ENERGY VALUES OP RATIONS FOR FARM ANIMALS. 7 



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 

 carbohydrates (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. 



PEED AS A SOURCE OP 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- 



