th of each session; and frequently the school-house is imperfectly
ventilated between the sessions at noon, and sometimes for several days
together.

Mark the following particulars: 1. The quantity of air breathed by
forty-five persons in three hours, according to the data just given, is
3375 cubic feet. 2. _Air once respired will not sustain animal life._ 3.
The school-room was estimated to possess a capacity of 3360 cubic
feet--_fifteen feet less than is necessary to sustain healthy
respiration_. 4. Were forty-five persons whose lungs possess the
estimated capacity placed in an air-tight room of the preceding
dimensions, and could they breathe pure air till it was all once
respired, and then enter upon its second respiration, _they would all
die with the apoplexy before the expiration of a three hours' session_.

From the nature of the case, these conditions can not conveniently be
fulfilled. But numerous instances of fearful approximation exist. We
have no air-tight houses. But in our latitude, comfort requires that
rooms which are to be occupied by children in the winter season, be
made very close. The dimensions of rooms are, moreover, frequently
narrowed, that the _warm breath_ may lessen the amount of fuel necessary
to preserve a comfortable temperature. It is true, on the other hand,
that the quantity of air which children breathe is somewhat less than I
have estimated. But the derangement resulting from breathing impure air,
in their case, is greater than in the case of adults whose constitutions
are matured, and who are hence less susceptible of injury. It is also
true in many schools that the number occupying a room of the dimensions
supposed is considerably greater than I have estimated. Moreover, in
many instances, a great proportion of the larger scholars will respire
the estimated quantity of air.

Again, all the air in a room is not respired _once_ before a portion of
it is breathed the second, or even the _third_ and _fourth_ time. The
atmosphere is not suddenly changed from purity to impurity--from a
healthful to an infectious state. Were it so, the change, being more
perceptible, would be seen and _felt_ too, and a _remedy_ would be
sought and applied. But because the change is gradual, it is not the
less fearful in its consequences. In a room occupied by _forty-five
persons_, THE FIRST MINUTE, _thirty-two thousand four hundred cubic
inches of air impart their entire vitality to sustain animal life,