624 INCOME OUTPUT. [BOOK n. 



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 portions of the outgoing 

 air are from time to time withdrawn and analyzed; and from the 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 faeces known, the quantity of oxygen consumed 

 may be arrived at by a simple calculation. For evidently the differ- 

 ence 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 how- 

 ever is also oper to objections, since minute errors in the anal} T ses 

 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 undesirable to leave the quantity used of so impor- 

 tant an element as oxygen to be determined by indirect calculations. 



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

 been completely carried out, that the animal's initial and ter- 

 minal weights have been accurately determined, the composi- 

 tion of the food satisfactorily known to consist of so much 

 proteid, fat, carbohydrates, salts, and water, and to contain 

 so much nitrogen and carbon, the weight of the faeces and the 

 nitrogen they contain ascertained, the nitrogen of the urine 

 determined, the carbonic acid and water given off by the whole 

 body carefully measured, and the amount of oxygen absorbed 

 calculated what interpretation can be placed on the results ? 



Let us suppose that the animal has gained w in weight 

 during the period. Of what does w consist ? Is it fat or pro- 

 teid material which has been laid on, or simply water which 

 has been retained, or some of one and some of the other ? Let 

 us further suppose that the nitrogen of the urine passed during 

 the period is less, say by x grammes, than the nitrogen in the 

 food taken, after deduction of course of the nitrogen in the 

 faeces. This means that x grammes of nitrogen have been 

 retained in the body; and we may with reason infer that they 

 have been retained in the form of proteid material. We may 

 even go farther and say that they are retained in the form of 



