About out-of-door airings we shall have occasion to speak more fully; but indoor airings are truly as important, because, if the tissues be nourished upon impure blood for all the hours the child spends in the house, the mischief will not be mended in the shorter intervals spent out of doors. Put two or three breathing bodies, as well as fire and gas, into a room, and it is incredible how soon the air becomes vitiated unless it be constantly renewed; that is, unless the room be well ventilated. We know what it is to come in and out of the fresh air and complain that a room feels stuffy; but sit in the room a few minutes, and you get accustomed to its stuffiness; the senses are no longer a safe guide.