peculiar unpleasant crowd odor with which all are familiar. Our sense of smell is subjected continuously to slight stimulation, but it is peculiarly and vividly responsive to unpleasant changes in our odorous environment. Thus on entering a crowded, close and stuffy room the odor often seems to us intolerable, and we at once assume that the air is very bad for any one who breathes it. We rush to the window and throw it open, or complain to the janitor, or retreat in disgust. Well, the air may indeed be very bad, but this is not because of its odor, except as to the odor's possible psychic effect. There is a peculiar relation between one's sense of smell and one's esthetic sense, and an unpleasant odor by rudely shocking the esthetic part of our nature may interfere with our efficiency; but there is no evidence in support of the idea that the odoriferous elements in crowd air are physically or chemically harmful to us. Our sense of smell, however it may disturb us, is probably the least valuable of all our senses in contributing to our physical welfare and it can the most readily be dispensed with—a too sensitive nose is really an affliction. This sense is in fact extremely subject to fatigue, and hence on confinement in crowd air our olfactory aversion to it soon ceases—a provision of nature which is not altogether an evil.
Strangely enough it is only within a period of scarcely more than thirty years that the occurrence and the significance of atmospheric dust have become accurately known. Dust has now been shown to exist in air everywhere: in uninhabited as well as inhabited regions, though the more where man and his works are; at the tops of lofty mountains; and over the largest of oceans. The numbers of dust particles found by different observers in a single cubic centimeter of air have varied from 157 at the summit of the Swiss Bieshorn to more than 200,000 in a Parisian garden. On dusty streets and within doors, especially in dusty trades, still more dust may exist, and it is estimated that a single puff of tobacco smoke discharges into the atmosphere 4,000,000,000 particles. Dust may consist of inorganic and lifeless organic matter, as well as bacteria and other living organisms. It may be carried long distances. The most striking known example of this is the fine pumice which was sent into the air to tremendous heights and in enormous quantities at the time of the extraordinary eruption of the East Indian volcano Krakatoa in 1883. This fine dust was carried completely around the earth and from the extreme north to the extreme south of the largest continents. Moreover, it continued to exist for several years as a component of the earth's atmosphere. To dust particles we owe the existence of clouds, fog and haze, the beautiful colors of the sunset, and in large part the blueness of the sky. Dust is thus our constant companion and with every breath we inhale much of it. Our bodies are prepared for this and possess defensive agencies for our protection. With these agencies in proper order the greater part of the ordinary inhaled
