although as we only drink a comparatively small quantity of water the whole of these few grains would not be swallowed in a single day. We have spoken of carbonic acid as an impurity in the atmosphere; and so it is, for it is unfitted to support animal life. An animal will die from suffocation in an atmosphere containing plenty of free oxygen if it contains over 10 or 12 per cent. of carbonic acid gas. But a minute increase of this gas in the atmosphere is of most importance from the fact that it always comes in bad company, and is found to be a measure of the many impurities which accompany it. Moreover, for every increase of carbonic acid there is a corresponding decrease in oxygen, so that we have in such cases a double effect, viz., a subtraction of the life-giving principle of the air, and the addition of noxious substitutes. These noxious substitutes consist for the most part of organic matter, either of animal or vegetable origin. The exact nature of the organic substances, which constitute the specific poisons of contagious diseases, still remains obscure. Whether they consist of inconceivably minute particles of decaying matter, or of living microscopic germs; whether in some instances they are conveyed by particles of skin and pus-cells from the diseased to the healthy, or are condensed with the watery vapor of the atmosphere and thus disseminated; all these are questions which have yet to be satisfactorily answered. It is, however, certain that almost invariably the atmosphere is made the vehicle of the contagion or deadly agent, whatever may be its nature; and hence the great importance of taking such precautions as will prevent the contamination of the air; or at all events, aid in dissipating or destroying its more hurtful impurities. An estimate of the amount of organic impurity in the air of our large cities may be formed by considering the enormous quantities of carbonic acid gas that are daily and hourly poured forth in these industrial centers. Dr. Angus Smith, whose investigations regarding “Air and Rain” have won for him a world-wide celebrity, has found that in the city of Manchester 15,066 tons of carbonic acid are daily passed into the air that envelops it; and Dr. de Chanmont states that 822,000,000 cubic feet of this gas are generated in London per day, or more than 9,500 cubic feet per second. Fortunately the operations of nature are in themselves calculated to restore a state of equilibrium in the constitution of the air. Injurious gases become diffused, diluted, or decomposed; animal emanations are absorbed in the processes of vegetation; suspended matters are washed down by rain, or fall by their own weight, while many organic substances are so acted on by oxygen as to render them innocuous. Thus the vast aerial sea maintains a uniformity of composition, owing to the mighty forces of nature, without which all our sanitary measures would be futile. But