te, the air of the room will be rendered unfit for respiration in
nine minutes.

508. _Railroad cars, cabins of steam and canal-boats, omnibuses, and
stage-coaches, require ample ventilation._ In the construction of
these public conveyances, too frequently, the only apparent design is,
to seat the greatest number of persons, regardless of the quantity and
character of the air to maintain health and even life. The character
of the air is only realized when, from the fresh, pure air, we enter a
crowded cabin of a boat or a closed coach; then the vitiated air from
animal excretions and noxious gases is offensive, and frequently
produces sickness.

509. The influence of habit is strikingly expressed by Birnan, in the
"Art of Warming and Ventilating Rooms:" "Not the least remarkable
example of the power of habit is its reconciling us to practices
which, but for its influence, would be considered noxious and
disgusting. We instinctively shun approach to the dirty, the squalid,
and the diseased, and use no garment that may have been worn by
another. We open sewers for matters that offend the sight or the
smell, and contaminate the air. We carefully remove impurities from
what we eat and drink, filter turbid water, and fastidiously avoid
drinking from a cup that may have been pressed to the lips of a
friend. On the other hand, we resort to places of assembly, and draw
into our mouths air loaded with effluvia from the lungs, skin, and
clothing of every individual in the promiscuous crowd--exhalations
offensive, to a certain extent, from the most healthy individuals; but
when arising from a living mass of skin and lungs, in all stages of
evaporation, disease, and putridity,--prevented by the walls and
ceiling from escaping--they are, when thus concentrated, in the
highest degree deleterious and loathsome."

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What suggestion when a school-room is not ventilated? 507. What is
said in regard to ventilating churches, concert halls, &c.? State the
illustration. 508. What remarks relative to public conveyances? 509.
State the influence of habit by Birnan.

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510. _The sleeping-room should be so ventilated that the air in the
morning will be as pure as when retiring to rest in the evening._
Ventilation of the room would prevent morning headaches, the want of
appetite, and languor--so common among the feeble. The impure air of
sleeping-rooms probably causes more deaths than int