Classes of Compounds 29 



as medicine and some in the arts. It is fortunate 

 that comparatively few must be considered in dis- 

 cussing the science and art of cattle -feeding. More- 

 over, it is convenient that the compounds which play 

 a leading part in animal nutrition are designated, es- 

 pecially for practical purposes, in classes rather than 

 singly, even though this custom tends to more or less 

 looseness of expression and definition. 



The same classification is used for the compounds of 

 both the vegetable and animal kingdoms, and it is now 

 customary to divide them into the following groups: 



Water, 



Ash (mineral compounds), 

 Protein (nitrogenous compounds), 

 Carbohydrates (and related bodies), 

 Fats (or oils). 



In this instance, accuracy is sacrificed to conven- 

 ience. The class names have come to be regarded, 

 more or less, as representing entities having fixed prop- 

 erties and functions, whereas each class contains numer- 

 ous compounds differing widely in their characteristics 

 and in their nutritive value and office . Moreover, these 

 terms have a variable significance as used under differ- 

 ent conditions. No one of them except water uniformly 

 represents just the same mixture of compounds when 

 applied to unlike feeding stuffs. 



Before passing to a detailed description of these com- 

 pounds, singly or in groups, it will be well to gain a 

 clear understanding of the relation which the fifteen ele- 

 ments mentioned sustain to these classes of substances. 

 This can be seen most readily by a tabular display: 



