the undefined impurity exists in large quantities, only rubbing can remove it rapidly, and this is done when we clean thoroughly walls and furniture. If very bad, and time presses, while elegance is not a demand, we cover the whole, and find whitewashing to be a ready mode.
The other reasons for ventilating have arisen from scientific inquiries. We wish to remove the carbonic acid; we might be able to find this gas by the senses, if it were very abundant; but it, is not so as a rule, until after our senses have informed us of the organic accompaniments. It furnishes, however, one of the most important and probably the most important of the reasons in every-day life, because carbonic acid is the most rapidly hurtful to animal life of all the emanations from the person. It lowers the vitality rapidly, and kills with indefinite warning. The best warning is the organic matter, which acts for both. The warning by carbonic acid is called indefinite, because people lose vitality, but do not observe that the cause is in the atmosphere, there being no smell connected with that gas. At night, when lights are burning, the carbonic acid warns better than the other impurities, by simply putting out the candles. This seldom occurs in private houses, but dim burning is common enough. Mr. James Napier, F. C. S., tells me that he has seen the candles beginning to go out in a small meeting-room in the country, which had a low ceiling and was crowded. It was needful to keep the door open. How blunt, then, is our perception of carbonic acid!
Another reason for ventilating is to remove solid floating bodies, including infectious matter, fungi, and peculiar emanations arising from disease or other anomalous conditions. These floating bodies can sometimes be distinguished by the sense of smell—in the case of mould, for example. If it were possible to describe a smell completely, this class would be largely subdivided, since experience has made many persons very learned on this subject. It is, however, a knowledge which we have not yet been able to receive from or communicate to others. This whole question requires careful examination. The knowledge of the smell cannot be taught, but it may guide us to much that can be taught. Ventilation for this class of bodies will be probably much more attended to in future; care must be taken to drive them to the nearest opening, and not to allow them to diffuse themselves through the room in which they may be produced. In some cases resort must probably be had to rapid artificial and heated currents.
We ventilate to remove smoke and ordinary dust; these are easily seen; and we use ventilation to procure dryness. Moisture rises constantly from the skin, and, if an inhabited apartment is not ventilated, that moisture accumulates. By opening the windows we cause floors to dry sooner, and we remove moisture from all the apartments, and that moisture has generally organic matter with it. If we ventilate with very moist air, we shall dry nothing; if we have very dry air, we may dry too thoroughly. It is, however, better for us to have what is
