_Elements of Chemistry familiarly explained and practically
illustrated_.
This is an excellent little work by Mr. Brande: it is not avowedly so,
although everyone familiar with his valuable Manual of Chemistry will
soon identify the authorship. The present is only the first Part of this
petite system, containing Attraction, Heat, Light, and Electricity.
It is, as the author intended it to be, "less learned and elaborate
than the usual systematic works, and at the same time more detailed,
connected, and explicit than the 'Conversations' or 'Catechisms.'"
It avoids "all prolixity of language and the use of less intelligible
terms;" and, to speak plainly, the illustrative applications throughout
the work are familiar as household words. Witness the following extract
from the effects of Heat:
_Ventilation--Heating Rooms_.
"In consequence of the _lightness_ of heated air, it always rises
to the upper parts of rooms and buildings, when it either escapes, or,
becoming cooled and _heavier_, again descends. If, in cold weather,
we sit under a skylight in a warm room, a current of cold air is felt
descending upon the head, whilst warmer currents, rising from our bodies
and coming into contact with the cold glass, impart to it their excess
of heat. Being thus contracted in bulk, and rendered specifically
heavier, they in their turn descend, and thus a perpetual motion is kept
up in the mass of air. This effect is attended with much inconvenience
to those who inhabit the room, and is in great measure prevented by the
use of double windows, which prevent the rapid cooling and production of
troublesome currents in the air of the apartment.
"We generally observe, when the door of a room is opened, that there are
two distinct currents in the aperture; which may be rendered evident
by holding in it the flame of a candle. At the upper part it is blown
outwards, but inwards at the lower part; in the middle, scarcely any
draught of air, one way or other, is perceptible.
"The art of ventilating rooms and buildings is in a great measure
dependent upon the currents which we are enabled to produce in air by
changes of temperature, and is a subject of considerable importance.
As the heated air and effluvia of crowded rooms pass upwards, it is
common to leave apertures in or near the ceiling for their escape. Were
it not, indeed, for such contrivances, the upper parts of theatres and
some other buildings would scarcely be endurable; but a mere aperture,
though it allows the foul air to escape, in consequence of its specific
lightness, is also apt to admit a counter-current of denser and cold
air, which pours down into the room, and produces great inconvenience.
This effect is prevented by heating, in any convenient way, the tube or
flue through which the foul air escapes. A constantly ascending current
is then established; and whenever cold air attempts to descend, the
heat of the flue rarefies and drives it upwards. Thus the different
ventilators may terminate in tubes connected with a chimney; or they
may unite into a common trunk, which may pass over a furnace purposely
for heating it.
"In some of our theatres, the gas chandelier is made a very effectual
ventilator. It is suspended under a large funnel, which terminates
in a cowl outside the roof; and the number of burners heat the air
considerably, and cause its very rapid and constant ascent through the
funnel, connected with which there may be other apertures in the ceiling
of the building. But in these and most other cases, we may observe that
the vents are not sufficiently capacious; and the foul air from the
house, and from the gas-burners themselves, not being able readily to
escape, diffuses itself over the upper part of the building, and renders
the galleries hot and suffocating--all which is very easily prevented by
the judicious adjustment of the size of the ventilating channels to the
quantity of air which it is requisite should freely pass through them.
"The small tin ventilators, consisting of a rotating wheel, which we
sometimes see in window-panes, are perfectly useless, though it is often
imagined, in consequence of their apparent activity, that they must be
very effectual; but the fact is, that a very trifling current of air
suffices to put them in motion, and the apertures for its escape are so
small as to produce no effectual change in the air of the apartment:
they are also as often in motion by the ingress as by the egress of air.
"From what has been said, it will be obvious that our common fires
and chimneys are most powerful ventilators, though their good services
in this respect are often overlooked. As soon as the fire is lighted,
a rapid ascending current of air is established in the chimney, and
consequently there must be a constant ingress of fresh air to supply
this demand, which generally enters the room through the crevices of the
doors and windows. When these are too tight, the chimney smokes or the
fire will not draw; and in such cases it is sometimes necessary to make
a concealed aperture in some convenient part of the room for the
requisite admission of air, or to submit to sitting with a window or
door partly open. Any imperfect action of the chimney, or descending
current, is announced by the escape of smoke into the room, and is
frequently caused by the flue being too large, or not sufficiently
perpendicular and regular in its construction. When there is no fire,
the chimneys also generally act as ventilators; and in summer there is
often a very powerful current up them, in consequence of the roof and
chimney-pots being heated by the sun, and thus accelerating the ascent
of the air. In a well-constructed house there should be sufficient
apertures for the admission of the requisite quantity of air into the
respective rooms, without having occasion to trust to its accidental
ingress through every crack and crevice that will allow it to pass.
These openings may either be concealed, or made ornamental, and by
proper management may be subservient to the admission of warm air
in winter."
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