412 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



the purposes of the body. The necessary vital functions must 

 first be subserved, such as the body heat, beating of the heart, 

 movements of the lungs, etc., and all other functions necessary 

 for the maintenance of the life of the animal. 



Any food values remaining after maintaining the animal, may 

 be used for productive purposes, such as work, building of fat or 

 flesh, production of milk, etc. 



The utilization of the various foods and nutrients by the body 

 is studied by means of exact experiments on animals. The 

 experiment must, of course, be adapted to the end in 

 view. A study of the income and outgo of nitrogen is 

 called the nitrogen balance. A loss of nitrogen means a 

 loss of flesh ; a gain of nitrogen, a gain of flesh. The 

 income and outgo of carbon, taken in connection with the nitro- 

 gen balance, gives the loss or gain of fat. This is called the car- 

 bon balance. The determination of the income and outgo of 

 energy is called the energy balance. 



The Nitrogen Balance. In order to determine the exact amount 

 of nitrogen the animal is gaining or losing, we proceed as in a 

 digestion experiment, but collect and analyze the urine in addition 

 to the solid excrement. 



The total quantity of nitrogen in the food fed is the income of 

 nitrogen. The nitrogen in the solid excrement is undigested 

 material; that in the urine, is the digested nitrogen which has 

 undergone complete metabolism in the body. The nitrogen in 

 the perspiration is so small that it is usually not considered. 



Income in food less outgo in solid and liquid excrement is the 

 loss or gain of nitrogen. If income is greater than the outgo 

 there is a gain ; if less, a loss of nitrogen. Since over ninety per 

 cent, of the nitrogen in the animal body is in the form of flesh, a 

 loss or gain of nitrogen represents a loss or gain of flesh. 



Water and fat-free flesh has been found to contain, on an 

 average, 16.67 P er cent - of nitrogen and 52.54 per cent, of carbon. 

 That is, i gram of nitrogen is contained in 6 grams of dry flesh, 

 which also contains 3.15 grams carbon. Flesh contains on an 

 average 77 per cent, water. Hence a gain of i gram of nitrogen 



