486 NUTRITION. 



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 differ- 

 ent 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 

 feces. 



It will be seen that the labor of such inquiries is considerable. The 

 urine, which must be carefully kept separate from the feces, 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 ac- 

 counted 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 espe- 

 cially 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 estima- 

 tion 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 res- 

 piration chamber. And in order that the comparison should be really com- 

 plete, 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 car- 

 bonic acid. 



In the plan originally adopted by Regnault and Reiset, and followed by some 

 other observers, the animal experimented on is allowed to breathe a limited and 

 measured atmosphere. The carbonic acid, as fast as it is formed, is fixed and 

 removed by a strong solution of caustic potash, and the normal percentage of 

 oxygen in the atmosphere is maintained by a supply of this gas from a gas-holder. 

 In this way both the oxygen consumed and the carbonic acid produced are directly 

 determined, while the continual supply of fresh oxygen prevents any evil effects 

 due to breathing a confined portion of air. In order, however, to avoid all possible 

 errors arising from a too restricted atmosphere, a different method has been adopted 

 by Pettenkofer and Voit. Their apparatus consists essentially of a large chamber, 

 capable of holding a man comfortably. By means of a steam-engine a current of 

 pure air, measured by a gasometer, is drawn through the chamber. Measured 



Eortions of the outgoing air are from time to time withdrawn and analyzed ; and 

 rom the data afforded by these analyses the amounts of carbonic acid (and other 

 gases) and of water given off by the occupant of the chamber during a given time 

 are determined. The oxygen consumed is not determined directly; but if the 

 total amounts of carbonic acid and of water given out by the lungs and skin are 

 ascertained, and the amount of urine and feces known, the quantity of oxygen 

 consumed may be arrived at by a simple calculation. For evidently the difference 

 between the terminal weight plus all the egesta and the initial weight plus all the 

 ingesta can be nothing else than the weight of the oxygen absorbed during the 

 period. This method in turn, however, is also open to objections, since minute 

 errors in the analyses of the small samples of air employed for the determinations 

 attain considerable dimensions when these are multiplied so as to give the changes 

 in the whole mass of air passed through the apparatus. It seems, moreover, unde- 

 sirable to leave the quantity used of so important an element as oxygen to be deter- 

 mined by indirect calculations. 



Let us imagine, then, an experiment of this kind to have been completely 

 carried out ; that the animal's initial and terminal weights have been accu- 

 rately determined ; the composition of the food satisfactorily known to con- 

 sist of so much proteid. fat, carbohydrates, salts, and water, and to contain 



