258 THE KOTHAMSTED EXPERIMENTS. 



the explanation and for the improvement of agricultural 

 practice. 



Thus, besides contributing much towards a better know- 

 ledge of the actual and comparative value of different foods, 

 he investigated the question whether animals either availed 

 themselves of the free nitrogen of the air as a source of some 

 of their nitrogen, or eliminated either free or combined nitro- 

 gen by the lungs or skin ; also whether the fat stored up by 

 the fattening animal was exclusively derived from the already 

 formed fat of the food, or whether it was produced within the 

 body, from other constituents of the food. 



From the point of view of the practical agriculturist, Bous- 

 singault seems fully to have assumed the utility of attempt- 

 ing to arrange stock-foods according to their nutritive value 

 compared with that of hay as a standard ; and, in fact, this 

 idea has given a direction to much subsequent investigation 

 also. 

 Nitrogen The first great advance made by Boussingault was, however, 

 m foods, fo determine the nitrogen in a large number of different foods ; 

 and, taking the amount of it as for the time the best measure 

 of nutritive value, on this basis to compare them with hay. 

 That is to say — supposing 100 parts of average good hay to 

 contain a certain amount of nitrogen, how much of each of 

 the other foods would be required to supply the same amount 

 of it ? These amounts would, on the supposition adopted, 

 represent the quantities by weight in which one food may be 

 substituted for another, and they may be considered as the 

 theoretical equivalents of 100 of hay. Accordingly, he deter- 

 mined the nitrogen in about seventy-six different descriptions 

 of food, which at that date involved a truly enormous amount 

 of labour. 

 Feeding Further, he selected a few typical articles of food for com- 



expen- parative feeding experiments, so as to be able to compare the 

 results obtained both with those indicated by theory accord- 

 ing to their contents of nitrogen, and with the estimates of 

 others, founded chiefly on somewhat similar practical trials. 

 He obviously fully recognised the difficulties and uncertainties 

 of such modes of experimenting, and took great care to obviate 

 error arising from them. 



He discussed the general results of some experiments with 

 milking cows; but gave in some detail the particulars and 

 results of ten experiments with the horse. The normal 

 food being hay, straw, and oats, he, in one case, substituted 

 half the hay by potatoes, in another by Jerusalem artichokes, 

 in another by mangels, in another by ruta-baga, and in another 

 by carrots. Again, in another the straw and oats were 

 replaced by potatoes ; in another half the hay was replaced by 



