254 THE FEEDING EXPERIMENTS 



than for oxen, the balance being made up by an extra proportion 

 of fat, which may amount to 75 per cent, of the increase. In 

 the case of really fat pigs the increase will contain about 70 per 

 cent, of fat and 7 per cent, of nitrogenous matter, being even 

 less nitrogenous and more fatty than with sheep. 



These experiments on the composition of whole animals, 

 which form the fundamental basis of our knowledge of the 

 nature of the animal's body and of the changes taking place 

 during growth and fattening, have never been repeated. 



VI. THE MANURE VALUE OF FOODS. 



In order to form any estimate of the value of different cattle 

 foods, it is of much importance to know how far their various 

 manurial constituents nitrogen, phosphoric acid, and potash- 

 find their way into the manure heap, and so back to the 

 farm. 



In the experiments previously described it is seen how 

 small a proportion of the nitrogenous constituents of food is 

 retained in the increased live weight of the animal during 

 fattening, by far the largest portion being passed undigested 

 into the faeces, or excreted as urea in the urine. When the 

 animal is producing milk, however, a much larger proportion of 

 the nitrogen will be removed in the milk than is retained in 

 fattening increase, and the manure made will be correspond- 

 ingly poorer. At the other extreme is the case of a working 

 horse or a store beast not gaining in weight, when the whole 

 of the nitrogen supplied in the food will be voided in the 

 faeces or the urine. 



As regards the mineral matters of the food, after the animal 

 has withdrawn a certain small proportion for increase or for 

 milk, the remainder must find its way into the manure ; but in 

 the case of the nitrogenous compounds there is always the 

 possibility of loss, because some of the nitrogen may pass into 

 volatile ammonia, or even into gaseous nitrogen, during the vital 

 processes. 



The question of the existence of this loss was investigated 



