owd
together in houses heated through the winter months by stoves, the germs
of tuberculosis take firm hold, and the deaths from this disease are
greater in proportion to population among this race than anywhere else.

_Effect of change in air._

This discussion illustrates another law of disease which makes the
necessity for ventilation particularly great among rural communities
where for nine months in the year outdoor life is freely enjoyed,
namely, that when either an individual or people are brought under
changed conditions, perhaps not unwholesome to those accustomed to them,
those unaccustomed will suffer severely. So a lack of ventilation during
the winter months in a farmhouse is very serious in its consequences to
those who have had the full enjoyment of fresh air through the rest of
the year.

Reference has already been made (in Chapter 1) to the prevalence of
influenza in rural communities, and it is quite probable that this would
be largely eliminated if the lungs were not deprived of their oxygen as
they are in most houses on the farm.

_Composition of air._

Ordinary air contains about 0.04 per cent of carbon dioxid; that is,
four parts in ten thousand parts of air, the other nine thousand nine
hundred ninety-six being made up of oxygen and nitrogen. Of course, it
is not possible to express any definite value for the amount of carbon
dioxid which is objectionable in air, because, in the first place, it is
not certain that the carbon dioxid in itself is the cause of diminished
vitality due to insufficient ventilation, and, in the next place,
insufficient ventilation affects different people in different ways. But
it is known that in the lungs the life-giving oxygen is changed to
carbon dioxid, and that just as carbon dioxid gas will prevent the
combustion of a candle flame, so carbon dioxid gas will destroy the life
of man.

When a deep well is to be cleaned out, the decomposition of organic
matter in the bottom of the well will have, in all probability, caused
the formation of this same carbon dioxid gas, and it is not uncommon for
a man descending into such a well to be overcome by the gas, which, in
some cases, even causes death. For this reason, it is common to lower
into a well, before it is entered by a man, a candle or lantern, on the
probability that if the lantern can stand it, certainly the man can,
while if the lantern goes out, it is wise to avoid the risk of having a
man's life put out