NITROGENOrs Toon and \\n\:w «ji.-, 



niti'Dgenoiis organic inatlci-. In oilier \v<>r<l>. tlir ii(»iMiiin.;^'riinu> 

 c'oiiiponnds aiv \\w main items to lu« taken into account in 

 making up tlic value of a cattle food, wliicli value camn.t In- 

 estimated on a basis of its nitrogen content onlv. 



11. — Relation of Xi'ri!o(;KNoi s 1''ooi> io W'okk. 



The very special im[)oi'tance that was originalK attaclie(l to 

 the nitrogenous constituents of food was also seen in the vie\N> of 

 J.iebig with regard to the source of tlie work, either external or 

 internal, performed l)y an animal. He put forward tin- \iew 

 that the amount of work done was determined by the amount 

 of nitrogenous material transformed in the body, and therefore 

 that it could he measured by the amount of nitrogen appearing 

 in the urine, since the albuminoids and other nitrogen com- 

 pounds in food which are digested and undergo change in 

 the animal are excreted as urea. Lawes and Gilbert, by their 

 studies of human dietaries, were led to conclude that this 

 view w\as mistaken, and that the fats and carbohydiates. 

 which are oxidised and leave the body in the respiration 

 products, supply the energy for the work performetl in and 

 l)y the body. Two experiments with pigs, carried out in lsr)4 

 and 1862 respectively, were adduced as further evidence. The 

 pigs w^ere confined in a frame ; the nitrogen in the food and the 

 nitrogen excreted in urine and fieces respectively were 

 determined. The food was so adjusted that one pig received 

 about twice as much nitrogen as the other (Table LXXW'II.). 



The animals were obviously under equal conditions a> 

 regards exercise, both being at rest, yet in each experiment the 

 animal receiving the highly niti-ogenous diet excri'ted rather 

 more than twice as nuich nitrogen as urea. Thus the amount 

 of nitrogen in the urine, which measinv> ihr amount of 

 allmminoids oxidised, could hardly be taken as a measure of 

 the amount of w^ork performed by the respective animals. 



The question was afterwards sy.stematically attacked in 

 various directions ])y other investigators, and diivct j)roof 

 was obtained that the energy require<l to cany on work is 



