“Why, father,” said Rollo, “I should think it would make it warmer.”
“No,” said his father; “it would keep the warmth from getting to it, because it is a slow conductor. If any thing already warm is covered up in hay, it keeps it from getting cool; if any thing already cool is covered up, it keeps it from getting warm.”
“What things are good conductors?” asked Rollo’s mother.
“The metals,” said his father, “are the best.”
“Whether they are polished or not,” replied his father. “I do not know that that makes any difference. Because, you see, in being conducted, the heat moves through the middle of the body, and not along its surface so much; and I don’t know that the character of the surface is of much importance. Some metals conduct better than others. The philosophers have a curious kind of an instrument to show the different conducting powers of different substances.”
“I thought plates were always round,” said Rollo.
“No,” replied his father; “a plate, in philosophical language, is any flat and comparatively thin piece of metal, or other such substance, whatever its shape may be. This plate that I was speaking of is square, or oblong,—I believe it is oblong,—and it is about as large as my hand, and perhaps half an inch thick.
“In this plate,” continued his father, “there are drilled a number of little holes.”
“Why, perhaps eight,” said his father. “They are in a row, running along the middle of the plate. The holes are about a quarter of an inch in diameter.”
“Why, I should think a common lead pencil might be a quarter of an inch in diameter. The holes are of such a size that you could just put a lead pencil in before it is cut.”
“Are the holes bored entirely through the plate?” asked Rollo’s mother.
“No,” replied his father, “a little more than half through.”
“Then,” continued he, “they have a number of little rods made of different substances,—such substances as they wish to try, in respect to their conducting power. One is made of brass, one of iron, one of glass, one of wood, &c., and these are fitted into the holes. That is, one end of each rod is made just large enough to go into its hole and take out, and yet not be loose in it.
“The upper ends of these rods are hollowed out a little, so as to form small cavities or cups, to put some phosphorus into.”