792 INCOME AND OUTPUT. [BOOK n. 



are precipitated, and so contribute to the quantitative result. 

 It has been and indeed still is debated whether the body may 

 not suffer loss of nitrogen by other channels than by the urine 

 and faeces, whether nitrogen may not leave the body by the 

 skin or indeed in a gaseous state by the lungs. The balance 

 of the conflicting 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 breaking 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 considerable. 

 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 

 variations 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. 



