56 Messrs, J. B. Lawes and J. II. Gilbert on Food in its 



the deposition of animal fat, he concluded that the relative 

 value of different foods, as such, was to a great extent depend- 

 ent on, and even measurable by, the proportion of nitrogenous 

 constituents which they contained. 



It was upon the assumption of the broad and fundamental 

 classification of the constituents of food according to their various 

 offices in the animal economy, as above stated, that numerous 

 analyses of food were undertaken, and, founded upon the results 

 obtained, Tables constructed professing to arrange current articles 

 of food, both of man and other animals, according to their com- 

 parative values as such; and whether the object were the feeding 

 of animals for the production of milk, the so-called fattening of 

 them for the production of meat, or the support of the body for 

 the exercise of muscular power, the proportion of nitrogenous 

 constituents was generally taken as the measure of that value. 



Omitting, for the sake of brevity, any special reference to the 

 labours or views of others, it will suffice here to make a few such 

 quotations from Baron Liebig's w r orks as will best convey shortly 

 in his own words a pretty clear indication of his own views, 

 and at the same time pretty fairly represent those of a large pro- 

 portion both of systematic writers and experimenters, on the 

 points in question. Speaking of the nitrogenous constituents of 

 food, he said : — 



" It is found that animals require for their support less of any 

 vegetable food in proportion as it is richer in these peculiar mat- 

 ters, and cannot be nourished by vegetables in which these mat- 

 ters are absent." (Chemical Letters, 3rd edition, p. 349.) 



Again : — 



"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." (Chemical Letters, 3rdedit. p. 369). 



In regard to the exercise of force, he said : — 



"As an immediate effect of the manifestation of mechanical 

 force, we see, that a part of the muscular substance loses its vital 

 properties, its character of life ; that this portion separates from 

 the living part, and loses its capacity of growth and its power of 

 resistance. We find that this change of properties is accompanied 

 by the entrance of a foreign body (oxygen) into, the composition 

 of the muscular fibre (just as the acid lost its chemical character 

 by combining with zinc) ; and all experience proves, that this 

 conversion of living muscular fibre into compounds destitute of 

 vitality is accelerated or retarded according to the amount of force 

 employed to produce motion. Nay, it may safely be affirmed, 

 that they are mutually proportional ; that a rapid transformation 



