614 



A MANUAL OF PHYSIOLOGY 



and metal equivalent to a mass of water M'. by (T" T) degrees. 

 .-. m (T / -T*)M(T*-T)4-M / (I W T). Everything in this equa- 

 tion except M' is known, and .'. M', the water-equivalent of the 

 calorimeter, can be deduced, and must be added in all exact experi- 

 ments to the mass of water contained in it. 



Secondly, all the excess of heat in the expired over that in the 

 inspired air is not given off to the calorimeter, for the air passes out 

 of it at a slightly higher temperature than that of the atmosphere. 

 At the beginning of the experiment this excess of temperature is 

 zero. If at the end it is i C., the mean excess is 0-5 C. Now, 

 when respiration is carried on in a room at a temperature of 10 C., 

 the expired air has its temperature increased by nearly 30 C. 

 About 3*0 of the heat given off by the respiratory tract in raising the 

 temperature of the air of respiration would 

 accordingly be lost in such an experiment. 

 But since the portion of the heat lost by the 

 lungs which goes to heat the expired air is 

 only ^ of the whole heat lost in respiration 

 (p. 579), the error would only amount to - : .-y of 

 the whole, and this is negligible. 



Thirdly, the air leaves the calorimeter satu- 

 rated with watery vapour at, say, io'5, 

 while the inspired air is not saturated for 

 10 C. Now, the quantity of heat rendered 

 latent in the evaporation of water sufficient 

 to saturate a given quantity of air at 40 C. 

 (the expired air is saturated for body-tem- 

 perature) is six times that required to saturate 

 the same quantity of air at 10. If, then, 

 the inspired air is half saturated, the error 

 ARRANGED FOR under this head is T T o, or 8% per cent. If the 

 WATER-VALVE. inspired air is three-quarters saturated, the 



error is %, or about 4 per cent. If the air 



is fully saturated before ' inspiration, as is the case when it is 

 drawn in through a water-valve (Fig. 202) by a tube fixed in one 

 nostril, the only error is that due to the slight excess of temperature 

 of the air leaving the calorimeter over that of the inspired air. 

 The latent heat of the aqueous vapour in saturated air at io'5 C. 

 is about T, 1 ^ more than the latent heat of the aqueous vapour in 

 the same "mass of saturated air at 10 C., or about y^y of the 

 latent heat in saturated air at 40. The error in this case would 

 therefore be under i per cent. The tubes must be wide and the 

 bottle large. 



FlG. 202. B O T T L E 



