ever, is not the case. A drop of water let fall into such a vessel rolls about like a raindrop upon the surface of a cabbage leaf, and evaporates very slowly. The reason is, that when the drop of water approaches near to the red-hot surface, it darts out a quantity of vapour of sufficient tension to bear the weight of the drop. The drop then rolls about upon a cushion of its own vapour, and does not come into contact with the heated metal at all. If the metallic vessel be allowed to cool gradually, a point soon arrives when the vapour is no longer of sufficient tension to support the drop. The water then comes into contact with the metal, and violent ebullition is the consequence. Thus you see how the vapour, as long as it has sufficient tension, forms a protecting layer round the drop. Have you ever seen a blacksmith lick a piece of white-hot iron? It can be done without suffering the slightest inconvenience. The tongue does not come into contact with the iron at all, the moisture forms a layer of vapour which protects the tongue. I have seen a gentleman pass his moistened hand through a stream of molten iron as it flowed from a furnace, and I have often dipped my own fingers into molten lead. The hotter the lead is the better, for the more certain you are that the vapour will possess a tension sufficient to protect you. Before making the experiment, it is well to moisten the fingers; of course it would be unwise to have the fingers immersed for more than an instant in the molten mass. Many wonderful escapes from the fiery ordeal of ancient days may be accounted for in this way.

The following remarkable experiment depends also upon the protective influence exercised by a layer of vapour. It is possible to obtain the gas which you see bubbling from soda water, that is to say, carbonic acid gas, in a solid condition, as white as snow. The cold of such solid carbonic acid is intense; and if we pour a little ether on the substance the cold becomes far more intense still. If a mixture of the carbonic acid and ether be placed in a red-hot crucible, a layer of vapour is developed around the mixture, which not only protects it from becoming intensely heated, but enables it to remain intensely cold in the red-hot vessel. If a spoon containing water be plunged into the mixture the water freezes instantly. The same occurs when a capsule containing mercury is dipped into the red-hot vessel. The mercury solidifies with extreme quickness, and when removed from the vessel it may be cut with a knife.

SECT. 6. Let me just say a few words on an effect of heat which is sure often to attract your attention in life: I mean its power of causing bodies to expand. Different bodies expand differently by heat; lead, for example, expands far more than iron, and some crystalline bodies expand differently in different directions. This force of expansion is almost irresistible; indeed, the contraction of iron bars by cooling has been applied to pull the massive walls of large edifices closer together. Bodies, then, contract by cooling, but water and some other substances form, under certain circumstances, an apparent exception to this rule. Water continues to contract by cooling until it reaches a temperature of 39° Fahrenheit; the contraction then ceases, and it commences slowly to expand. When it reaches the temperature of 32° Fahrenheit it freezes, and the passage of the substance from the liquid to the solid condition is accompanied by a sudden increase of volume. The particles in the solid state require more room than in the liquid state. The force of expansion here is enormous. If the water be enclosed in the stoutest bomb-shell, it will burst the shell in freezing. This is the cause of the bursting of leaden pipes by frost; when the thaw comes, the injury done by the act of freezing becomes first manifest. From what has been said you will see that ice is lighter than water, bulk for bulk, and hence it is that ice swims on water. The metal bismuth is another substance which exhibits, in a striking manner, this increase of volume on cooling. I have filled a strong iron bottle with the molten metal, and closed it by a screw-tap; on cooling, the metal expanded, and rent from top to bottom the iron vessel which contained it.

ELECTRICITY.

POSITIVE AND NEGATIVE ELECTRICITY-THUNDER AND LIGHTNING-THE

ELECTRIC TELEGRAPH.

SECT. 1. In order to repeat for yourselves the little experiments which I am now about to describe, I would recommend you to get two small sticks of sealing-wax, two pieces of glass tube, a piece of flannel, and a piece of silk. In all experiments, the wax, the glass, the flannel, and the silk, must be perfectly dry.

If you rub the stick of sealing-wax with the flannel, or against the cloth sleeve of your coat, and if you rub the glass with the bit of dry silk, you will find that the wax and the glass have acquired by the rubbing the power of attracting very small bits of paper or other light bodies. This effect was first observed with rubbed amber, which the Greeks called electron; and this is the reason why we call the force or agent which produces the attraction electricity.

