"Bubbling," suggested the young philosopher's mamma.
"Yes; but ginger-beer bubbles," said Harry, "but you wouldn't exactly call that boiling. Boiling is the escaping of steam. That causes the bubbling; so the bubbling of water over the fire is only the sign that the water boils. But what occasions the escape of the steam?"
"The heat, of course—the—what is the right word?—the caloric," answered Mr. Bagges.
"True; but what heat? Why, the excess of heat over two hundred and twelve degrees—taking that as the average boiling point of water. You can heat water up to that point, and it remains water; but every degree of heat you cause to pass into it above that, turns a quantity of the water into steam; and flies off in the steam, unless the steam is hindered from escaping by extraordinary pressure. Blow the fire under that kettle as much as you will, and you will make the water boil faster, but you won't make it a bit hotter than two hundred and twelve degrees."
"Well, to be sure!" Mrs. Wilkinson exclaimed.
"If water," continued Harry, "could keep on getting hotter and hotter above the boiling point, why, we might have our potatoes charred in the pot, or our mutton boiled to a cinder. When water is confined in a strong vessel—and strong it must be to prevent a tremendous blow-up—confined, I say, so that no steam can escape, it may be heated almost red-hot; and there is a vessel made for heating water under pressure, called Papin's Digestor, which will digest almost any thing."
"What an enviable apparatus!" exclaimed Mr. Bagges.
"Well," resumed Harry; "so the boiling point of water depends on the degree of force which the air, or what-not, is pressing on its surface with. The higher the spot on which you boil your water, the lower the point it boils at. Therefore, water boiling at the top of a mountain is not so hot as water boiling at the mountain's base. The boiling point of water on the summit of Mont Blanc, is as low as one hundred and eighty-four degrees. So, if water must be at two hundred and twelve degrees, to make good tea, don't choose too high a hill to build a temperance hall on. The heavier, also, the air is, from the quantity of moisture in it, the hotter water becomes before it boils. If the atmosphere were carbonic acid gas, water would get much hotter without boiling than it can under—"
"Present arrangements," interposed Mr. Badges.
"Consisting of a mixture of nitrogen and oxygen," continued Harry. "Water requires only a very low heat to make it boil in an exhausted receiver, out of which the air has been pumped, so as to leave none to press upon its surface. Owing to boiling depending upon pressure, you can actually make water boil by means of cold."