An average adult expires 3∕5 (·6) cubic foot of carbonic acid per hour. Now as the carbonic acid in air to be breathed must not contain more than two parts in 10,000 (·02 per cent.) in excess of what is present in external air (·04 per cent.), it follows that if _x_ = the amount of fresh air required by an adult per hour in order to keep the carbonic acid in the room down to .06 per cent., then:— ·02 : ·6 :: 100 : _x._ _x_ = 3,000 cubic feet. =Relation of Air Required to Cubic Space of Room.=—If we accept 3,000 cubic feet of air as the amount required per head per hour, this may clearly be furnished by having a large room with comparatively little circulation of air, or by having a small room with frequent interchanges. Thus, supposing the cubic space allowed to each individual is 1,000 cubic feet—that is, 10 feet in every direction—the atmosphere will require changing three times per hour. Now, it is found that when a current of air, at the temperature of 55°-60° Fahr., is moving at the rate of less than one mile per hour, it is not perceptible—that is, produces no draught. The rate of a breeze, which is just perceptible, is 18 inches per second, or one mile per hour. As draughts are objectionable, ventilation, in the best sense of the word, means the supplying of abundant fresh air at a rate of less than one mile per hour, or warmed air at a higher rate. Air moving at the rate of 2½ miles per hour, or 3½ feet per second, is perceived as a slight draught by all, at the average temperature of our climate (about 50° Fahr.) Where natural ventilation is employed, the difficulties of thoroughly ventilating a small space, without draught, are very great. A change of air three or four times in an hour is all that can be borne under ordinary conditions in this country, and this necessitates a supply of 1,000 or 750 cubic feet of space respectively for each individual. And a change of this frequency is commonly not effected; the ventilating apparatus may fail temporarily, or may be wilfully stopped up, or there may be no means of ventilation; it is essential therefore to have as large a cubic space as possible. A large cubic space, does not obviate the necessity for efficient circulation of air. It is, however, advantageous, not only on account of the initial longer time before the air reaches the limit of impurity, but also because there are less draughts, and there is a larger wall surface and larger windows for unperceived ventilation.