Household Administration Part 4

PHYSICAL SCIENCE IN THE HOUSEHOLD

We may for this purpose examine some of the laws of common physical and chemical phenomena, neglect of which has resulted in much needless discomfort in daily life, and even more serious consequences. For instance, the laws of expansion of gases and liquids with heat, and their subsequent behaviour, are phenomena that are often imperfectly realised. There is probably no person who is unacquainted with the law of gravitation, but there are many persons who accept literally the statements that smoke rises and that balloons ascend. A clear understanding of what actually takes place when gases and material ma.s.ses appear to move in opposition to the law of gravitation is essential to any scheme for warming and ventilating the house.

A very simple experiment will serve to reconcile the apparent contradiction of the universal law by the observed fact. Suppose we have two fluids, oil and water, of which oil is, bulk for bulk, lighter than water. If the oil be poured into a gla.s.s beaker, it will be seen to rest at the bottom of the beaker; if water be now poured into the same beaker the water will go to the bottom of the beaker and will displace the oil and lift it up so that the oil will float on the water; the oil may be lifted to any height we please if sufficient water be poured in to lift it to that height. If a single drop of oil be introduced into the water by means of a pipette and be liberated at the bottom of the beaker the water will close in under it, and lift it up to the surface.

In both cases the oil "rises" through the water. Oil, however, has no tendency to "rise" by itself, and in this case it lay motionless until it was lifted by the heavier fluid. We may use colloquial language when describing phenomena if we bear in mind what is really taking place.

A balloon "rising" through the air is exactly a.n.a.logous to the drop of oil in the water. The balloon is, bulk for bulk, lighter than air; the air therefore closes in under it and lifts it just as the water lifted the bubble of oil.

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 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 s.p.a.ce 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 subst.i.tuted 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.

[Ill.u.s.tration: 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 cla.s.ses--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 pa.s.s 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 pa.s.s through pa.s.sages 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 pa.s.ses 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 pa.s.ses 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.

[Ill.u.s.tration: 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 pa.s.s into the atmosphere and will give rise to the peculiar odour a.s.sociated 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 quant.i.ty 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 ill.u.s.tration 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.

EFFECT OF CHANGES OF TEMPERATURE ON WATER

Accidents have occurred in houses owing to ignorance of the full effects of heating or cooling water from its ordinary temperature. Water at any ordinary temperature expands when subjected to the action of heat; it contracts on cooling till it reaches a temperature seven degrees above the freezing point; from this temperature it expands until it becomes a solid ma.s.s of ice. At still lower temperatures ice contracts.

Let us consider first the effect of heating water. If water at the ordinary temperature be poured into a vessel which is placed on a fire or other source of heat the water at the bottom of the vessel will be warmed and will expand; it will therefore be lighter, bulk for bulk, than the water nearer the top of the vessel. The cold water will therefore descend, and the warm water will rise. All ordinary water contains air; presently the air in the water will become visible as small bubbles which rise to the surface of the water and escape noiselessly into the atmosphere. As more heat is applied some of the water in the bottom of the vessel will be formed into steam, and bubbles of steam will expand and rise into the cooler water above and collapse there with a rattling noise which is characteristic of the state known as simmering. These bubbles of steam rising and bursting aid the convection currents in stirring and mixing the water so that it presently becomes of even temperature throughout. When this occurs the bubbles of steam rise to the surface and burst explosively into the atmosphere, throwing the water violently about; the water is then boiling. It is an important point to remember in cookery that boiling water will not become any hotter with the application of more heat, but it will "boil away;" that is, it will be completely converted into steam. The steam resulting from any volume of water occupies a s.p.a.ce 1700 times that of the water from which it is produced, but what concerns the housewife most seriously is that the change of water into steam is accompanied with the evolution of tremendous mechanical force that will burst any vessel in which the water is enclosed. It is the fact of this tremendous exercise of mechanical force that has led to serious accidents when hot-water bottles have been put into the oven to keep warm. It has been a.s.sumed by some people that if the hot-water bottle be not completely filled, that if what they consider to be sufficient room is left for the expansion of the water, no harm can result from putting the bottle into the oven, but no arrangement can make such a course safe.

