360 THE POPULAR SCIENCE MONTHLY. 



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 slowe) 

 currents. We may next ask, Is there any advantage in rapid cur- 

 rents 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 phy- 

 sician. The first reason is for his own sake. Even in health we poi- 

 son ourselves, and in disease we tend more rapidly in the same direc- 

 tion. 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 attend- 

 ants. 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 sup- 

 pose 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, espe- 

 cially 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 influ- 

 ence. As a rule, it would be unnecessary to purify the air of the day- 

 time, 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 cli- 

 mates, 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 or- 

 ganic matter, and the condition of the air is similar to that of the even- 

 ings 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 vegeta- 

 tion. In both cases ventilation is wanted. Our walls become satu- 

 rated with moisture if they are porous, if not porous they are covered 

 with streams of water. The moisture has organic matter in it which 



