CHAP, v.] NUTRITION. 623 



evidence seems however in favour of the view that no such loss 

 takes place. It would appear that though nitrogen, the pivot, 

 so to speak, of the chemical changes of living beings, forms so 

 large a portion of the atmosphere and moreover is physically 

 diffused through the bodies of both plants and animals, free 

 nitrogen is of no chemical use to either of them. It enters into 

 and remains in their bodies as an inert substance, and the nitrogen 

 which leaves a plant or animal, in a gaseous state, is simply a 

 part of the same inert supply and does not come from the break- 

 ing up of the nitrogenous substances of the body or of the food. 



Of these elements of the income and output, the nitrogen, 

 the carbon, and the free oxygen of respiration are by far the 

 most important. Since water is of use to the body for merely 

 mechanical purposes, and not solely as food in the strict sense 

 of the word, the hydrogen element becomes a dubious one, 

 the sulphur of the proteids and the phosphorus of the fats are 

 insignificant in amount ; while the saline matters stand on a 

 wholly different footing from the other parts of food, inasmuch 

 as they are not sources of energy, and pass through the body 

 with comparatively little change. The body-weight must of 

 course be carefully ascertained at the beginning and at the end 

 of the period, correction being made where possible for the faeces. 



It will be seen that the labour of such inquiries is con- 

 siderable. The urine, which must be carefully kept separate 

 from the faeces, requires daily measurement and analysis. Any 

 loss by the skin, either in the form of sweat, or, in the case of 

 woolly animals, of hair, must be estimated or accounted for. 

 The food of the period must be as far as possible uniform in 

 character, in order that the analyses of specimens may serve 

 faithfully for calculations involving the whole quantity of food 

 taken ; and this is especially the case when the diet is a meat 

 one, since portions of meat differ so much from each other. 

 But the greatest difficulty of all lies in the estimation of the 

 carbonic acid produced and the oxygen consumed. In some 

 of the earlier researches this factor was neglected and the varia- 

 tions occurring were simply guessed at, through which very 

 serious errors were introduced. No comparison of income and 

 output can be considered satisfactory unless at least the carbonic 

 acid produced be directly measured by means of a respiration 

 chamber. And in order that the comparison should be really 

 complete, the water given off by the skin and lungs must be 

 directly measured also ; but this seems to be more difficult than 

 the determination of the carbonic acid. 



In the plan originally adopted by Kegnault and Reiset and fol- 

 lowed by some other observers, the animal experimented on is 

 allowed to breathe a limited and measured atmosphere. The car- 

 bonic acid, as fast as it is formed, is fixed and removed by a strong 



