PRACTICAL EXERCISES 



695 



more 



is carried on in a room at a temperature of 10 C., the expired air has 

 its temperature increased by nearly 30 C. About ^ 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. 658), the 

 error would only amount to 3^ of the whole, and this is negligible. 



Thirdly, the air leaves the calorimeter saturated with watery vapour 

 at, say.. 10-5, while the inspired air is not saturated for 10 C. Now, 

 the quantity of heat rendered latent in the evaporation of water suffi- 

 cient to saturate a given quantity of air at 40 C. (the expired air is 

 saturated for body-temperature) is six times that required to saturate 

 the same quantity of air at 10. If, then, the inspired air is half 

 saturated, the error under this head is ^ , or 8 per cent. If the inspired 

 air is three-quarters saturated, the errors ^, 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. 218) by a tube fixed in one nostri), 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 

 than the latent heat of the aqueous vapour in the same 

 mass of saturated air at 10 C., or about T ^ of the 

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

 case would therefore be under i percent. The tubes 

 must be wide and the bottle large. 



ii. In the observations on the blood-flow in the 

 hands (Experimen 32, p. 218) data on the quantity 

 of heat given off by the hands when immersed in water 

 at a given temperature have already been obtained. 

 Additional data should be got by putting the hand 

 into the calorimeter without previous immersion in 

 the bath, and comparing the heat given off during 

 the period when the hand is acquiring the temperature 

 of the calorimeter with that given off when the steady 

 state has been reacted. Different calorimeter tem- 

 peratures should be employed. It will be found that 

 as the calorimeter temperature is diminished the 

 quantity of heat given off may be increased although 

 the blood-flow is diminished, each gramme of blood passing through 

 the hand giving off more heat the lower the calorimeter temperature. 



The quantity of heat lost by the hand, at a given temperature 

 of the calorimeter, per square centimetre of skin surface can be cal- 

 culated. If no special instrument for measuring the area of irregular 

 surfaces is available, the surface of the hand can be arrived at ap- 

 proximately by covering it with strips of gummed paper of known 

 breadth, and noting the length used to cover the whole hand up to the 

 lower level of the styloid process of the ulna. Or an old thin glove 

 which fits the hand can be cut off at this level and weighed. As large 

 a piece as possible of regular shape is then cut from the glove, weighed, 

 and its area deduced by measuring it with a rule. The area of the 

 whole glove, on the assumption that it is of uniform thickness, is thus 

 known. Or, without cutting the glove, it may be laid flat on a piece 

 of paper, an outline of it traced, and the paper cut out. The weight 

 of the paper cut out is compared with that of a piece of paper of known 

 area, and its area deduced. Obviously this is approximately equal to 

 half the surface of the hand. 



Fig. 218. Bottle 

 arranged for 

 Water-Valve. 



