;

rather to powder. The wood of our tables and chairs would fall to powder, and we should have no furniture and the bricks of our houses would do the same, so that we should have no houses. We should do the same ourselves, and in fine all things would resolve themselves into a huge mass of dust.

Finally, let us think what would happen if there were no such thing as chemical attraction. In the first place the fire would cease to burn because the carbon of the coals would no longer care to unite with the oxygen of the air.

In the next place no two simple or elementary substances would unite together to form a compound substance, but we should have nothing but about sixty simple substances consisting of a great number of metals and a small number of gases. There would be

no variety in such a world, and indeed there would be no living in it, for our own bodies are compound; and if chemical affinity were destroyed part of them would go up into the air and mix with it, while another part, consisting of a quantity of carbon, a little phosphorus, and some one or two metals, would fall to the ground, and thus we should come to an end.

HOW GRAVITY ACTS.

8. Centre of Gravity. EXPERIMENT 3.-Let us now endeavour to find out what sort of a force gravity is, and for this purpose let us take this irregular sheet of iron and hang it up by a thread. You see it hangs in a particular way, and you also see that the line already drawn with white paint on the sheet is in

the same direction as the line of the thread. Next let us hang the sheet freely from some other point; here again you have another white line in prolongation of the thread, and you further see that these two white lines cut each other in a point marked G.

Fig. 2.

Now let us hang up the sheet by some third point in its rim. As before, you have a white line in prolongation of the thread. Now you will easily see that these three white lines all cut one another at the same point G; in fact, if you suspend the sheet from any point freely by a thread, and draw a white line in prolongation of the thread, all such lines will cut one another in the same point G, so that this point will always be directly under the point from which the sheet is hung, and if you push the sheet to one side it will return again to its old position. Now what is this peculiar point G? To find out let me

attach a string to G, and hang the sheet by the string; you see that the sheet will balance round G in all directions just as well as if its whole weight were condensed into the point G. Now G is what we call the centre of gravity of the sheet; and if

G.

Fig. 3.

I hang up the sheet freely by a string, it will put itself in such a position that its centre of gravity G shall be as low as possible. If instead of hanging the sheet by a string I suspend it loosely upon a peg, it will still try to place the point G as low as it possibly can, and it will not hang as in fig. 3.

9. The Balance.-Every substance has a point G of this kind, which we call its centre of gravity. The balance which you see on page 28 has, like everything else, its point G-its centre of gravity. And it will endeavour, just like the sheet of iron, to place this point as low down as it possibly can.

Now when there are equal weights in both scalepans, this point G is somewhere directly under the point upon which the balance is swung; and hence, if by pushing it I try to tilt it to a side, when freed it will ultimately return to its old position. In fact, when the weights in each pan are equal, it will always

keep this position, with the pointer pointing exactly in the middle; so that if I am weighing a substance, and place this substance in the one scale-pan, and the weights in the other, and if the pointer points exactly in the middle, I am then quite sure that the weights in the one scale-pan are exactly equal to the weight of the substance in the other. But if the weights are not heavy enough, the beam of the balance will be tilted over by the substance in one direction; while if the weights are too heavy, they in turn will tilt over the beam in the other direction.

EXPERIMENT 4-Suppose that I put this piece of metal into one of the scale-pans, and put weights equal to 150 grains into the other, the scale-pan with the metal in it sinks down, thereby showing that the metal is heavier than the weights. Next let me put weights equal to 250 grains into the other scale-pan. Now again it is these 250 grains that are too heavy, and you see that the scale-pan containing them sinks. down, whereas before it was the other that sank. Thus the weight of the metal is somewhere between 150 and 250 grains. Let us therefore try a 200-grain weight, and you see that now the pointer points exactly in the middle, and the beam of the balance is exactly horizontal, showing that the weight of the metal is exactly 200 grains.

THE THREE STATES OF MATTER.

10. You have seen that we cannot do without the various forces of nature, and that if one piece of matter were not drawn or attracted to another piece,

there would be no such thing as a world at all. You have seen, too, that if there were no cohesion, there would be nothing but powder. I may now proceed to tell you that if everything possessed cohesion to a great extent, we should be nearly as badly off, for we should in such a case have neither liquids nor gases, neither water nor air.

The particles of a bar of iron or steel possess very great cohesion, and it is very difficult to force them apart. But water and mercury have hardly any cohesion whatever, and the very slightest touch will scatter in all directions a quantity of water or of mercury. Yet these two liquids have still a little cohesion left, as you may see by the following experiments.

EXPERIMENT 5.-Take a very small quantity of mercury from the bottle containing it, and put it on a flat glass surface. By pressing it you may split it up into small globules. Now these globules are a proof that the particles of mercury cling together. For, put another plate of glass above them, and you may by this means squeeze them flat; but if you take away the glass, the mercury will resume its previous globular shape.

EXPERIMENT 6.-Sprinkle a few drops of water on an oily or greasy surface, and these will be found to have a rounded form, not unlike drops of mercury, showing that the particles cling to one another.

On the other hand, the particles of gases, such as the air we breathe, have no tendency to keep together, but rather the reverse. Indeed they will separate from one another unless there is some force which keeps them from doing so.

So that, you see, we have three very different states

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