if nature be so powerful as a sanitary agent, how is it that we still require to cope with that formidable enemy which we call foul air? It is because we ourselves are constantly vitiating the atmosphere around us whether we live, or work, or die, and because the impure products thus generated are not sufficiently provided against by efficient ventilation. Let us glance briefly at the principal sources from which these impurities rise. These may be grouped under three heads, viz.: (1) Respiration; (2) Putrefaction; (3) Trades and Manufactures. _Respiration._—The air which we draw into our lungs with every breath contains twenty-one per cent. of oxygen, but when we expire it again it only contains 13 parts. We have, in fact, abstracted 8 per cent. of oxygen and given back in its place a poisonous mixture of carbonic acid, organic matter, and watery vapor. We breathe out this poisonous mixture at the rate of one gallon each minute, but (even apart from the organic matter) it is so impure, owing to the amount of carbonic acid which it contains, that each gallon would require to be diluted with one hundred gallons of pure air before being again fitted for respiration. In such a city as London then, the air is being polluted even by the carbonic acid which we exhale at the rate of nearly six hundred million of gallons per minute, to such an extent as to render it unfit to be breathed again. Surely this should make us welcome every strong gale as an angel sent from heaven bearing healing on its wings. But it is in dwellings, and especially in the dwellings of the poor, that the polluting effects of respiration are greatest, for in these it is too often the case that man places every possible obstacle in the way of nature’s methods of ventilation. Moreover, in apartments that are crowded it is practically impossible to maintain the air in a state of purity, and thus they become hot-beds of disease. The very interesting experiments made by Dr. Angus Smith upon himself in an air-tight leaden chamber, led him to the conclusion that, in air containing an increased amount of carbonic acid, this gas alone, even without the other hurtful ingredients, such as organic matter, rapidly produces poisonous effects—indicated by feebleness of the circulation, extreme slowness of the heart’s action, and great rapidity of the breathing—and that when men are exposed to it they are really gasping for breath, without being aware of the cause.