The bursting of the kitchen boiler is an accident resulting from disregard of the phenomena of heated water. It sometimes happens that the hot-water supply of the various taps in the house fails. If the boiler supplying the water is a hand-fed one some one whose duty it was to fill it has neglected that duty. An empty boiler with a removable lid will do no harm, but it is not advisable to leave it empty, as the heat of the fire will destroy the iron of which it is made. No attempt, however, should be made to fill the boiler while it is hot, as the result of pouring cold water into it will be the sudden and violent conversion of the water into steam, and the person pouring in the water will a.s.suredly be scalded. If the boiler be one that is filled automatically, one of two things has probably occurred: either the pipes are blocked by fur--that is to say by sediment from the boiled water--or the supply-pipe is frozen. In neither case is it safe to light the fire.

If the pipes are blocked by fur steam will be formed in the boiler and it will burst; if the supply-pipe is frozen the heat may thaw the ice, and the inrush of cold water will at any rate crack the boiler.

[Ill.u.s.tration: FIG. 3.]

When water expands with heating convection currents are formed in it, and the hot water rises to any height we please if cold water be available to take its place. This law of convection is applied to maintain a circulation of hot water in pipes used for warming a house.

The general arrangement of such a system is shown in Fig. 3. The furnace heats a boiler in the bas.e.m.e.nt or on the lowest storey of the house; HB and HL' are parallel vertical pipes connected with a horizontal pipe H'H at the top of the house; C is a small cold-water cistern which is furnished with a ball-tap to maintain the supply of cold water to the pipe H'L if any water is drawn off at any part of the circuit. The short pipe A acts as a valve for the escape of air from the pipes. The pipes H'L, H'H, and HB are filled with water. When the fire is lighted in the furnace, hot water is driven up the pipe HB by cold water descending through H'L, and this circulation goes on so long as a difference of temperature is maintained in the pipes; that is, so long as the fire is burning. Any number of coils of pipes may be introduced into the circuit between the boiler and the top of the pipe HB. In filling the pipes with water allowance is made in these coils for the expansion of the water with heat and for the air which we have seen escapes from heated water, and a tap is fixed in each coil for letting out any air that may have lodged in it. If free air remains in the pipes the circulation of the water will be hindered and the boiler may become dangerously overheated. It is therefore necessary when the heating apparatus is in use to examine these taps and see that water and not air escapes from them.

The installation of a heating apparatus in middle-cla.s.s houses is fairly common, and where one is not found many persons use gas or oil stoves in the pa.s.sages in the winter, for it is now realised that it is not possible to heat rooms by means of open fires without creating cold draughts in them from the cold pa.s.sages into which they open. And, moreover, the constant change of temperature encountered in pa.s.sing from one warm room to another through cold pa.s.sages is not only disagreeable, but is not found to be conducive to health.

Let us turn to the cooling of water. Water expands about one-eleventh of its volume on becoming ice. This change of state, like that of change into steam, is accompanied by the evolution of tremendous mechanical force. If water freezes in pipes it bursts the pipes, and on a thaw taking place the pipes are found to leak. The appropriate remedy for this state of things is to protect the pipes from cold or to empty them when a frost is apprehended. In all properly built houses there is a tap by means of which the water supply can be cut off from the house, thus allowing the pipes to be emptied on a frosty night. The custom of leaving the taps dripping is effective, because the pipe is generally liable to freeze at some particular point where it is in immediate contact with the cold air, probably in the unclosed c.h.i.n.k where the pipe pa.s.ses through the wall; keeping the water moving in the pipe prevents any part of it getting cold enough to freeze, but the practice should not be resorted to, as it wastes water.

RADIANT HEAT

[Ill.u.s.tration: FIG. 4.--Section of a Convex lens.]

