"What next?" sighed Mrs. Wilkinson.
"You can, indeed. Put a little boiling water in a salad-oil flask; so that the flask may be a quarter full, say. Cork the flask tightly. The boiling stops; and the upper three-fourths of the flask are full of vapor. Squirt a jet of ice-cold water upon the flask, above where the water is, and the water below will instantly begin to boil. The reason why, is this. The vapor in the flask presses on the surface of the hot water. The cold condenses the vapor—turns it back to water. That takes off the pressure for the time; and then the hot water directly flies into vapor, and boils, and so on, till it cools down too low to boil any longer. What reduces the boiling point of water on a hill or a mountain is, that the pressure of the atmosphere decreases as you ascend. A rise of five hundred and thirty feet in height above the level of the sea, makes a difference of one degree; so, give me a kettle of water and a thermometer, and I'll tell you exactly how near the moon we are."
"I shouldn't think one could make good hot mixed punch up in a balloon, now," observed Mr. Bagges, reflectively.
"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.
"So," said Harry, "strictly pure water would not be quite so great a blessing to us as you might think. Of course, you know, uncle, I don't mean to say that there is any advantage in the impurity of such water as the Thames, except when used for the purpose of fertilizing the earth. I am speaking of water so pure as to contain no air. Water of such severe purity would be very unmanageable stuff. No fishes could live in it, for one thing. I have already given you one good reason why it would be unsuitable to our kettle; and another is, that it would not be good to drink. Then water, as we find it in the world, has a very useful and accommodating disposition to find its own level. Pump all the air out of water, however, and it loses this obliging character in a great measure. Suppose I take a bent glass-tube, and fill one arm of it with airless water. Then I turn the tube mouth upward, and if the water were common water, it would instantly run from one arm into the other, and stand at the same level in both. But if the water has been exhausted of its air, it remains, most of it, in the one arm, and won't run till I give the table a smart rap, and shake it. So, but for the air contained in water, we could not make the water run up and down hill as we do. If water were deprived of air, London would be almost deprived of water."