So Rollo and his father and mother took their places at the tea-table to eat their supper. Nothing more was said on the subject, until, just before they rose from the table, Rollo happened to think that his father had said, at first, that a stove like the one in that room warmed the room both by radiation and conduction, and he asked his father how that was.
“In the first place,” replied his father, “it warms the room by radiation just like any other fire, as you observed by holding your hand before it. But then it also warms the air in the room by conduction, in this way; the heat of the fire heats the iron of the stove and of the pipe. The heat is conducted through the iron to the outside, and there some of it radiates, and the rest is conducted into the air which happens to touch it. Exactly as if you put your hand against the stove pipe, the heat is conducted through to your hand. So it is conducted to the air, which touches the pipe. This air becomes heated, and swells, and so grows light, and rises out of the way, and more cold air comes in to the stove pipe, and gets heated in the same way by conduction. So the air is continually coming up against the stove and pipe at all parts, and it gets heated by conduction, and then rises and floats away over the room, and thus helps warm the room. But in a common fireplace all the heat which is conducted off goes back among the bricks of the chimney, and does not help warm the room. If we had a light tuft of down or cotton, and were to put it near the pipe, perhaps it would be carried up to the top of the room, by the current of air which is heated by conduction of the pipe.”
“I wish I had a tuft to try it,” said Rollo. “It would be a very good experiment.”
Rollo had no tuft, but he had that evening an opportunity to witness the effect of the ascending current of heated air, in a way which he did not anticipate; for, after tea, they had the table out, for Rollo’s father to do some writing, while his mother sat in a rocking-chair near, reading. Rollo wanted to write, and so his father let him have a pen and ink. He said that he would write a little note to his cousin Lucy. It was a very small note, on a little piece of paper, and when he had written down one side, he undertook to dry what he had written by means of the radiation from the lamp, and accordingly held up his paper very near the flame of the lamp, upon one side. The writing, however, did not seem to dry very fast, and in changing the position of it, he held it over the flame, though so high that he thought that there was no danger of burning it. But the current of heated air which rises upward from a fire, even so small a fire as the flame of a lamp, is hotter, and rises higher, than most children would suppose. At any rate, before Rollo was aware of the danger, his note was on fire, and he was obliged to turn around suddenly, and throw his paper upon the hearth, to escape burning his fingers.