women are accustomed to spend the greater portion of the day in the
house, and on that account special attention must be given to keeping
the air of the house pure. Ventilation takes care of itself in
summer, when the windows are open, but in cold weather, when in our
anxiety to keep the temperature comfortable we may overlook the need
of fresh air, it demands close attention. The necessity of
ventilation at all times is due, of course, to the composition of the
atmosphere and to the changes produced in it as we breathe.

The air about us is a mixture of gases, of which oxygen and nitrogen
are the most important. Although nitrogen, which constitutes four-
fifths of the atmosphere, is taken into our lungs in breathing, we
make no use of it, but breathe it out in precisely the same condition
as we take it in. As chemically combined in the food-stuff known as
protein, nitrogen is indispensable to animal life; but our bodies
make no use of the gaseous form of nitrogen. Oxygen, on the other
hand, supports life; and though it forms less than one-fifth of the
atmospheric air, it is present in ample amount for our needs. After
we draw air into our lungs, the oxygen it contains is absorbed by the
blood and used by the tissues. In return our tissues give up a waste
product, carbonic acid gas, which is thrown off by the lungs. It is
interesting to observe that the carbonic acid gas which animals
exhale supports the life of plants, and that the plants, under the
influence of sunlight, give back pure oxygen to the atmosphere.
Obviously, the complementary relation exhibited here is of mutual
benefit.

The average person uses about four bushels of air a minute.
Consequently, rooms that are occupied must be constantly replenished
with fresh air; otherwise the point is quickly reached where the