24 THE SCIENTIFIC PRINCIPLES 



two individuals, in the same temperature, is propor- 

 tional to the amount of nitrogen in their urine ; 

 whether the mechanical force has been employed in 

 voluntary or involuntary motions, whether it has been 

 consumed by the limbs or by the heart and other 

 viscera.' 



Thus, apparently influenced by the physiological 

 considerations which he had adduced, and notwith- 

 standing that in some passages he seemed to recog- 

 nise a connection between the total quantity of 

 oxygen inspired and consumed, and the quantity of 

 mechanical force developed, Liebig nevertheless very 

 prominently insisted that the amount of muscular 

 tissue transformed — the amount of nitrogenous sub- 

 stance oxidated — was the measure of the force 

 generated. He accordingly draws the conclusion, 

 that the requirement for the nitrogenous constituents 

 of food will be increased in proportion to the increase 

 in the amount of force expended. 



It will be obvious that the question whether, in 

 the feeding of animals for the exercise of mechanical 

 force, that is, for their labour, the demands of the 

 system will be proportionally the greater for the 

 nitrogenous, or for the non-nitrogenous constituents 

 of food, is one of considerable interest and practical 

 importance. 



In reference to this point, to that of the sources 

 in the food of the fat of the animal body, as well 

 as to the requirements for the different constituents 

 of food for the maintenance, and for the general 

 increase, of the body, in the feeding of the animals of 

 the farm, a great deal of experimental evidence has 

 been acquired during the last forty years, both in this 

 country and on the Continent, and to this I shall 

 have to refer in some detail on a future occasion. 



