part
of the room, while that which is colder and purer descends. The reason
for this may not be readily conceived, especially when we consider that
carbonic acid, the vitiating product of respiration, is specifically
heavier than common air. Three considerations, however, will make it
apparent. 1. Gases of different specific gravity mix uniformly, under
favorable circumstances. 2. The carbonic acid which is exhaled from the
lungs at about blood heat is hence rarefied, and specifically lighter
than the air in the room, which inclines it to ascend. 3. The ingress of
cold and heavier air from without is chiefly through apertures near the
base of the room. Raising the lower sash of the windows allows a portion
of the purer air of the room to pass off, while the more vitiated air
above is retained. Lowering the upper sash allows the impure air above
to escape, while the purer air below remains unchanged.

Lowering the upper sash is also the _safer method of ventilation_. It
not only allows the impure air more readily to escape, but provides also
for the more uniform diffusion of the pure air from without, which
takes its place through the upper part of the room. The renovated air
will gradually settle upon the heads of the scholars, giving them a
purer air to breathe, while the comfort of the body and lower
extremities will remain undisturbed. This is as it should be; for warm
feet and cool heads contribute alike to physical comfort and clearness
of mind. Raising the lower sash of the windows endangers the health of
scholars, exposing those who sit near them to colds, catarrhs, etc.
Indeed, when it is very cold or stormy, it is unsafe to ventilate by
lowering the upper sash of the windows. At such times, provision should
be made for the escape of impure air at the upper part of the room, and
for the introduction of pure air at the lower part, as will be shown
while treating upon the means of warming.

MEANS OF WARMING.--Next in importance to pure air in a school-room is
the maintenance of an even temperature. This is an indispensable
condition of health, comfort, and successful labor. It is one, however,
that is very generally disregarded; or, perhaps I should say, one that
is not often enjoyed. School-houses are generally warmed by means of
stoves, some of which are in a good condition, and supplied with dry,
seasoned wood. The instances, however, in which such facilities for
warming exist, are comparatively few. It is muc