26 
to .04 per cent. The maximum amount of oxygen is to be found in 
the air on the sea shore and mountain sides. 
OTHER CONSTITUENTS OF AIR. 
We have said that air conists chiefly of oxygen and nitrogen, 
but norma/ air always contains small and variable quantities of 
vapor of water, carbonic acid, organic matter, and ammonia 
and ozone. Air vitiated by breathing, as we shall see, contains 
some of these constituents in excess, while others of its con- 
stituents are diminished. In addition to the above it should be 
mentioned that in the vicinity of large smelting and chemical works, 
certain gases, e.g. Sulphuretted hydrogen, Hydrochloric acid, etc., may 
be present, and pollute and poison the air. Owing to the law of the 
diffusion of gases, and the prevalence of air currents, there is always 
present the tendency to preserve a constant composition of the atmos- 
phere, and thus it is that noxious gases cannot accumulate to a dan- 
gerous degree, save under extremely artificial circumstances. 
THE MOISTURE OF THE AIR. 
Moisture or vapor of water, always present in the atmosphere, is 
the result of the evaporation of water from the ocean, lakes and rivers, 
as well as from the soil and vegetation. Its amount is directly depen- 
dant on the temperature. Hot air can hold or absorb more moisture 
than cold air. When saturated air is cooled, moisture is deposited, of 
which the well known condensation on the outside of a glass of cold 
water in summer, is an illustration. Our breath is loaded with moisture, 
and hence the determination of the amount of moisture in the air of 
a room may sometimes serve as a guide to a correct diagnosis of its 
condition. Over the hygrometric state of the atmosphere, of course, 
we have no control, though to a certain extent, and especially in winter, 
we can regulate the amount of moisture in the air of our houses. 
EFFECT OF EXCESS OR DEFICIENCY OF MOISTURE. 
It might be well here to note that an excess or deficiency (above or 
below the normal amount) exerts a decided action upon the health. An 
excess of moisture is more prejudicial than a deficiency, since, in the 
first place, it tends to preserve the organic matter, which is one of the 
