is not removed by mere drying, and the effects are very long in leaving. We may know this from breathing the air of any bedroom in a London hotel, or in most private houses in towns. People are afraid to keep their windows open, because of the smoke without, and so they retain the organic matter. We can readily smell this, even if arising from healthy persons, and it has ceased to be a matter of surprise. If unhealthy persons are present, unhealthy matter may be expected to diffuse.
If, then, any disease is propagated by organic germs living on the organic matter of the atmosphere, or associated with it, it is not at all wonderful that the disease should lurk in corners of houses, in clothes, or other porous matters, simply because we can trace floating matter to its lodgment in such places.
It is remarkable how readily porous bodies absorb the moisture of the air, and substances with it. I find that the leather on the bookcases in my study, where gas has been used, is made rotten, and in exact proportion to the height, the highest being so frail that it can scarcely be handled, while the lowest is still pretty firm, although much less so than at first. The amount of sulphuric acid in the pieces is also in proportion. The intermediate are affected in an intermediate way. No better proof can be had of the absorptive action of these porous substances, and of the unequal state of the atmosphere in various parts of a room.
When rooms which have absorbed organic matter have been shut up, the original peculiar smell ceases, and a musty one takes its place; we recognize something which instantaneously brings that of mould to our minds. We cannot doubt that the air in such cases is full of the spores of such plants; the plants themselves grow in abundance, and we know well that when they grow they readily send out colonies. The leather of the bookcase was said to show the inorganic bodies; the books themselves are covered with the organisms when care is not taken, so that one small room gives an epitome of the whole subject. We have here, therefore, no mysterious agent, but one that is perfectly plain. Why should the agent be mysterious in the case of the infectious disease? It is only so far a mystery—we do not know the different plants or organisms, and so cannot tell whether we have health or disease in them by merely examining them through a microscope.
If porous substances have the characteristics alluded to, why use them? There are some difficulties here. If a wall is to be cleaned frequently, and rubbed when wet, it is better that it should not be porous. That seems quite clear; but when these processes cannot be undertaken, it really seems as if it were better to have it porous. Such substances absorb moisture in some seasons, and give it out slowly at others. Our clothes are of this kind. It is not possible to have warm coverings not porous. Porous bodies hold also a good deal of air, and they cause oxidation more readily. Nature has employed them for
