166 ELEMENTARY AGRICULTURAL CHEMISTRY 



satisfactory, but is still the usual way of expressing analytical 

 results. 



Protein. — This is obtained by multiplying the percentage of 

 total nitrogen present by 6*25, on the assumption that all the 

 nitrogen is present as albuminoids and that the latter contain 

 IB per cent, of nitrogen. Both these assumptions are generally 

 wrong. Many food-stuffs contain a considerable portion of 

 their nitrogen in the form of amides, which are less valuable 

 for feeding purposes than albuminoids (see chap. v.). How- 

 ever, in recent analyses distinction is made between albu- 

 minoids and amides. 



Fat, or ether extract, as it is sometimes more accurately 

 called, is, as the latter name indicates, the proportion of the 

 material which is soluble in ether. It generally includes true 

 fat or oil, chlorophyll, and other colouring substances and 

 resinous bodies. 



The value of a food depends partly upon its composition, 

 but also upon its palatability and digestibility. The last can 

 be determined experimentally by experiments with animals. 

 Weighed quantities of the food are fed to animals kept in such 

 a way that their excrement can be collected and analysed for 

 a considerable period of time. In this way the proportion of 

 each food constituent digested out of 100 parts by weight 

 supplied can be determined. This proportion is known as the 

 " digestion coefficient," but differs with the kind of animal, and 

 even with different individuals of the same kind. , 



The ** digestibiHty " of a food constituent, however, in this 

 sense, has no reference to the ease or rapidity of its assimilation, 

 nor to its effect upon the health or comfort of the animals 

 consuming it. 



Animals differ in their power of digesting any given food or 

 food-constituent. Thus ruminants, by their more thorough 

 and repeated mastication, are better able to digest bulky fodder 

 than are pigs and horses. 



In the following table are given the average digestion 

 coefficients of the constituents of various foods according to 



