or window is opened, and we do not warm the house itself. Builders make the walls thinner in these days, and we sit at a fire very much as savages do over a blaze in the open air.
This is less the case with large rooms, where we require slower currents. We may next ask, Is there any advantage in rapid currents at any time? There is; in the case of infectious diseases, it would seem in the abstract to be of the greatest importance that the patient should be in a current, speaking as a chemist, and not a physician. The first reason is for his own sake. Even in health we poison ourselves, and in disease we tend more rapidly in the same direction. Infectious emanations may be collecting round a patient, and, if so, the still air will keep them more carefully near him. I speak only generally, and do not enter on the hospital controversy.
Perhaps we cannot have rapid currents in large rooms very easily, so much air is required; but we can have frequent changes of air. It is clear, however, that the rapid removal of the air collecting around patients with infectious diseases, and probably also non-infectious, is most likely to promote health, both in the patients and in the attendants. Few people can stand the rapid motion of cold air, and, if we must have rapid currents, they must be heated.
The source of the air with which we ventilate ought probably to be high in all cases, but even here we must move slowly. We are not quite sure that any infectious disease ever sends its emanations high into the air. Disease seems to creep along the ground; the causes may be at a considerable height, but we are compelled to suppose them very thinly disseminated there; and the action seems to be according to quantity as well as intensity; toward the surface they congregate and are active. This we see from the evening air, especially in marshy places; it is only after a certain repetition of the attack of the more thinly diffused wandering substances falling down from the atmosphere and accumulating, that men yield to the influence. As a rule, it would be unnecessary to purify the air of the daytime, if in an open place, even in average towns; and in most places it would be unnecessary to purify the air of the night in this country. It would, however, be better to warm it in northern and damp climates, and even in temperate climates, in order to produce a difference of temperature between the air entering the room and that within it, even if the necessity arising from the cold of rapid ventilation did not occur. In inhabited rooms the moisture increases as much as the organic matter, and the condition of the air is similar to that of the evenings of summer: whenever the temperature goes down a little, there is a deposit of dew; but, when the warmth increases, the air is laden with moisture, and the condition resembles that near a warm close vegetation. In both cases ventilation is wanted. Our walls become saturated with moisture if they are porous, if not porous they are covered with streams of water. The moisture has organic matter in it which
