This can only be done successfully by turning the people out of doors, at least every two or three hours if grown-ups, and every hour or so if children.

That is what school recesses are for, and they might well be longer and more frequent.
[Illustration: VENTILATING THE PUPILS, AS WELL AS THE CLASSROOM] The first and chief thing necessary for the good ventilation of houses and schools is plenty of windows, which are also needed to give proper light for working purposes, and to let in the only ever-victorious enemy of germs and disease--sunlight.
Secondly, and not less important, the windows should fit properly, and be perfectly hung and balanced, so that the sash will come down at a finger's touch, stay exactly where it is put, and go up again like a feather, instead of having to be pried loose, wrested open, held in place with a stick, and shoved up, or down again, only with a struggle.
[Illustration: A WELL-AIRED CLASSROOM The windows to the left of the pupils cannot, of course, be shown in the picture, but it can be seen that the lighting of the room is chiefly from that side.

Notice that the windows are both down from the top and up from the bottom.] There should be, if possible, windows on two sides of every room, or, if not, a large transom opening into a hall which has plenty of windows in it.

With this equipment and a good supply of heat, any room can be properly ventilated and kept so.

But it _will not ventilate itself_.
Ventilation, like the colors of the great painter Turner, must be "mixed with brains"; and those brains must be in the room itself, not down in the basement.