CHAP, v.] NUTRITION. 793 



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 gaso- 

 meter, is drawn through the chamber. Measured portions of the out- 

 going air are from time to time withdrawn and analysed ; and from the 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 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 multi- 

 plied 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 important 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 terminal 

 weights have been accurately determined, the composition 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 interpreta- 

 tion 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 proteid 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 



