Mr. W. Tom, have you brought the small cork I told you to bring?

Tom. Yes, father; here it is.

Mr. W. Put it in the kettle spout.

Tom. Why, it blows it out again, as soon as it is in.

Mr. W. You did not half press it in. Hold it fast—press with all your strength.

Tom. See there—the lid is blown off!

Mr. W. Blown off! How is this?—nobody has put gunpowder into the kettle!

Ella. I am sure there is nothing but clean water; I saw it put in.

Mr. W. But, is it not very extraordinary that simple, clean water, should blow the kettle lid off?

Tom. Not at all, father. When you told us about the expansion of cold water below forty degrees, we wondered, because we could not think ice was more bulky than water; but there seems no reason to doubt, that the hotter water becomes, the more room it takes up.

Mr. W. How does the heat of the fire do this?

Tom. By expanding it.

Mr. W. We know that; but how?

Tom. By driving the particles of steam farther and farther asunder.

Mr. W. Precisely. The moment the particles of a drop of water become steam, they occupy eighteen hundred times as much room as they did before.

Tom. And press the lid eighteen hundred times more forcibly than water?

Mr. W. Its force is altogether irresistible. If this kettle were composed of iron, an inch thick or more, if steam could not escape, it would burst it with ease.

Tom. Is that the reason why steam boilers burst?

Mr. W. It is one reason, but not the principal one. If the water in the kettle were all boiled out, and it was full of steam, and we corked it tightly up, and soldered the lid down, and still kept the fire blazing fiercely about it, it would burst at the weakest part: perhaps the lid would fly off, or the side burst: the steam would rush out, and, if we were near, we might be scalded.

Tom. Then, when a boiler grows old and thin, if the pressure is very great, it bursts in the weakest part?

Mr. W. Just so; and ingenious men have made some portion of the boiler of a weaker metal—so that, if it burst from the pressure of the steam, it should hurt no one.

Amelia. I cannot understand what you mean.

Mr. W. You see this kettle on the fire:—if we cork up the spout, and fasten the lid down, and let it boil, it will, probably, blow the cork out, and hit some of you; but if, at the back part of the kettle that touches the chimney, we have a part of it made of lead, or tin, it will explode there.

Amelia. Oh! I see now.

Tom. But, father, this cannot account for the tremendous explosions, by which the boiler itself is thrown a great distance, and even factories are blown down.

Mr. W. I think not. I will try to make you understand this, to-morrow.

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