
FOODS, IN RELATION TO RESPIRATION AND FEEDING. 323 
On the Composition of Foods, in relation to Respiration and the Feeding 
of Animals. By J.B. Lawes, Esq., of Rothamsted; and J. H. 
GiLBeErRT, PA.D., F.C.S. 
Durine the last twelve years our knowledge of the adaptation of food, 
according to its composition, to the various exigences of the animal system, 
has assumed much of definiteness ; and it is to the experiments and writings 
of MM. Boussingault, Liebig and Dumas, that we must attribute, either 
directly or indirectly, much of the progress that has been made. There are, 
however, connected with this important subject still many open questions ; 
and it is with the hope of aiding the solution of one or two of these, and thus 
providing a new starting-point for further inquiry, that we propose in the 
present paper to bring forward some results of our own which bear upon 
them, and to point out the conclusions to which they appear to us to lead. 
The writers to whom we have above referred, as well as many others, 
whether themselves experimenters or more systematic writers on the subject 
of the chemistry of food, may, with few exceptions, and with some limita- 
tions, be said to agree on two main points, viz. on the one hand, as to the 
connection of the nitrogenous constituents of the food, with the formation in 
the animal body of compounds containing nitrogen, and with the exercise of 
force ; and on the other, as to the general relationship of the mon-nitrogenous 
constituents of the food with respiration, and with the deposition of animal 
fat. It is indeed upon the assumption of this broad and fundamental classi- 
fication of the constituents of food, according to their varied offices in the 
animal ceconomy, that a vast series of analyses of foods have of late years 
been made and published; whilst, founded upon the results of these analyses, 
numerous tables have been constructed, professing to arrange the current 
articles of diet both of man and other animals, according to their comparative 
values as such. Among the labourers in this field of inquiry, we are much 
indebted to MM. Liebig, Dumas, Boussingault, Payen, Playfair, R. D. 
Thomson, Horsford, Schlossberger and Kemp, and others. 
When speaking generally then, of the various requirements of the animal 
organism, the more special adaptations of the several proximate compounds 
and ultimate elements of which our vegetable and animal aliments are made 
up, are, as we have already said, fully admitted ; but in attempting to apply to 
practice the principles herein involved by the construction of tables of the 
comparative value of foods, it seems to have been generally assumed, that our 
current food-stuffs are thus measurable rather by their flesh-forming than by 
their more specially respiratory and fat-forming capacities. Hence, with 
some limitations, the per-centage of nitrogen has always been taken as the 
standard of comparison. 
Founded upon their per-centage of nitrogen, M. Boussingault first arranged 
tables of the comparative values of different articles of food, chiefly in refer- 
ence to the dieting of the animals of the farm; and with this method Professor 
Liebig has expressed his concurrence. At page 369 of the 3rd edition of his 
Chemical Letters, 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.” And at page 349 of the same, speaking of the nitrogenous 
compounds of food, he says—“TIt is found that animals require for their 
support less of any vegetable food in proportion as it is richer in these 
peculiar matters, and cannot be nourished by vegetables in which these 
matters are absent.” 
In like manner, various specimens of flour and of bread have been arranged 
by Dr. R. D. Thomson ; other articles of vegetable diet by Mr. Horsford ; and 
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