amount of the acid in the external atmosphere, we can calculate how much of the latter must go on mixing itself with the room-air, to which carbonic acid has been added, in order that the proportion of the acid may decrease by so and so much in a definite time. The action of diffusion or absorption may generally be left out of consideration in this calculation. I do not consider this method to be absolutely correct, but I have found it quite satisfactory when a building was a few years old, and quite dry. At all events, until a better method has been found, we must keep to this one, even if it were still less complete than it is.
By researches which are too complicated to lie explained in a popular lecture, it has been found that the ventilation of the same room or space, when the doors and windows are shut, undergoes considerable and definite alterations under different circumstances. Ventilation has been found to be much greater than had been supposed before. On an average, in spaces in which the air kept good, there existed a ventilation of more than 2,100 cubic feet per head and hour. It is known that a person does not inhale and exhale more than eighteen cubic feet of air per hour, and so it was thought that 2,100 cubic feet per hour was a ridiculously large quantity for one person.
But it has been shown, first in France, not by calculation, but quite empirically by simple experimenting, that this quantity of ventilation is not more than is absolutely indispensable. After the epidemic of cholera of 1848, the erection of a model hospital in the Faubourg Poissonnière was decided upon, and the Hôpital la Riboisière was erected, which was to be furnished with artificial ventilation. The quantity of air which was required from the ventilating apparatus was stated in the plan. It was believed that the demands put under Nos. 4, 5, and 7, of the plan for ventilation, were extraordinarily large:
4. Continuous ventilation of warm air in winter and cold air in summer at least 700 cubic feet per hour and bed in the large wards.
5. Ventilation during the day only at 350 cubic feet per bed in the rooms of the corresponding pavilion.
7. The ventilating apparatus must have a surplus of strength, in order to be able to produce in all or some wards a ventilation double that stated.
The air was partly propelled by fan-wheels, partly by ventilating flues. It flowed to and fro through pipes in the wards, and its velocity could be measured easily by anemometers.
In preliminary experiments, a ventilation of 350 cubic feet per bed and hour was tried, but the air was found already by the smell to be so bad that the authorities congratulated themselves in having provided for double the strength. This was now tried, but with the same result, and it was a comfort to know that, for extraordinary cases, another 700 cubic feet per bed and hour could be obtained; but