fact that the ice is at present floating upon the water. In passing from ice to water there is, therefore, a great contraction of substance. Cast iron contracts like ice when it melts, or, which is the same thing, expands like water when it freezes or gets solid, and in consequence, if liquid iron be run into a mould, when it gets solid it will expand so as to fill all the crevices of the mould; it can thus be cast in a mould. On the other hand, gold, silver, and copper expand when they melt, and contract when they become solid; they will not therefore, like cast iron, run into the crevices of a mould, and therefore coins made of these metals cannot be cast in a mould, but must be stamped.

All substances, however, expand very greatly when they are converted into gas, and a cubic inch of boiling water will be converted into steam occupying nearly 1,700 cubic inches.

62. Other effects of Heat.-You have now seen that heat expands bodies or makes them larger, and that it also causes them to change their state, passing from solids to liquids and from liquids to gases as the heat continues to be applied. You have seen how powerful an agent heat is; how the strongest and hardest bar of iron will by it be changed into a whitehot mass as soft as treacle, and if heated still more will be driven off in the shape of gas.

Heat affects bodies in many other ways, and more especially it promotes the operation of chemical attraction. Thus at a low temperature coal will not combine with the oxygen of the air, and we may keep our coals for any length of time in our coal-cellar. But when heat is applied combination takes place; and as this

combination in its turn produces heat, the process of combination goes on, and the coal is said to burn.

In like manner in the experiment (Chemistry Primer, Art. 6) where sulphur and copper combine together, heat is first of all applied in order to promote combi nation, but when this has begun heat is generated, and the process goes on of itself, without requiring any more heat from a lamp.

63. Freezing mixtures.-Again, chemical union, you have been told (Chemistry Primer, Art. 7), produces heat, and this is always true; nevertheless sometimes two substances which have a tendency to form a solution mix together with the production of cold and not of heat. Thus common salt and snow have a tendency to form a solution, and they will do so with. the production of very considerable cold, or, to speak more correctly, with the absorption of a very considerable quantity of heat.

EXPERIMENT 44.-To prove this, let us rapidly mix some melting ice or snow and some salt together, and place in the mixture the bulb of our thermometer. The mercury in the tube will soon fall below o°, thereby showing that this mixture is colder than melting ice.

Now what is the reason of this ? it is to be found in the fact that after these two substances have become mixed together we have a liquid and not a solid--in fact, we have strong brine. Now you have been told that heat is swallowed up, or becomes latent, when Dodies pass from the solid into the liquid state-for instance, when ice becomes water. The brine, therefore, being a liquid, swallows up part of the heat of the snow and salt, and the consequence is that we have a

very cold liquid as the result of the union of two solid bodies. Thus when two solid bodies dissolve each other, we have very frequently a lowering of temperature on account of the heat which is swallowed up by the liquid. Such bodies are said to form freezing mixtures.

In like manner, if we have a liquid that evaporates very fast we find it to be intensely cold, because in order to become a vapour or gas it requires a great deal of heat, and gets it where it can thus if you drop some ether upon your hand it feels very cold, and soon flies away in the shape of gas; in fact, it has robbed your hand of a large quantity of heat in order to produce this vapour or gas. Very low temperatures, very intense cold, may sometimes be produced by causing certain liquids to evaporate very rapidly.

EXPERIMENT 45.-To prove this let me pour some water into a shallow vessel, and place it along with a pan containing strong sulphuric acid under the receiver of the air-pump, and exhaust the air. As the pressure of the air is withdrawn the water will evaporate very rapidly, and in order to do so will take away so much heat from its own substance that it will be turned into ice.

64. Distribution of Heat.-Let us now proceed to another part of our subject, and consider the tendency which heat has to distribute itself.

A hot body will not always remain hot, but it will part with its heat to the colder bodies that are around it; and it will always insist upon doing this, but it will do it in different ways according to circumstances.

EXPERIMENT 46.-For instance, let us put a poker into the fire; some of the heat of the fire gets into

that part of the poker which is in the fire, and this passes along the poker until it heats that end which is farthest away from the fire, and you will at last find it too hot to touch. This passage of heat along the poker is called conduction of heat.

EXPERIMENT 47.—Again, let us take a flask two-thirds full of water, and heat it from below. As the lower particles of water are heated they expand, and therefore get lighter; they consequently rise to the top for the same reason that a cork rises in water, and are replaced by colder and, therefore, heavier particles from above. A new set of particles are thus continually subjected to the heat of the lamp, and in process of time the whole water will get heated and begin to boil. This process is called convection of heat.

Neither of these processes will, however, account for the heat that reaches us from the sun. Whether in conduction or convection the heat is carried by means of particles of solid or liquid matter, but we have reason to think that there are no such particles between us and the sun, while we know that the sun's light and heat takes less than eight minutes to come from the sun to us over a distance of 90 millions of miles. Evidently, then, the heat which comes to us from the sun moves with an immense velocity, and does not reach us in virtue of heating up the particles between the sun and ourselves. In fact, in a very cold day, when the air is very cold and anything but heated, the sun's rays may be very powerful. Now the process by which heat comes to us from the sun or any other very hot body is called radiation of heat.

We have thus three very different ways in which a heated body communicates its heat to a cold one; namely, conduction, convection, and radiation. us now consider these in order.

Let

65. Conduction of Heat.-We have spoken about thrusting a poker into the fire, and told you that at last the other end of the poker will be too hot to hold. But if, instead of a metal poker or rod, a glass or stoneware rod were thrust into the fire, the other end of this rod would never get very hot, because stoneware does not conduct heat nearly so well as metal.

Wool and feathers are still worse conductors, and this is why these substances have been provided by nature as the clothing of animals; for the heat of an animal is generally greater than that of the surrounding substances, and this heat is not readily conducted off through the garment of wool, feathers, or fur, with which the animal is clad. So in the case of boilers of engines; when we wish to keep in the heat, we supply them with steam jackets or coverings inade of a nonconducting substance.

A bad conductor may be used not only to keep in heat, but also to keep it out; flannel, for instance, may be used to wrap round our bodies in order to keep the heat in, or it may be used to wrap round a block of ice which we wish to preserve in order to keep the heat out. In fact, heat cannot readily pass through flannel whether it be going from within outwards or from without inwards.

EXPERIMENT 48.-It is very easy to show you that different substances have different conducting powers for heat. You see, as in the figure, two rods or wires, one