CHAP, v.] NUTRITION. 447 



the other organic constituents of sweat amount to very little ; and 

 (3) of the urine, which is assumed to contain all the nitrogen really 

 excreted by the body, besides a large quantity of saline matters, 

 and of water. Where great accuracy is required the total nitrogen 

 of the urine ought to be determined; it is maintained, however, 

 that no errors of serious importance arise when the urea alone, as 

 determined by Liebig's method, is taken as the measure of the 

 total quantity of nitrogen in the urine, since, in this method, other 

 nitrogenous bodies besides urea 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 break- 

 ing up of the nitrogenous substances of the body or of 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, cor- 

 rection being made where possible for the feces. 



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 con- 

 sumed. In some of the earlier researches, this factor was 

 neglected and the variations occurring were simply guessed at, 



