217. How the Atmosphere is Made Impure. The air around us is constantly being made impure in a great variety of ways. The combustion of fuel, the respiration of men and animals, the exhalations from their bodies, the noxious gases and effluvia of the various industries, together with the changes of fermentation and decomposition to which all organized matter is liable,—all tend to pollute the atmosphere. The necessity of external ventilation has been foreseen for us. The forces of nature,—the winds, sunlight, rain, and growing vegetation,—all of great power and universal distribution and application, restore the balance, and purify the air. As to the principal gases, the air of the city does not differ materially from that of rural sections. There is, however, a vastly greater quantity of dust and smoke in the air of towns. The breathing of this dust, to a greater or less extent laden with bacteria, fungi, and the germs of disease, is an ever-present and most potent menace to public and personal health. It is one of the main causes of the excess of mortality in towns and cities over that of country districts. This is best shown in the overcrowded streets and houses of great cities, which are deprived of the purifying influence of sun and air. The fatal effect of living in vitiated air is especially marked in the mortality among infants and children living in the squalid and overcrowded sections of our great cities. The salutary effect of sunshine is shown by the fact that mortality is usually greater on the shady side of the street. 218. How the Air is Made Impure by Breathing. It is not the carbon dioxid alone that causes injurious results to health, it is more especially the organic matter thrown off in the expired air. The carbon dioxid which accompanies the organic matter is only the index. In testing the purity of air it is not difficult to ascertain the amount of carbon dioxid present, but it is no easy problem to measure the amount of organic matter. Hence it is the former that is looked for in factories, churches, schoolrooms, and when it is found to exceed .07 per cent it is known that there is a hurtful amount of organic matter present. The air as expelled from the lungs contains, not only a certain amount of organic matter in the form of vapor, but minute solid particles of _débris_ and bacterial micro-organisms (Chap. XIV). The air thus already vitiated, after it leaves the mouth, putrefies very rapidly. It is at once absorbed by clothing, curtains, carpets, porous walls, and by many other objects. It is difficult to dislodge these enemies of health even by free ventilation. The close and disagreeable odor of a filthy or overcrowded room is due to these organic exhalations from the lungs, the skin, and the unclean clothing of the occupants.