its. So does the growing lad
require more air than an adult of the same weight, for the reason that
he consumes more food than a person of mature years. Habit also exerts
a controlling influence. A man who works in the open air suffers more
when placed in a small, unventilated room, than one who is accustomed
to breathe the confined air of workshops.

-=-=-=-=-=-=-=-=-=-=-=-=

Observation 3d. 501. What questions may be asked respecting the
inspired air? Give the remark of Birnan. 502. How many cubic feet of
air are adequate for a man to breathe each minute? How much does Dr.
Reid allow? 503. Mention some reasons why different persons do not
require the same amount of air.

-=-=-=-=-=-=-=-=-=-=-=-=

504. _Air, in which lamps will not burn with brilliancy, is unfitted
for respiration._ In crowded rooms, which are not ventilated, the air
is vitiated, not only by the abstraction of oxygen and the deposition
of carbonic acid, but by the excretions from the skin and lungs of the
audience. The lamps, under such circumstances, emit but a feeble
light. Let the oxygen gas be more and more expended, and the lamps
will burn more and more feebly, until they are extinguished.

_Illustrations._ 1st. The effects of breathing the same air again and
again, are well illustrated by an incident that occurred in one of our
halls of learning. A large audience had assembled in an ill-ventilated
room, to listen to a lecture; soon the lamps burned so dimly that the
speaker and audience were nearly enveloped in darkness. The
oppression, dizziness, and faintness experienced by many of the
audience induced them to leave, and in a few minutes after, the lamps
were observed to rekindle, owing to the exchange of pure air on
opening the door.

-=-=-=-=-=-=-=-=-=-=-=-=

How is it with the laborer? With the gormandizer? With the person that
works in the open air? 504. What effect has impure air on a burning
lamp? Give the illustration of the effects of impure air on lighted
lamps.

-=-=-=-=-=-=-=-=-=-=-=-=

2d. In the "Black Hole of Calcutta," one hundred and forty-six
Englishmen were shut up in a room eighteen feet square, with only two
small windows on the same side to admit air. On opening this dungeon,
ten hours after their imprisonment, only twenty-three were alive. The
others had died from breathing impure air.

505. _Air that has become impure from the abstraction of oxygen, an
excess of carbonic acid, or the excretions from