover, this inter- 
nal work involves the wear and tear of the animal machine, 
which unlike purely mechanical devices, makes its own repairs 
at the expense of the raw materials it is its mission to convert 
into animal products. 
An important question here presents itself; how is the food 
consumed by animals disposed of, and what purpose does it 
serve in the animaleconomy? ‘The correct answer to this is 
of great practical importance and interest to every farmer, and 
especially to breeders of improved stock. 
In the first place, materials are provided for growth, and 
for the needed repairs of the system, but only a small propor- 
tion of the food constituents are utilized for these purposes, as 
will be seen from the following table giving the results of 
experiments at Rothamsted. 
Each 100 pounds of food constituents consumed by fatten- 
ing animals were disposed of as follows : 
VERSUS AT C Ne CREAN RU TS OA Ree Sap SCRI IMS o8 ee gk PME | 
