Rollo did so. He said he could feel the heat very plainly, though he had to hold his hand nearer than was necessary when he was feeling the heat of the fire.
“Yes,” said his father; “that is because the pipe is not so hot as the fire. If the pipe was as hot as the fire, it would be red hot, and then you could feel the radiation as far as you do that of the fire. However, there is some radiation, though the pipe is not very hot. There is always radiation when the thing is warmer than the air around it. There is radiation from your face.”
Here Rollo put his hand up towards his face, to try whether he could feel the radiation.
“You can’t feel it very well with your hand,” said his father, “because your hand is warm, and so that radiates too. But if you hold your hand up very near to your face, perhaps you will feel both to be warmer, for your hand will radiate to your face, and your face will radiate to your hand.”
Rollo then put his hand up very near to his cheek, and, after holding it there a minute, he said that he perceived that it felt warm.
“At any rate,” said his father, “you understand what radiation is. The heat that comes from the sun is radiated.”
“Yes, father,” said Rollo, “I understand what radiation is; now tell me about conduction.”
“Very well,” said his father; “suppose you had an iron crowbar here, and should put one end into the fire, among the burning coals, and let it remain there while you took hold of the other end with your hand. Now, the heat would gradually extend along the bar, from the hot end in the fire towards your hand, through the substance of the iron.”
“Yes,” replied his father; “so that at last the end in your hand would begin to feel hot. Did you never try it with a pin in the lamp?”
“Yes, sir,” said Rollo; “if I hold a pin in the lamp, it burns my fingers.”
“The heat,” said his father, “is conducted along through the pin to your fingers. There is a radiation from the lamp at the same time, but the radiation is not enough to burn you at that distance. That is, if you hold your finger near the lamp, as near as the length of a pin, you would feel the warmth produced by the radiation of the heat through the air, but it would not be enough to burn you. But when you take a pin, and put one end into the flame, it conducts off a great deal more heat than will come by radiation, and your fingers are burned. But then it takes more time for the heat to come along the pin, than for it to come through the air. Thus, in that case, the heat that is radiated comes very quick, though there is but little that comes; while that which is conducted comes slowly, but in a great quantity.”