"Some time since I visited an establishment where one hundred and fifty girls, in a single room, were engaged in needle-work. Pale-faced, and with low vitality and feeble circulation, they were unconscious that they were breathing air that at once produced in me dizziness and a sense of suffocation. If I had remained a week with, them, I should, by reduced vitality, have become unconscious of the vileness of the air!" There is a prevailing prejudice against _night air_ as unhealthful to be admitted into sleeping-rooms, which is owing wholly to sheer ignorance. In the night every body necessarily breathes night air and no other. When admitted from without into a sleeping-room it is colder, and therefore heavier, than the air within, so it sinks to the bottom of the room and forces out an equal quantity of the impure air, warmed and vitiated by passing through the lungs of inmates. Thus the question is, Shall we shut up a chamber and breathe night air vitiated with carbonic acid or night air that is pure? The only real difficulty about night air is, that usually it is damper, and therefore colder and more likely to chill. This is easily prevented by sufficient bed-clothing. One other very prevalent mistake is found even in books written by learned men. It is often thought that carbonic acid, being heavier than common air, sinks to the floor of sleeping-rooms, so that the low trundle-beds for children should not be used. This is all a mistake; for, as a fact, in close sleeping-rooms the purest air is below and the most impure above. It is true that carbonic acid is heavier than common air, when pure; but this it rarely is except in chemical experiments. It is the property of all gases, as well as of the two (oxygen and nitrogen) composing the atmosphere, that when brought together they always are entirely mixed, each being equally diffused exactly as it would be if alone. Thus the carbonic acid from the skin and lungs, being warmed in the body, rises as does the common air, with which it mixes, toward the top of a room; so that usually there is more carbonic acid at the top than at the bottom of a room. [Footnote: Prof. Brewer, of the Tale Scientific School, says: "As a fact, often demonstrated by analysis, there is generally more carbonic acid near the ceiling than near the floor."] Both common air and carbonic acid expand and become lighter in the same proportions; that is, for every degree of added heat they expand at the rate of 1/480 of their bulk.