Household Administration, Its Place in the Higher Education of Women

by Unknown

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 glass 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 passes remains 101 ice. From this we may surmise that the heat of the sun does not affect the medium through which it passes.

Clerk Maxwell suggested yet another experiment in illustration 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 pass, and that they heat bodies through which they do not pass. 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 passes 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, pass 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 102 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 passage 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 passed from our hand into the ice. The process by which heat passes from one body to another in contact with it is called conduction. The fundamental law of conduction is, that heat always passes from a warm body to a cold one. Clerk Maxwell illustrated 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 passed 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 103 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 passes at different rates through different substances. Substances through which heat passes quickly are called good conductors of heat. The law of the conductivity of heat is that in a homogeneous 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 104 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 assign 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, glass, 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 105 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.