of cast-iron stoves, effects from which sheet-iron stoves, which are not pervious to it, are free; and it is one of the products of the combustion of poor illuminating gas. It is, nevertheless, customary to measure the degree of insalubrity which any atmospheric medium has reached by the quantity of carbonic acid it contains. This is found to increase rapidly in school-rooms, hospital-wards, and assembly-rooms of all kinds, but not nearly so rapidly, unless the room is extremely close, as the gas is actually developed by the life-processes of the inhabitants of the rooms. This fact indicates that, even in rooms regarded as close, a considerable renewal of air is all the time going on by natural or spontaneous ventilation.
Dr. Pettenkofer has made an ingenious use of the estimation of the proportions of carbonic acid to measure the spontaneous ventilation, or the speed with which the air gradually renews itself in rooms. It is sufficient for this purpose to develop artificially in a room an exactly ascertained quantity of the gas, and to determine by repeated analyses the quantity of acid that disappears in a certain time. The method is a good one, provided there is no opportunity for the acid to be absorbed by fresh mortar. By gauging in this manner the ventilation of a number of places, and then observing in the same places the degree of alteration in the atmosphere resulting from the presence of a given number of persons, Dr. Pettenkofer found that the atmosphere remained of a satisfactory quality when it was renewed at the rate of sixty cubic metres an hour per head. The proportion of carbonic acid continued under these conditions to be less than a thousandth. Experiments were made in a room with brick walls, and having a capacity of seventy-five cubic metres. On the first day when the temperature was 66° in the room and below the freezing-point out-of-doors—the difference being nearly 36°—the rate of change (seventy-four cubic metres) was sufficient to renew all the air in the room in an hour; with a good fire in the stove, the rate of ventilation was raised to ninety-four cubic metres an hour. With paper pasted over the joints of the doors and windows, it fell to fifty-four cubic metres. On another day, when the difference between the inner temperature and that out-of-doors was about seven degrees, the rate of ventilation was only twenty-two cubic metres an hour; and with a window half open it was only increased to forty-two cubic metres; thus an opening of eighty square decimetres was of less effect upon ventilation than the simple transpiration through the walls assisted by a difference of about 36 between the outer and inner temperatures. A calculation based on these experiments indicates that a difference in temperature of 1° C. (1·8° Fahr.) causes to pass every hour about two hundred and forty-five litres of air for each square metre of exposed wall-surface.
The question of the volume of air needed by a man for free respiration is a complex one, on which hygienists do not readily agree. The answer to it must depend, not only on the exterior conditions in view,