94 BOARD OF AGKICULTURE. 



Liebig's services to animal physiology are prominent in 

 two directions, namely : First, on account of his extensive 

 analytical examinations of numerous organic substances, of 

 the proximate constituents of plants which serve as food for 

 animals, and of the chemical changes they undergo during 

 their passage through the animal system ; and, second, on ac- 

 count of the direction he has given to the modes of observa- 

 tion to be applied in the study of animal physiology, by sub- 

 stituting the empirical experimental methods for the specu- 

 lative philosophical one of preceding periods. A cursory 

 study of the views of leading physiologists before 1840 can- 

 not fail to concede to him a controlling influence on our pres- 

 ent views regarding the principles which underlie a rational 

 system of animal nutrition. Liebig's classification of the 

 constituents of the animal food into three distinct groups, 

 namely, Nitrogenous substances, non-nitrogenous substances, 

 and mineral substances, furnishes the frame-work of the 

 more rational system of stock-feeding of to-day. 



Although it had been noticed for years that some articles 

 of animal food contained the element nitrogen as one of their 

 constituents, wdiilst others contained none — nitrogenous 

 and non-nitrogenous substances — experience has furnished 

 ample proof during famine, in war, and under other excep- 

 tional circumstances, that one single article of food, in par- 

 ticular those which contained no nitrogen, could not sustain 

 life beyond a limited period of time ; yet no satisfactory ex- 

 planation of the real cause of death in those cases had been 

 advanced. The force of this statement may be deduced 

 from a subsequent brief enumeration of some feeding exper- 

 iments carried on by distinguished scientists between 1830 

 and 1840. Several of the following experiments were made 

 in connection with a prize question offered for general com- 

 petition by the "French National Academy," one of the 

 foremost scientific associations of Europe : 



" Is the animal gelatine obtained by the boiling of bones, 

 a suitable animal food ? " 



