"Prove that water is heated by the rising and falling of hot currents? Get a long, slender glass jar. Put a little water, colored with indigo, or any thing you like, into the bottom of it. Pour clear water upon the colored, gently, so as not to mix the two, and yet nearly to fill the jar. Float a little spirit of wine on the top of the water, and set fire to it. Let it blaze away as long as you like; the colored water will remain steady at the bottom of the jar. But hold the flame of a spirit-lamp under the jar, and the colored water will rise and mix with the clear, in very little time longer than it would take you to say Harry Wilkinson." "Ah! So the water gets colored throughout for the same reason that it gets heated throughout," Mr. Bagges observed, "and when it gets thoroughly hot--what then?" "Then it boils. And what is boiling?" "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.