“I’ll tell you,” said his father. “When they are going to use the instrument, they put all the rods into their sockets, and a little bit of phosphorus into the top of each one, in the cavity. Each piece of phosphorus is about as big as the head of a pin. Now, when these rods are all arranged in this way, the whole apparatus is put upon some hot place, where the whole plate will be heated alike, as, for example, upon the top of a stove. The heat from the stove is conducted into the iron plate; from the iron plate it is conducted into the ends of the rods which are in the sockets; thence it is conducted up the rods towards the phosphorus at the tops of them, only it goes up much more slowly through some than through others. And we can tell which rod conducts the heat the most rapidly, for the phosphorus upon the top of that rod will be the first to take fire.”
“Will the phosphorus take fire?” said Rollo.
“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.”