TRANSMISSION OF HEAT,

HEAT passes from one body to another, or from one part to another of the same body, in three ways-by conduction, convection, and radiation,

CONDUCTION.

Heat is conducted when it passes through a body, the molecules communicating heat to one another. Different substances have different powers of conducting heat. Liquids and gases conduct heat very slightly, and are therefore known as bad conductors.

If you were to place a stove in the roof of a building, it would never warm it well; and yet, why should it not, if the air could conduct heat readily? Again, if you place a tin cup, containing flaming spirit, to float on the top of a vessel of water, it will never make the water boil, and yet it surely would if water were a good conductor of heat. If you fasten a lump of ice at the bottom of a test-tube nearly full of water, and then apply the heat of a spirit lamp to the upper part of the water, you may actually set the water at the top a-boiling, while the ice remains unmelted at the bottom. How could this be if water freely conducted heat?

Of solids, some are bad and some are good conductors. The metals are the best, but they have not all equal powers. Take two spoons—one of silver, the other of German silver, -and place on the extremity of the handle of each a small piece of solid butter. Then balance them on the edge of a cup of hot water-the bowls of the spoons resting on the surface of the liquid. Silver conducts heat about seventeen times as rapidly as German silver: consequently, the butter on the handle of the silver spoon will melt (with the heat conducted from the water) long before the other piece of butter is affected by it.

Stone, wood, cotton, wool, and many other substances are bad conductors. Therefore, a metal coffee-pot is provided with a wooden handle. When you go to a shop to buy a metal teapot, do not be tempted by cheapness to take one which has the handle joined close to the body; look out for one which has pieces of glass, or earthenware, or wood, between the teapot and the handle, to break the conduction of the heat. A laundress uses a square of thick calico or cloth to protect her hand from the hot handle of her iron. The sheep's wool keeps within him the warmth of his body: in summer-time he does not need this non-conductor, therefore it is shorn off, and by next winter is probably woven into cloth or flannel, to serve the same useful purpose on the body of some human being.

The following list of solid substances commences with the best and ends with the worst conductors:-Silver, copper, gold, iron, lead, stones, earths, wood, charcoal, silk, linen, cotton, wool, fur, down.

You now see why wool and furs make such warm clothing. It is not that they have heat in themselves, but simply that, being bad conductors, they keep in the warmth of the body. For the same reason you will be able to preserve a lump of ice a long time in summer by wrapping it in flannel, which keeps out the heat.

CONVECTION.

We said that gases and liquids conduct heat very badly : yet they may be rapidly heated. There must, therefore, be some other way of transmitting heat.

Put into a flask of water a little bran, or some oak sawdust. Apply heat, and you will see that the bran or sawdust has a rapid motion-rising in the centre of the flask, and falling down again at the sides (see the dots in the figure). This can only result from the movement of the water: in other words, the movement of these particles proves that

the molecules of water are moving in the same direction. Hence we learn that when a liquid is heated at the bottom the heated molecules rise, making room for other cooler molecules to come near the flame to be heated in their turn. Presently the heated molecules, having travelled to the top of the liquid, are themselves driven away downwards by hotter molecules, and thus the heat is conveyed by the molecules hither and thither throughout the whole of the liquid. This is called convection of heat.

Why do the heated molecules rise? The answer is plain : when heated they expand; thus they become lighter, and rise to the surface.

Gases are heated in the same way; for the molecules of a gas are even freer than those of a liquid, and are capable of flowing hither and thither on the very slightest occasion. This is not so with the molecules of a solid, which adhere together more or less firmly, so

that force is required to separate them.

RADIATION.

A heated body throws out its heat in all directions (like a candle giving light in all directions). This transmission of heat takes place in straight lines in every direction,—so that, for instance, if you wish to prevent the radiation of heat from a fire to your face you have nothing to do but place something as a screen in a straight line between your face and the hot fire.

Radiation is not conduction-that is to say, it does not depend upon the surrounding air or other objects for transmission; for if a heated body be placed under the receiver of an air-pump, and the air be exhausted, radiation will continue as before.

The most remarkable example of radiation of heat is from the sun, which gives out its heat in all directions through space, If a spectator could be stationed on the sun itself he would see our earth as a tiny speck among the other planets and stars. Therefore the proportion of the sun's heat which the world receives by radiation is extremely small compared with the great flood of heat which is pouring forth in every direction. Yet the heat we do get is enormous. It has been calculated that if we were to kindle a fire every nine yards all over the earth's surface, and feed each fire with a sack of coal daily, we should only produce an amount of heat equal to that which we daily receive from the sun.

From what has been already said it will be understood that when we speak of a radiator we do not mean a medium for heat to pass through (that would be a conductor), but the heated body itself, which gives out heat. Some substances are good radiators-i.e., they give out their heat freely; others very slowly. The following list begins with the best radiators, and ends with the worst:-Lampblack, paper, sealing-wax, glass, ice, black-lead, tarnished metals, bright metals.

Hence you see that metal teapots, coffee-pots, urns, and dish-covers should be kept as bright as possible, for then very little heat is lost by radiation.

THE BAROMETER.

If you take a wine glass or tumbler and plunge it into a tub of water, and then raise it gently with its mouth downwards,

you will find that as it is raised the water will not sink in the glass to the level of the water in the tub, but will continue to fill the glass so long as its mouth remains under water,

This curious fact was known for a long time before its real cause was discovered. Philosophers used to say that the water filled the glass because nothing else could get in to do so, and because, as they taught, "Nature abhors a vacuum;" by which they meant that it was contrary to nature for a space to remain unoccupied. They explained the working of the pump by the same rule: the air is pumped out, they said, and the water rushes up to prevent the formation of a vacuum.

But when some workmen at Florence were one day fixing a pump to an unusually great depth, they found that they could by no means make the water rise in the pump more than about thirty-two or thirty-three feet. Galileo, one of the greatest philosophers then living, began to see that this curious discovery might lead to important results: evidently here was Nature seemingly permitting a vacuum above thirtythree feet without taking any trouble about it.

But Galileo died without solving the puzzle, and his pupil, Torricelli, continued the inquiry. He soon saw that this column of water, measuring thirty-three feet in length, must have considerable weight, and that some force must sustain that weight. What could it be? He then reasoned that if this unknown force could sustain a column of water up to thirty-three feet, and no higher, its power was limited to that weight of water. He next conceived the happy idea of trying a column of mercury, which, as it is thirteen-anda-half times as heavy as water, he calculated could only be sustained (by this unknown force of limited power) to the height of about twenty-nine inches. He therefore took a glass tube about a yard long, closed at one end; and, after filling it with mercury, applied his finger to the open end, and inserted this end in a vessel of mercury. On removing