alations 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
_debris_ 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