Let us apply this to air. Air when warmed expands, and therefore warm
air is, bulk for bulk, lighter than cold air. Warm air behaves in the
presence of cold air as the balloon: it is displaced and lifted by the
cold air, the result being an ascending stream of warm air, which is
called a convection current.
The movement of ascending smoke is essentially the same as that of the
warmed air. Smoke is warm air made visible by the particles of soot with
which it is laden. The particles of soot would fall to the ground except
that they are carried upwards in the stream of warm air. Dr. W. N. Shaw
has called attention to the importance of these phenomena in his book on
"Air Currents and the Laws of Ventilation," in the Cambridge Series of
Physical Text-books. He there says: "The dominant physical law in the
ventilated space is the law of convection. It is at once the condition
of success and the cause of most failures. Without convection,
ventilation would be impossible; in consequence of convection, nearly
all schemes of ventilation fail.
"The law of convection is the law according to which warmed air rises
and cooled air sinks in the surrounding air. Its applications are truly
ubiquitous. Every surface, _e.g._ a warm wall, or a person warmer than
the air in the immediate neighbourhood, causes an upward current; every
surface colder than the air in contact with it causes a downward
current.
"Ventilation would be much easier if warmed air or cooled air could be
carried along at any height required; but the law of convection is
inexorable: warmed air naturally finds the ceiling, cooled air the
floor."
It is true that the ventilation of a house is generally considered to be
the business of the builder and architect, yet there are many unpleasant
phenomena that come under the observation of the housewife which are
due to this law of convection, and it will be useful to consider a few
of them.
Let us take first the universal annoyance to housewives caused by the
sight of _dirt on the ceiling_. That all air is full of dust is seen
when a stream of sunlight crosses a room; the particles of dust are then
clearly perceived moving rapidly in all directions in the air. These
dust particles, when air is at rest, constantly fall to the ground under
the action of gravity, and are deposited on shelves and ledges, from
which they have to be removed daily by the housemaid. When air is warmed
and ascends it carries the dust particles with it, and these particles
striking against any cold surface with which they come into contact
stick to it. This is the cause of the necessity for the periodical
sweeping of chimneys. The walls of the chimney are colder than the smoke
that comes into contact with them, and the particles of soot in the
smoke striking against them are deposited on them. In the house the
effect of the bombardment of surfaces by dust-laden streams of air is
seen most conspicuously over burning gas-lights. Burning gas does not
itself produce all the dirt which is found on the ceiling above it, but
it causes upward streams of hot air, which carry up the dust and deposit
it on the ceiling. The practice of suspending a shade over the gas-light
does not lessen the amount of dust and smoke in the air, but the shade
serves to spread out the air over a larger surface, and thus to render
the dirt on the ceiling less apparent. That the shade itself remains
clean is due to the fact that it gets hot. A heated surface promotes the
activity of the motion of the air-particles in its neighbourhood, and by
this local activity the dust is repelled, so that a surface remains
clean or becomes coated according as it is more or less hot than the
invading current. The validity of this explanation may be tested by
holding a cold spoon over a lighted candle when it will be seen that the
spoon becomes blackened; if a hot spoon be substituted for the cold one
it will remain clean.
In order that the hot, vitiated air of a room may escape easily, it has
been in many cases the custom to place an exit opening for it in the
chimney over the room fireplace. The wall in the neighbourhood of this
ventilator invariably becomes black; but as this wall is warm it is not
probable that dust is deposited on it by the outgoing air, the
explanation given by the housewife that the smoke from the chimney gets
through the ventilator into the room is probably correct, though these
ventilators are supplied with mica flaps which should swing open when
air from the room strikes against them, and close when the air from the
chimney does so.
When a house is heated by hot-water pipes and radiators, the walls over
these pipes are another source of trouble (Fig. 1). A good deal of
scientific ingenuity is required if the walls are to be kept clean.
[Illustration: FIG. 1.]
That some ceilings appear striped with broad light and dark lines is
due to inequalities in the temperature of the ceiling. The light stripes
are under the joists, which prevent to some extent the escape of heat
from the ceiling, and the dark correspond to the unprotected parts of
the ceiling. The dust rising from the room is slightly repelled by the
currents from the warmer parts of the ceiling, and sticks more readily
to the colder parts.
