While Jonas had been saying these things, he had taken more ashes from the ash-hole, and had covered the stick over entirely. He then put away the shovel, and was brushing up the hearth, when Rollo, after standing a moment, as if in thought, said,— “Jonas, what do you mean by _combustion_?” “Did I say _combustion_?” rejoined Jonas. “Yes, you said you wanted slow _combustion_.” “Well, I meant burning. I want the wood to burn slowly all night.” “Then why did not you say _burning_,” said Rollo, “so that I could understand you. I don’t see where you get all your learned words. I suppose it is out of some of your books.” “Yes,” said Jonas, “they call it _combustion_ in the books that I read, but I don’t know exactly why. I think there must be some difference between _combustion_ and _burning_, but I don’t know exactly what it is.” “I mean to ask my father,” said Rollo. “But do you expect that that stick of wood will burn, Jonas?” continued Rollo, after a moment’s pause. “Yes,” said Jonas, “it will burn slowly. A little air will get through the ashes so as just to keep it burning slowly. It is very dry.” “Suppose that there could not any air get through at all?” said Rollo. “Why, then,” said Jonas, “it couldn’t burn at all. It would go out. Sometimes I have buried up a fire so deep in ashes that it has gone out, and then I find nothing but black coals there in the morning, when I rake it open. That’s the way they make charcoal.” “How?” said Rollo. But Jonas had no opportunity to answer this question then, for they were just going out into the yard when Rollo asked it, and the attention of both the boys was attracted to the fire-flies. They, however, soon had looked at the fire-flies as much as they wished. Rollo tried to catch one, but he could not. He would see a flash at a little distance from him, and he would run to the place with his cap in the air; but, by the time that he got there, the fire-fly would of course have gone on to another place, though Rollo could not tell where, without waiting to see him flash again. Then he had to run again; but before he got to this second place the fire-fly would be gone again. One of the fire-flies led him a zigzag chase, in this way, all around the yard, and finally flashed at last just over the garden fence, so that Rollo gave up in despair. In the meantime, Jonas had gone to the barn; and now Rollo went to see what had become of him. He found him shutting the doors up for the night, and then they both came back towards the house, and sat down upon the edge of the platform, under the piazza, and Rollo asked Jonas to tell him how they made charcoal. “Why, they only bury up wood, as I did my log, lightly, so that enough air can get in, until it is burnt through; and then they cover it up tight, so that no air can get in, and so it goes out; and when it is all cold, they open it, and find the heap is all black coals.” “How big a heap do they make?” asked Rollo.