192 THE BODY AT WORK 



impossible to keep a pipe alight inside the bag. Not that any 

 man so placed would desire, one would imagine, to add the 

 combustion-products of tobacco to those given off from the 

 lungs ! The survival of the explorers proves that it is im- 

 possible to fix a limit of safety even for the carbonic acid in air 

 vitiated by respiration. It is, however, a matter of common 

 observation that air which is moist and warm, owing to respira- 

 tion, and tainted with the odours of humanity, is extremely 

 prejudicial to those who live in it. Such an atmosphere is a 

 favourable medium for the conveyance of germs, whether of the 

 common cold or of a more virulent type. At one time it was 

 supposed that the volatile emanations which can be condensed, 

 along with water, by hanging a vessel of ice to the ceiling of a 

 crowded room, were actively poisonous ; but this statement has 

 not been confirmed by recent research. It is unnecessary to 

 call any such evidence in support of the thesis that human 

 beings thrive better in fresh air than in foul. The admirable 

 results achieved by the " fresh-air cure " show that there is no 

 degree of vitiation which can be pronounced innocuous. Never- 

 theless, public opinion demands that sanitarians should give 

 some figure as a guide. Commonly they fix the maximum of 

 carbonic acid compatible with health at 0-06 per cent., the 

 quantity of carbonic acid being taken as the measure of all 

 impurities present. An adult exhales about 0-6 cubic foot of 

 C0 2 per hour. Fresh air already contains about 0-04 per cent. 

 If, therefore, the percentage is not to rise higher than 0-06 per 

 cent., each adult must be supplied with 3,000 cubic feet of air 

 per hour. With good ventilation air may be changed four 

 times an hour, and therefore 800 cubic feet is regarded as 

 sufficient space for each occupant of a room. The figure may 

 pass. It is a reasonable basis from which to calculate the 

 packing capacity of a dormitory. So long as a man has 

 800 cubic feet of air to himself, he may safely feel that he has 

 room to stretch his lungs. Dwelling on this figure may make 

 him feel uncomfortable when he finds himself in a railway 

 carriage, seated five on a side, with the windows closed. In the 

 theatre or in church he may doubt whether he has all the fresh 

 air to which his humanity entitles him. But, as a philosopher 

 rather than as a physiologist, he reflects that, whether on the 

 Antarctic icecap in a sleeping-bag or standing on a summit in 



