this latter substance, but it seems to be a necessity in the growth of
vegetation. There are other things in the air which, while they are
unnecessary for breathing purposes, it will be well for us to
understand, as some of them are things to be avoided rather than
inhaled.

As before mentioned, air contains moisture, which is a very variable
quantity. In a cold day in winter it is not more than one-thousandth
part, while in a warm day in summer it may equal one-fortieth of the
quantity of air in a given space. There is also a small amount of
ammonia, perhaps not over one-sixty-millionth. Oxygen also exists in the
air in very small quantities in another form called ozone. One way to
produce ozone is by passing an electric spark through air. Anyone who
has operated a Holtz machine has noticed a peculiar smell attending the
disruptive discharges, which is the odor of ozone. It is what chemists
call an allotropic form of oxygen, just as the diamond, graphite, and
charcoal are all different forms of carbon, and yet the chemical
differences are scarcely traceable. It is more stimulating to breathe
than oxygen and is probably produced by lightning discharges.

As has been before stated, the oxygen of the air is consumed by all
processes of combustion, and in this we include the breathing of men and
animals and the decay of vegetable matter, as well as the more active
combustion arising from fires. A grown person consumes something over
400 gallons of oxygen per day, and it is estimated that all the fires on
the earth consume in a century as much oxygen as is contained in the air
over an area of seventy miles square. All of these processes are
throwing into the air carbon dioxide (carbonic acid), which, however, is
offset by the power of vegetation to absorb it, where the carbon is
retained and forms a part of the woody fiber and pure oxygen is given
back into the air. By this process the normal conditions of the air are
maintained.

One decimeter (nearly 4 inches) square of green leaves will decompose in
one hour seven cubic centimeters of carbon dioxide, if the sun is
shining on them; in the shade the same area will absorb about three in
the same time.

There is another substance in the form of vegetable germs in the air
called bacteria. At one time these were supposed to be low forms of
animal life, but it is now determined that they are the lowest forms of
vegetable germs. Bacteria is the general or generic name for