but also upon the limit of variation, or tolerance, which is regarded as admissible in the normal composition of the air. In a room hermetically closed, where the volume of available air is limited by the capacity of the in closure, 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 addition to this of the twenty litres exhaled will bring up the whole amount to thirty-three litres, or the one-thousandth part of the volume of air, which we have accepted as the tolerable limit. Consequently 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 produced by combustion is not so dangerous as are the exhalations from a living being. The case of a perfectly close room will, however, 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 centers 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 windows, on the condition of the chimneys, and lastly on the permeability 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 notable degree the danger of the air within houses stagnating, and will always prevent its becoming vitiated to the extent that might otherwise 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 subjected to a regular ventilation, there is established, at the end of a certain time, a permanent régime; 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
