Household Administration, Its Place in the Higher Education of Women

by Unknown

EFFECTS OF CHANGES OF TEMPERATURE ON AIR

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 84 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 85 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. 86 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.

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 88 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 89 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 90 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 91 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.

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, 92 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 93 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 94 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.