Let us take for our second example the apparently trivial matter of
_smells in the house_. Smells may be of various kinds from various
causes. The best judge of the kind, and therefore of the cause, is the
nose. Suppose the smell to be the common one in houses of all
classes--the smell of cookery! The smell of cookery in the house is
generally a winter phenomenon. The air in an inhabited house is always
in a state of motion, induced by the inequalities of temperature caused
by the inhabitants themselves, and to a greater extent by the fires, of
which there will certainly be one in the kitchen. We must remember that
cold air will get into the house through all available openings, to take
the place of the air which supplies the fires. The most obvious
available openings in an ordinary dwelling-house are the casual ones of
the open chimneys of unused grates, and the loosely fitting doors and
windows. In cold weather fires are lighted in the sitting-room grates;
these fires when lighted should warm the air in the chimneys above them
and cause an upward draught in the chimney. Sometimes however the
chimney will be found to be occupied by a current of air coming down to
feed fires in other rooms, and so long as this goes on the smoke from
the newly lighted fire comes into the room. The down-draught can be
stopped by opening a window to supply sufficient cold air to counteract
it, otherwise we have to adopt special devices to make the smoke go up
the chimney in the first instance. Sometimes a newspaper is burnt in the
grate to give the necessary amount of warm air, but this is a dangerous
practice by which the chimney may be set on fire. Sometimes air is
supplied by the bellows. A newspaper is often held in front of the grate
so as to close the opening above the fire and cause the cold air to pass
through the fire, thus promoting combustion and the supply of hot air in
the chimney. In any case, the warm air of the fire is carried up the
chimney by the cold air of the room, and this cold air is drawn from the
casual openings already referred to. It has been demonstrated by
laboratory experiments that the amount of draught in any chimney depends
on the height of the chimney and the fire in its grate.
Smells are conveyed about a house by the flow of air to feed the fires,
and they nearly always find their way from all parts of the house to the
ground-floor sitting-rooms when the doors are left open and the fires
are burning. On their way they pass through passages and are therefore
nearly ubiquitous. The air of any room in the house is in communication
with that of every other room, and it is only by the nature of the smell
that we can tell its probable source. There are people who like when
they open the bedroom door in the morning to know that coffee and bacon
await them downstairs, or on coming into the house from a cold winter's
walk to meet a "delicious smell of Irish stew." To other people all
smell of cookery is abhorrent, and they feel a sense of irritation that
their guests should on entering the house be regaled with the odour of
the preparation of food. To many mistresses the only remedy that
suggests itself is a message to the cook, who is powerless in the matter
and returns an answer that she is sorry, but that she doesn't know why
there should be a smell of cooking upstairs as there is none in the
kitchen. A visit to the kitchen will generally confirm the cook's
statement as to that particular spot, but a considerable smell will be
encountered on the kitchen stairs. We may inquire into the cause of
this. The usual equipment of the kitchen includes a closed range,
supplemented in many cases by a gas stove. The kitchen fire draws a
plentiful supply of air from casual openings, and this air for the most
part passes with the smoke up such flues as are open. The oven is
provided with a ventilator, which carries off the odour of baked or
roasted meats. The odour in the hot air over the closed range has no
escape except into the kitchen--the cook says that ever so slight an
opening in the top of the range will prevent the oven from heating. This
odour-laden air therefore comes directly into the kitchen, and being hot
is directed to the ceiling, thus escaping the cook who is in the draught
of the fresh air supply. Travelling along the ceiling the hot air passes
through the opening at the top of the door and mingles with the fresh
air on its way upstairs. The same thing happens when the gas stove is
in use. The only remedy is to provide some exit for the hot air of the
kitchen which will be more easily accessible than that by way of the
door, for the hot air will travel by the easiest path. A considerable
knowledge of science is required to achieve this object.
[Illustration: FIG. 2.]
