“It burns me pretty quick, father,” said Rollo.
“Yes,” replied his father, “because the pin is very short. But if it was a knitting-needle, or an iron rod longer still, it would take some time for the heat to pass from one end to the other. Heat moves quickly by radiation, and slowly by conduction.
“Now, if you should go to the stove,” continued Mr. Holiday, “and put your hand upon it, you would burn it by conduction; that is, the heat would be conducted through the iron to your hand. But if you should hold your hand before the fire so near that the heat would come from the coals through the air to it, burning you, then you burn your hand by radiation.
“But you must understand,” continued his father, “that there may be conduction from air, as well as from iron. If you come into a room which is very warm,—that is, where all the air in it is very warm, although the fire may be gone out,—then the warmth you feel comes to your face and hands by conduction from the air which touches your face and hands, just as your hand would be burned by touching the iron that is hot. But if you come into a room where the air is not very warm, but where there is a great blazing fire, then you feel warmed by radiation. Do you understand this?”
“Yes, sir,” said Rollo. “I believe I do.”
“I will ask you some questions then, and see if you can answer them. When we toast bread, is the heat applied by radiation or conduction?”
“Yes; when we bake bread in an oven, is it by radiation or conduction?”
“Yes,” said his father, “because you perceive that the fire has all been taken out from the oven, and the bread is baked by the heat of the air which surrounds it. I don’t know but that there may be also some radiation from the sides of the oven, which are very hot; but no doubt a great deal of the heat is conveyed to the bread by conduction.
“When things are boiled, is the heat applied by radiation or conduction?” said his father.
“By conduction from the water,” said Rollo.
“Very well,” rejoined his father. “I believe you understand the distinction very well. Some other time, I will tell you more about both radiation and conduction. But now I see the tea is coming in.”
Rollo looked around, and saw a girl coming in, bringing a waiter with two tea-pots upon it, and a plate of toast covered with a bright cover, and some other things. She put them all upon the table, and then said to Mr. Holiday,—