Bursting of a steam-engine.
Sometimes water is changed into steam so quickly that it is like the changing of powder into gas in its effects. This is seen in the way that the boiler of a steam-engine is sometimes burst, as I will explain to you. By carelessness, there is not a proper supply of water in it. The fire will, of course, heat the boiler very hot. Now see what must be the consequence when more water is let into it. The boiler, being so very hot, changes this fresh supply of water all at once into steam, and you know it takes but little water to make considerable steam, just as it takes but little powder to make a great deal of gas. All this steam so suddenly made acts precisely like the gas made by burning powder. It must have room, and as there is not room enough for it in the boiler, it must get out somewhere. The strong boiler can not hold so much steam in, and it bursts.
But perhaps you will ask, Is it nothing but air or gas that throws the ball out of the cannon, or the bullet out of the gun, so fast that you can not see it? Can such a light, thin thing as gas drive a ball through even thick beams of wood? Yes, the gas that the powder turns into can do all this.
How powder sends the ball out of a cannon.
Now see the reason why the powder and the ball must be put into a cannon to do this. If the powder should be laid upon the ground, with the ball lying upon it, and fire should be touched to it, there would not be much of a sound, and the ball would not be moved much. Why? Because the gas that the powder turns into has a chance to escape in every direction; but when the powder and the ball are put into a cannon, the gas is all shut in, so that it can escape but one way, instead of every way, as it did when the powder was on the ground. It goes out of the mouth of the cannon, pushing the ball before it. It does to the ball just what the air does to you when it blows against you and pushes you along. It is a very hard blowing of gas that throws out the ball so fast. The gas is made all at once, as I have before told you, and it must find room somewhere. There is not room for it in the cannon, and in going out to find room it throws the ball out.
If you should blow a little ball of paper from your mouth, it would not go far. This is for the same reason that a ball laid upon a heap of powder is not moved much when the powder is exploded. But put the paper ball into a quill, and blow through it, and you can send it across a room quite swiftly. The reason is, that the air which you blow out can escape only through the quill, just as it is with the gas in the cannon.
When the gas comes out of the mouth of the cannon, it spreads out in all directions, because it has room now. It is exactly as it is with a crowd of people coming through a door; as fast as the crowd gets through, it spreads out.
How rocks are blasted.
Observe, now, how rocks are rent in pieces in blasting. Quite a large hole is drilled into the rock. It is like the space in the barrel of a gun when it is done. This is filled with powder. Why, now, when the powder explodes, does not the gas come out of this in the same way that it does out of a cannon or a gun? Why, instead of this, does it break the rock in pieces? It is because the hole is not large enough for so much gas to come out. If we should put as little powder into it as we do into a gun, the gas would all come out, as it does out of a gun, without breaking the rock at all; but it is filled quite full of the powder, and so a great deal of gas is made. If we should put as much powder into a gun, it would burst like the rock, because there would not be room enough for the escape of so much gas unless it went out slowly, and that it will not do.
How a rocket goes up in the air.