It is pleasant on a dry, still day in winter, when the ground is covered with crisp snow or glistens with hard frost, to feel the warmth of the sun's rays, and it is becoming quite a fashion for people of leisure to spend the winter months at the pleasure resorts amid the snow-laden mountains of Switzerland. It is a matter of some interest to inquire how it happens that the sun's rays are warm when the thermometer tells us that the temperature of the air is below freezing-point. There is an old and pretty experiment in which a burning gla.s.s is made of ice; it is not a difficult thing to do. If the scale-pan of an ordinary balance be made hot and be pressed against a slice of ice (the concave side of the scale-pan towards the ice), first on one side of the slice and then on the other, the ice can be formed into a convex lens (Fig. 4). If now this lens be placed in the path of a sunbeam and the light be brought to a focus, that is, to a bright spot on a piece of paper, the paper will be heated and will take fire while the lens through which the heat pa.s.ses remains ice. From this we may surmise that the heat of the sun does not affect the medium through which it pa.s.ses.

Clerk Maxwell suggested yet another experiment in ill.u.s.tration of this law. By means of an ice lens he collected the sunlight to a focus in the middle of a basin of clear water, and observed that no effect was discernible in the water. He then directed the focus (the spot of light) on to a mote in the water. The mote became hot, the water was agitated, convection currents were formed, and the mote was carried up in them.

This showed that rays of light from the sun do not affect the substances through which they can pa.s.s, and that they heat bodies through which they do not pa.s.s. It has been demonstrated by laboratory experiments that all hot bodies emit rays of heat, whether we see the rays or not.

When we see the rays the bodies are said to be red or white-hot. The process by which heat pa.s.ses from one body to another without warming the intervening medium is called radiation. Radiation takes place only through transparent bodies. Rays of heat, like rays of light, pa.s.s through transparent bodies; whereas they are absorbed by, that is they make hot, opaque bodies. Heat rays travel in straight lines and are reflected from polished surfaces; their intensity varies inversely as the square of the distance of the object on which they fall from their source. The heat of an ordinary fire is radiant heat; when we sit round the fire we act as opaque bodies and absorb the heat, and are what we call scorched if the fire is very bright. If we move away from the fire, still letting the same firelight shine on us, we are not scorched; this is because the heating power of the rays varies inversely as the distance from their source, therefore if we move away double the distance we receive one quarter of the heat that we received before we moved. If we draw our chairs to one side we are not scorched, because the rays of heat do not travel round a corner.

CONDUCTION OF HEAT

We have seen that the ice-lens was not affected by the pa.s.sage of heat through it. If we now take hold of the lens we shall experience a feeling of cold, and the lens will begin to melt. Heat has pa.s.sed from our hand into the ice. The process by which heat pa.s.ses from one body to another in contact with it is called conduction. The fundamental law of conduction is, that heat always pa.s.ses from a warm body to a cold one.

Clerk Maxwell ill.u.s.trated this law in a series of very simple experiments. He placed a silver teaspoon in a cup of hot tea, and noted that the handle became warm gradually from the hot tea; the heat pa.s.sed from the bowl of the spoon in the tea to successive parts of the handle until the whole spoon was hot. His second experiment was to put two cold spoons, one of silver and one of German silver, into the tea, when he found that the same phenomenon took place, but that the silver spoon became hot much more quickly than did the German silver one. He then put three spoons into the tea, made respectively of silver, of German silver, and of bone. In the result, he found that when the other two were hot, the bone spoon hardly showed any sign of heat at the end of its handle.

The conclusion to be drawn from these experiments is that heat pa.s.ses at different rates through different substances. Substances through which heat pa.s.ses quickly are called good conductors of heat. The law of the conductivity of heat is that in a h.o.m.ogeneous body the flow is continuous, and is from the region of high temperature to the region of low temperature, and that it continues until the body is of uniform temperature throughout. The law is the same for bodies of different materials when in contact one with another.

