“Yes,” replied his father. “If the stove is pretty hot, enough heat will be conducted up the rods to set the phosphorus on fire. It will look as if the tops of the rods were getting on fire. First one will blaze up, and then another, and then another, according as the rods convey the heat up more or less readily. Perhaps the phosphorus upon the top of the glass or wooden rod will not take fire at all.”
“I should think,” said Rollo, “that the heat would get up some time or other, if it went up ever so slowly.”
“No,” said his father, “for some of the heat is conveyed off into the air, up and down the sides of the rods; and so, if the rods are very long, or if they are very bad conductors, they would radiate the heat from their sides so fast that not enough would get up to the top to set the phosphorus on fire.”
“What is the slowest conductor of heat?” said Rollo.
“The air is a very slow conductor,” replied his father, “if we can only keep it still. So is wood. A heap of shavings is a very slow conductor, because it consists of wood and air together.”
“Why, the little interstices between the shavings are all filled with air; and the heat, in passing through, has to go through the thickness of a shaving first, and then through a small space of air; then through another shaving, and then a little more air; and so it works its way along very slowly, and with great difficulty. Therefore, if any thing is covered with shavings, heat neither gets into it nor out of it very easily. It is the same with hay.”
“Only the blades of hay are not wood,” said Rollo.
“Not exactly,” replied his father, “and yet they are very near it. The fibres of the stems of grass are very similar to the fibres of wood. At any rate, they are both slow conductors; so that hay, like shavings, will keep the heat from coming away from any thing that is hot, and from getting into any thing that is cool.”
“It is very strange,” said Rollo, “that the same thing should be good to keep things hot, and to keep them cool too.”