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ENERGY VALUES OF RATIONS FOR FARM ANIMALS. a 
If the fuel materials supplied in the feed are just adequate to the 
work to be done, they are virtually burned. up as a source of power. 
If more are supplied than are immediately needed, the body is able to 
store away the surplus for future use, much as we may fill up the 
gasoline tank of an engine. To a small extent the body stores up 
-earbohydrates (in the form of glycogen), but most of its surplus fuel 
it converts into fat. The fat of the body, therefore, is its reserve of 
fuel. In fattening, the body is accumulating a surplus against future 
needs which man diverts to his own use as food. - If the feed becomes 
insufficient, this store is drawn upon and the animal gradually be- 
comes lean. Similarly, in growth and in milk production, the animal 
sets aside a part of the supply of both repair and fuel material in its 
feed for its own growth or for the use of its young, and man appro- 
priates the resulting meat or milk as repair and fuel material for 
his own body. 
FEED AS A SOURCE OF FUEL MATERIAL. 
We can run an engine with various kinds of fuel. For the steam 
engine we may use coal or wood or petroleum; for the internal-com- 
bustion motor, gas, alcohol, or gasoline may be employed. Similarly, 
we supply the animal body with a great variety of feeding stuffs from 
which it has to extract its supply of fuel, and even the materials 
which it actually burns up are of various sorts. 
These fuel materials are not all of equal value. A pound of good 
anthracite coal, for example, is, other things being equal, about 14 
per cent more valuable as fuel than the same weight of alcohol, while 
a pound of fat in the feed has over twice the fuel value of a pound of 
starch. Evidently, it will greatly simplify comparisons of different 
feeding stuffs and rations as sources of fuel material to have some 
simple method by which we can learn not only the amount of fuel 
material which the feed contains, but also the quality of that fuel. 
Such a basis of comparison is afforded by a study of the energy 
values. 
MEASUREMENT OF ENERGY. 
Anything which has the capacity to do work is said to possess 
energy. Hence we say that the fuel of the engine and the feed of the 
animal possess energy, since they enable the engine or the body to 
do work. They hold this energy stored up in the ‘‘latent”’ or ‘“poten- 
tial’ form of chemical energy. When they are burned in the engine 
or the body, this chemical energy is set free, part of it being converted 
into work and the rest escaping as heat. 
Plainly, then, the value of a fuel, or of a feeding stuff so far as it 
serves as fuel, depends, in the first place, on how much chemical 
energy it contains. This can be measured without difficulty by con- 
