"Aha!" cried her brother, "that's not the way _we_ account for things, Harry, my boy, eh? Now, convince your mother; explain the boiling of a kettle to her: come." "A kettle boils," said Harry, "by means of the action of currents." "What are you talking about? Boiling a plum-pudding in a tea-kettle!" exclaimed the mystified mamma. "Currents of heated particles--of particles of hot water," Harry explained. "Suppose you put your fire on your kettle--on the lid of it--instead of your kettle on your fire--- what then?" "You would be a goose," said his mother. "Exactly so--or a gosling"--rejoined her son; "the kettle would not boil. Water is a bad conductor of heat. Heat passes through the substance of water with very great difficulty. Therefore, it would have a hard matter to get from the top of a kettle of water to the bottom. Then how does it so easily get from the bottom to the top?" "Ah!" sighed Mr. Bagges. "In my young days we should have said, because the heat rises, but that won't do now. What is all that about the--eh--what--law of ex--what?--pansion --eh?" "The law of expansion of fluids and gases by heat. This makes the currents that I spoke of just now, mamma; and I should have spelt the word to explain to you that I didn't mean plums. You know what a draught is?" "I am sorry to say I do," Mr. Bagges declared with much seriousness, instinctively carrying his hand to the region of the human body from the Latin for which is derived the term, Lumbago. "Well," pursued Harry, "a draught is a current of air. Such currents are now passing up the chimney, and simply owing to that trifling circumstance, we are able to sit here now without being stifled and poisoned." "Goodness!" ejaculated Mrs. Wilkinson. "To be sure. The fire, in burning, turns into gases, which are rank poison--carbonic acid, for one; sulphurous acid, for another. Hold your nose over a shovelful of hot cinders if you doubt the fact. The gases produced by the fire expand; they increase in bulk without getting heavier, so much so that they become lighter in proportion than the air, and then they rise, and this rising of hot air is what is meant by heat going upward. The currents of hot air that go up the chimney in this way have currents of cold air rushing after them, to supply their place. When you heat water, currents are formed just as when you heat gas or air. The heated portion of water rises, and some colder water comes down in its place; and these movements of the water keep going on till the whole bulk of it is equally hot throughout." "Well, now," interrupted Mr. Bagges, "I dare say this is all very true, but how do you prove it?"