The conduction of heat is in operation in every department of domestic life. People live in houses and are clothed to protect them from the vicissitudes of the weather, including the cold of winter and the heat of summer; use is made of the phenomenon in warming the house and in the preparation of food.

In selecting materials for various purposes, account has to be taken of their conductivities, for in some cases it is desirable that the transfer of heat should take place slowly, and in others that it should take place quickly. It might be thought that the conductivity of a substance could be estimated by touch, but a little reflection will show that this cannot be the case. The flow of heat between two bodies depends upon the difference of temperature between them, and if there should be no difference of temperature between them at the moment of touch there will be no flow of heat, though both are bodies of greater or less conductivity. Let us take, for example of the uncertainty of estimation by touch, a well-known experiment. Suppose we have a basin of hot water and a basin of cold water, and place a hand in each for a few moments; suppose we withdraw the hands and plunge them into a basin of tepid water, we shall find that the tepid water feels cold to the hand that was in the hot water and warm to the hand that was in the cold water.

Luckily, it has been found possible in the laboratory to refer substances to a common standard and to a.s.sign numerical values to them in order of their conductivities, so that substances can be compared and a selection made for any desired purpose. Pure silver has the highest conductivity; other useful materials take the following order: copper, zinc, lead, iron, steel, marble, gla.s.s, brick, slate, wood, fur, cotton, flannel, water, air. Fur and wool no doubt owe much of their warmth to the fact that they consist of fibres which enclose a good deal of air, but as a matter of fact the warmth of loosely woven woollen and knitted articles in general is often overrated; they are very warm as under garments or in calm weather, but in windy weather the air in them is rapidly changed and the cold seems to blow through them. If for any purpose we select a material from its place in a table of comparative conductivities, and use it without reference to the law of conduction of heat, we shall probably be disappointed with the result. We know that cotton burns easily; if we stretch a cotton handkerchief over the back of a gold watch and place a red-hot cinder from the fire on the handkerchief on the watch, the handkerchief will not be burnt.

Many interesting problems present themselves when a house has to be built or rented. There is often opportunity for some choice of material in walls or roof, and some peculiarities to be considered. Are the top rooms of a thatched cottage warmer or colder than the top rooms of a house covered with slates? Is a wooden or an iron building warmer? What difference does it make if the iron building is lined with wood? If the iron walls were twice as thick, what would be the effect inside the room? Would the walls of such a building be always dry inside? It sometimes happens that the end wall of a row of houses is covered with slates to preserve it from the effects of storms of wind and rain; will that inside wall be always dry?

But the housewife is probably more interested in those articles in use in the house which it is her business to provide. Shall the stoves be of slate or iron? In olden days warming-pans were made of copper. What change in the manner of use justifies making them of earthenware or India-rubber? The slow transmission of heat through thick woollen materials has been applied to the construction of Norwegian cooking-stoves (Fig. 5). These stoves consist of a wooden box, lined with well-padded felt. The cooking vessels are of metal; the food when at boiling point is placed in these vessels and the lids put on, a thick padded felt is placed on the vessels and entirely fills the wooden lid of the box which is then closed; the heat is preserved so that the cooking is continued without further attention. Would it be possible to use the Norwegian stove as a refrigerator? Would it keep an ice pudding cold without any alteration? In connection with this we may ask why freezing machines have the inner vessel in which the freezing takes place of zinc, and the outer vessel which contains the ice and salt of wood? What would be the effect of interchanging the materials?

[Ill.u.s.tration: FIG. 5.]

It is possible that the excellence of some continental cookery is due to the extensive use on the continent of earthenware cooking utensils through which heat pa.s.ses very slowly. The growing fashion of using enamelled cooking vessels must have some effect on the food cooked in them as heat certainly pa.s.ses quickly through them. Reference has been made to them simply to demonstrate the universality of the application of physical laws, and we may now return to the house and its arrangement for the comfort of the inmates.