828 INCOME AND OUTPUT. [BOOK 11. 



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 begin- 

 ning 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. 



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 Petterikofer 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 



