VENTILATION, AND THE REASONS FOR IT. 361 



is not removed by mere drying, and the effects are very long in leav- 

 ing. 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 book- 

 cases 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 infec- 

 tious disease ? It is only so far a mystery we do not know the differ- 

 ent 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 



