THE HABITATION AND THE ATMOSPHERE. 175 



but also upon the limit of variation, or tolerance, which is regarded as 

 admissible in the normal composition of the air. In a room hermeti- 

 cally closed, where the volume of available air is limited by the ca- 

 pacity of the inclosure, the proportion of carbonic acid will soon reach 

 the one thousandth, which we have adopted as the tolerable limit ; 

 and the more speedily as the size of the room is diminished, the more 

 tardily as it is enlarged. The volume of air required will also evidently 

 be proportioned to the time the man stays in the room. Assuming that 

 about twenty litres of carbonic acid are exhaled in an hour from the 

 lungs of an adult man, we find that he will require about thirty-three 

 cubic metres of fresh air every hour ; for this quantity of air already 

 has a normal content of thirteen litres of carbonic acid ; and the addi- 

 tion to this of the twenty litres exhaled will bring up the whole 

 amount to thirty-three litres, or the one-thousandth part of the vol- 

 ume of air, which we have accepted as the tolerable limit. Conse- 

 quently the space a person must have, if he is to live in a really close 

 room for an hour, is thirty-three cubic metres ; if he is to live there 

 two hours, sixty-six cubic metres. More will be needed if lamps or 

 gas-lights are kept burning in the room, for a candle in burning 

 will consume as much oxygen as a man ; but the carbonic acid pro- 

 duced by combustion is not so dangerous as are the exhalations 

 from a living being. The case of a perfectly close room will, how- 

 ever, never be realized ; for, however tightly we may close the doors 

 and windows, the air will always get in through some crack, and, if 

 there are no cracks, it will penetrate through the walls. The most 

 thoroughly calked room is not proof against the natural ventilation 

 that results from inequalities of temperature. Houses are great cen- 

 ters of draughts in cold weather, and are permeated by a spontaneous 

 ventilation that is dependent at once on the degree to which the outer 

 atmosphere is agitated, on the number and sizes of the doors and win- 

 dows, on the condition of the chimneys, and lastly on the permea- 

 bility of the walls. It may be increased by a suitable distribution of 

 ventilators, and is aided by the draught of the chimneys when fires 

 are kindled in them ; but fires may be regarded as artificial means of 

 ventilation. These agencies of natural ventilation diminish in a nota- 

 ble degree the danger of the air within houses stagnating, and will 

 always prevent its becoming vitiated to the extent that might other- 

 wise be apprehended from the causes of contamination which we have 

 reviewed. Their effect should be taken account of in estimating what 

 extent of artificial ventilation may be required ; otherwise, we might 

 make exaggerated provisions for it. 



When an inclosure containing a given number of persons is sub- 

 jected to a regular ventilation, there is established, at the end of a 

 certain time, a permanent regime ; the adulteration of the air, having 

 reached a certain limit, does not vary any more, the noxious gases 

 being eliminated as fast as they are developed. The proportion of 