Closely allied with the smell of cookery is _the smell of the gas
stove_. Many persons consider that the use of a gas stove either in the
kitchen or in a bedroom is inseparable from the peculiar odour of
partially consumed gas. It may therefore be useful to consider how the
gas supplied to stoves and incandescent lights differs from that of an
open gas fire or that of an ordinary burner. Gas stoves and incandescent
lights get their supply of gas through what are known as Bunsen burners,
so called after the German chemist whose invention they are. In an
ordinary burner the gas mixes with atmospheric air at the opening at
which it burns; the supply of air obtained in this way is insufficient
for complete combustion until the outer layers are reached; the interior
part of the flame is bright and smoky. In the Bunsen burner the gas
issues from the main through a nozzle which opens inside a bulb. The
bulb is perforated to allow of the ingress of atmospheric air; the gas
and air mix in the tube which is a prolongation of the bulb, and the
mixture is lighted at the top of the tube. Fig. 2 shows a representation
of the Bunsen burner as applied to a gas stove. In this the gas escapes
from the main at the nozzle _n_, into a bulb of which the tube A is a
prolongation, air is admitted to the bulb at the openings _a a_, and
the mixed gas and air is burnt at the openings in the tube A. The amount
of air supplied is regulated by the size of the openings _a a_ and the
holes where the gas is lighted. The gas thus supplied with air is
completely consumed where combustion begins, and a clear, blue,
non-luminous flame is the result. If the holes through which the mixture
of gas and air issues are partially closed by rust or by accretions from
the "boiling over" of saucepans it is evident that, the gas supply being
unchanged, less air can be drawn through them; consequently the gas will
not be entirely consumed, and acetylene (C2H2, one of the products of
partially consumed coal gas) will pass into the atmosphere and will give
rise to the peculiar odour associated with gas stoves. This product of
partially consumed gas is very poisonous, and all gas stoves should be
furnished with chimneys to carry off the fumes to the open air. The
phenomenon known as "burning back," that is, the ignition of the gas at
the nozzle in the bulb, is caused by the pressure of gas being too small
for the supply of air. The gas should at once be turned out and
relighted till it burns at the proper places. The simple remedy for
smell from a gas stove is the cleansing of its burners, unless indeed
the kettle is too close to the holes from which the gas issues for
complete combustion to be possible.
There is another winter phenomenon which is very disagreeable--the
presence of _fog in the house_; and the perplexed housewife asks, Where
does the fog get in when all outside doors and windows are closed? We
have already pointed out that the sitting-room fires must have air, and
that that air will be drawn from casual openings. Among these openings
are the chimneys of fireless grates; the greater part of the fog in the
house comes down these chimneys. On a foggy day it is wise to close the
chimneys of fireless grates and provide some other opening for the
supply of air; but all air from the outside is full of fog. The problem
of how to let in air and keep out fog suggests the question, What is
fog? Fog consists of material particles (dust or smoke) on which vapour
has condensed; if these particles can be removed the air will be clear.
The problem for the housewife is how to free a sufficient quantity of
air from these particles.
_A smell of gas_ in any part of the house may be very dangerous if no
one on the premises has any scientific knowledge, for it may be premised
that the escape of gas is not where the smell is first perceived. Gas
being lighter than air is carried upwards, and the smell is at first
above the place of escape; it may even be in a room over where the gas
is escaping. The only safe detector of the source of mischief is the
nose; the mixture of coal gas and atmospheric air is explosive, and no
light must be struck. The upper sash of the window should be pulled down
to allow the gas to escape, and if the accident is at night time must be
allowed before searching for the source of escape further than can be
done by feeling the taps in the dark or following the scent by the
nose.
Further illustration of the effect of convection currents in the air of
a dwelling-house are needless, but the student may profitably spend time
and thought in considering how fresh air may be introduced into a room
without causing cold air to lie on the floor or hot, vitiated air to
cling to the ceiling. It is the old problem (with a difference) of
teaching a grandmother to suck an egg. He may also interest himself in
seeking answers to the questions (1) What action is expected to take
place when a poker is placed against the bars of a grate to make the
fire draw? and (2) Does the sun put the fire out, and if so how? In
connection with the expansion of air with heat he may consider the
popular fallacy that an inverted empty pot in a pie keeps in the juice.