174 ELEMENTARY AGRICULTURAL CHEMISTRY 



&c., and to maintain the animal in health, it is possible to assess 

 their relative values as sources of mechanical power when con- 

 sumed by the animal by a determination of their heat-producing 

 power. The amount of heat produced by the burning of 1 part by 

 weight of the three classes of food constituents may be stated 

 in terms of the weight of water which that heat would raise 

 through 1° C. These are found to be : 



Fat . . . 9300 



Protein 4100 



Starch 4100 



Amides {e.g., asparagine) 3500 



If it is desired to calculate the heat-producing or calorific 



power of a food, the proportions of the digestible constituents 



in 1 part by weight should be multiplied by their respective 



heats of combustion. The sum of these products will give the 



calorific power of the food when fed to animals — i.e., the 



quantity of water which will be raised 1°C, in temperature by 



the heat evolved from 1 part by weight of the food consumed 



in the animal. Suppose, for example, we wish to calculate the 



calorific power of oats. 



1 part by weight of oats contains "092 part digestible protein 



„ „ „ -473 „ „ carbohydrates 



•012 „ „ fat 



-, ,,. , . , ^, ,. , ^.- ,. 4100 = 377-2 



Multiplying by the respective J .473 ^ 4100 = 1939-0 

 heats of combustion, 1-042x9300= 390-6 



r-092 X 

 A -473 X 

 t-042 X 



The calorific power of oats thus is 2706*8 



Calculated in this way, the heat-producing powers of food- 

 stuffs show great variation, maize being nearly 3500, while 

 roots are very low, turnips being about 300. 



In an animal, food is used chiefly in three ways : 



1. To promote growth and increase. 



2. To repair and renew tissue. 

 ^ 8. To furnish heat and energy. 



The proportion used in these three ways varies greatly with 

 the animal. Thus in young and growing animals, as also in 

 fattening animals, kept quiet, the first will consume a larger 

 proportion of the total food than in adult animals doing hard 



