VENTILATION. 
173 
in our houses quite as good for breathing as the air 
outside; yet we can do much to keep it from 
becoming harmful to us. 
4. We can cause currents of air to pass through 
the room, that they may not only carry away or 
drive out the used-up and im- 
pure air, but may bring pure air 
to take its place. 
5. This is what is meant by 
ventilation. It is the means by 
which the air in and beneath 
our dwellings is made to move 
on. But it cannot do this 
unless there is a sufficient outlet 
for the foul air and an inlet for 
the fresh air. In warm countries 
the doors and windows serve 
i • , l i , • Structure of the Skin 
these purposes m the day-time, 
but there ought to be other openings for use when 
doors have to be closed. 
6. Of course we should take care what sort of air 
we let into the house. An exchange of one bad 
servant for another equally bad would leave us no 
better for the exchange. We ought to see that we 
get a supply of the driest and purest air that is to 
be had, and that it does not come in as a chilling 
draught. 
7 . It is chiefly at night, when our rooms are 
closed, that the air in them becomes foul. There- 
