A boy was once much surprised to see the melted lead which he poured into a piece of elder, from which he had scooped the pith, thrown with great force against the ceiling. The reason was, that the elder was moist, and the moisture inside being changed all at once into steam, the expansive force of the steam threw out the lead, just as the expansive force of the gas made all at once from powder throws the ball out of a gun.
Explosion of a foundry.
It takes but a little water to make a good deal of steam, and this explains an explosion that once occurred in a cannon foundry in London. There happened to be some water in one of the moulds, and, therefore, when the melted metal was put into it, this water was at once made into steam, and this, in trying to get free, made such an explosion as to blow up the whole foundry. Perhaps you can hardly believe that so little water could do so much when turned suddenly into steam. But you must remember that the steam occupies, if set free, about 1700 times as much room as the water does from which it is made. It tries to get this room, and in doing this it exerts great force, especially if it be made very suddenly.
How the sound of the steam-whistle is made.
You will like to know how the sound of the steam-whistle is made. In the chapter on the hearing, in Part Second, I told you that sound is always caused by the vibration or shaking of something. Now in the steam-whistle there is a sort of bell-shaped thing with a thin edge or rim. The steam, as it is let out through the whistle, strikes against this rim, and makes it vibrate, and so produces the sound. The sound is very loud, because the steam comes out with great force.
Questions.—In what three respects is steam like air? Tell about the steam-fog. How do we know that steam can not be seen? What is said about the steam that comes from the nose of a tea-kettle? Describe how steam is made. In what way can you see the whole process? What is said about the force of steam? How is its force shown in the locomotive when it is stopped at a station? Tell about the rattling of the lid of a boiling tea-kettle. Explain how boilers are commonly burst. How does the safety-valve operate? How is it that the safety-valve does not always keep boilers from bursting? What other way in which boilers are burst is mentioned? Tell about the accident with the melted lead. Tell about the blowing up of an iron foundry. How is the sound of the steam-whistle made?
CHAPTER XXVI.
LIGHT.
As I told you about heat, that we do not know what it is, so, also, we do not know what light is. But we know many things about light, just as we do about heat.
The chief use of light.