FEEDING OF ANIMALS. 261 



that is to say, more by their flesh-forming than by their more 

 specially respiratory or fat-forming capacities. Thus he says Liebig on 

 (v 45) ■ ^ n ^ ro ~ 



W' ' ' genous con- 



Chemical researches have shewn, that all such parts of vegetables as Ij^ W * 

 can afford nutriment to animals contain certain constituents which are 

 rich in nitrogen ; and the most ordinary experience proves that animals 

 require for their support and nutrition less of these parts of plants in 

 proportion as they abound in the nitrogenous constituents. 



Again, at p. 369 of the third edition of his Chemical Letters 

 (1851), he says: — 



The admirable experiments of Boussingault prove, that the increase 

 in the weight of the body in the fattening or feeding of stock (just as 

 is the case with the supply of milk obtained from milch cows), is in 

 proportion to the amount of plastic constituents in the daily supply of 

 fodder. 



Liebig would probably be somewhat biassed in favour of 

 the conclusion here stated, by the view he held — that the 

 amount of force exercised in the animal body was measurable 

 by the amount of nitrogenous substance transformed, and this 

 again by the amount of urea found in the urine. To Liebig's 

 views on this latter point, as well as on the question of the 

 sources in the food of the fat of the animal body, and on some 

 other points of scientific as well as practical interest, reference 

 will be made further on, when considering each of these 

 several questions independently. In the meantime our 

 special object is to show, what were the prevailing opinions 

 on the subject of the adaptation of foods according to their 

 composition, to the sum of the requirements of the animals of 

 the farm, which iu eludes not only those for the mere main- 

 tenance of the body, but those for increase in live-weight, for 

 the production of milk, or for the exercise of force, as the case 

 may be. It was, however, not only in regard to the foods of 

 the animals of the farm, but to human foods also, that the 

 system of estimating their comparative value according to 

 their percentage of nitrogen came to be applied. Thus, dif- 

 ferent descriptions of flour and bread, and numerous other 

 aliments, both vegetable and animal, were examined, and 

 their comparative food-values were assumed to be indicated 

 by their richness iu nitrogen. 



The Eothamsted Feeding Expekiments. 



It was in 1847, after Boussingault had published his first ^rf/SS- 

 tables of the comparative nutritive values of differeut foods, ing expert- 

 founded on their percentage of nitrogen, and after Liebig had ]^J in 

 substantially endorsed Boussingault's conclusions on the point, 1847. 