The experiment which I have just described, though usually the first made, is by no means the most simple. We must know a good deal of the laws of electric action before we can explain the attraction of the bits of paper; and these laws I will now endeavour to make plain to you.

You must try and construct a little paper or wire loop, in which you can lay your bit of glass tubing, or your stick of sealingwax, so as to suspend it horizontally by means of a fine string. In Fig. 1, I have sketched such an arrangement, where A B is supposed to be the stick of wax laid upon a loop of wire.

A

FIG. 1.

B

Rub one stick of wax with the flannel, and place it on the wire, taking care that the surface of the wax is not scraped by the wire. Now, rub the second stick of wax, and bring its end gradually near to the end of the suspended stick. You will find that the suspended stick will be repelled.

Remove the sealing-wax, and put a piece of glass tube rubbed with silk in its place; rub the second glass tube with silk, and

bring it near to the suspended one; you will find, in this case also, that repulsion will take place. Thus we see that rubbed wax repels rubbed wax, and rubbed glass repels rubbed glass.

Let the rubbed glass remain on the loop, and cause the rubbed wax to approach it, you will now find that the wax is attracted by the glass.

Let the rubbed wax be placed on the loop, and the rubbed glass brought near, you will find that the rubbed wax attracts the rubbed glass.

Instead of the wax, I might choose from a great number of other resinous substances, amber, rosin, sulphur, gutta-percha, shellac, &c.; and instead of the glass, I might choose from a great number of vitreous bodies, crystals and stones of various kinds. On submitting the substances to the experiment just described, we should always find that the resinous bodies, when rubbed, repel each other, and that the vitreous bodies, when rubbed, repel each other; but that the rubbed resinous body attracts, and is attracted by, the rubbed vitreous body.

For the sake of making you understand me more clearly, I must now place an image before your mind. You often hear of the electric fluid. Some people believe in such a fluid: I confess to you that I do not; but still the conception of a fluid will be extremely useful in helping you to understand me.

It has been imagined that, by the act of rubbing, a thin layer of this fluid is diffused over the glass or over the sealing-wax. That diffused over the glass has been called vitreous electricity, and that upon the wax has been called resinous electricity. The former is also often called positive electricity, and the latter negative electricity. Indeed, the two last terms are the most common, and I shall make frequent use of them.

Remembering, then, the experiments which have been just described, we arrive at the fundamental law of electric action, which is usually thus expressed: Electricities of the same name repel each other, and electricities of opposite names attract each other; positive repels positive, and negative repels negative, but positive and negative attract each other.

SECT. 2. Some bodies permit this electric fluid to pass over them with great facility; such bodies are called conductors of electricity. The metals are all good conductors. Other bodies hold the fluid

fast upon their surface, and do not permit it to pass freely over them; such bodies are called non-conductors, and sometimes insulators. Resinous bodies are all non-conductors.

If you rub a rod of metal held in the hand, and examine it afterwards as we examined the wax and the glass, you find no trace of electricity upon it. Electricity, however, is developed on the metal, but it flows instantly away through the hand to the earth, and leaves no trace behind.

But fix your rod of metal on a shellac, or a sulphur, or a gutta-percha handle, and take this in your hand while you rub the metal. The fluid cannot now escape over the insulator, and on examining the metal rod you will find that it can attract light bodies, and behave in other respects like the glass or the sealing

wax.

Lay a quill, or a bit of wood, upon the wire-loop, Fig. 1. It will be attracted by either the rubbed glass or the rubbed wax. Let it strike against either of them, or rub either of them carefully against the wood or quill, you thus transfer a portion of the electricity of the glass or the wax to the suspended object; and if you make the experiment properly, you will afterwards find that the object is repelled by the body which has given it the electricity. This is what you would expect from the law already mentioned.

But why is the wood attracted without being rubbed? Why are the small pieces of paper attracted in the experiment referred to at the commencement of this article? These are difficult questions, and you must listen attentively.

the piece of light wood placed upon the wire loop, and let c be a