NITROGENOUS COMPOUNDS. 219 



plete without giving the valunble results to which he 

 has arrived. 



It is now very well established that the nutritive 

 value of the food of an animal depends greatly upon the 

 proportion of nitrogenous substances contained in it. 

 Without doubt, the sugar which is found to be an ingre- 

 dient of most vegetable substances at some periods of 

 their growth in some degree contributes to it also ; 

 so do the starch and other ingredients which combine 

 to promote heat and respiration; for no doubt there is 

 a mutual relation existing between the various elements 

 of food, some going to sustain and nourish one part of 

 the animal system, and others forming some other part, 

 equally important to health and existence. Each may 

 perform its function, and be not only important, but 

 indispensable ; but chemists have been accustomed to 

 base the nutritive value of articles of food chiefly upon 

 the nitrogenous compounds. 



The nitrogenous constituents of any substance, as 

 grass or hay, for instance, may be determined with little 

 difficulty, and with great exactness, since it has been 

 found, by abundant research, that, when present, they are 

 of nearly the same constitution, and do not vary in their 

 combinations. The determination of the sugar is some- 

 what more difficult. 



The constituents of plants may accordingly be divided 

 into two classes: one class embracing all those sub- 

 stances of which nitrogen or azote forms a part, and the 

 other consisting of non-nitrogenous bodies. Gluten, al- 

 bumen, gelatine, casein, legumen, and fibrin, belong to 

 the former class, being nitrogenous substances; while 

 starch, gum, sugar, woody fibre, mucilage, <fcc., are desti- 

 tute of nitrogen, or non-nitrogenous. 



Only a small quantity of nitrogen is found in vege- 

 table substances, and it is derived in part, at least, from 



