"Then," Harry proceeded, "it requires more heat to make water boil in a glass vessel than it does in a metal one. A metal vessel's inner surface is made up of very small points and dents. Scratching the inside of the glass so as to give it a roughness something like what the metal has, makes the boiling point lower; and a few iron filings thrown into water boiling in glass at two hundred and fourteen degrees, will bring it down to two hundred and twelve. The filings, and the roughness of the glass, are so many more points for the heat to pass into the water from, and form steam, and the water does not cling to them so hard as it clings to a smooth surface. Throw a lot of hay into a pan of hot water, and it makes a quantity of steam rise directly; and I have heard a doctor say that some poor people are in the habit of giving themselves cheap steam-baths by this means." "A very good thing for rheumatic pains, I should think; certainly a much more rational remedy than patent medicines or Government poison," Mr. Bagges remarked. "There are some salts," continued Harry, "which, if dissolved in water, will prevent it from boiling till it is heated to two hundred and sixty-four degrees, as if they held the water back from flying into steam. So, then, the boiling of water may be hindered, more or less, by pressure from without, and attraction from within. The boiling point of water depends on another important fact which the kettle always mentions before it boils, although we don't all of us understand the kettle's language. The singing of the kettle tells us--" "That the water is going to boil," interrupted mamma. "Yes, and that water contains air. The singing of the kettle is the noise made by the escape of the air, which is driven off by the heat. The air sticks and hangs in the water, till the heat expands it and makes it rise. Put a glass of water under the receiver of an air-pump, and exhaust the receiver. As you pump, the water begins to bubble, as if it were boiling; but the bubbles are the air contained in the water, being pumped out. The air-bubbles act like wedges between the little invisible drops that make up the whole water. If it were not for them, the water would be a mass which would hold together so hard that it would not go into steam, or boil, till it was heated to two hundred and seventy degrees, as may be proved by boiling some water quite deprived of air. And not only that, but when it did boil, it would boil all at once, and blow up with a tremendous explosion; which would be a still greater inconvenience in boiling a kettle." "A pretty kettle of fish, indeed!" Mr. Bagges observed.