“Well,” replied his father, “I will explain it to you. It is a very good apparatus to illustrate the subject. The tea-kettle is a radiator, and the heater is a conductor, and so they show both modes of conveying heat.
“For,” continued his father, “as the sides of the tea-kettle do not come in contact with any thing but the air, they do not conduct away the heat much; for the air does not take heat easily by conduction. But the tea-kettle radiates heat continually. If you put your hand near it, you will feel the heat passing off into the air all around.”
Here Rollo, who was seated not very far from the tea-kettle, put out his hand towards it to feel the radiation.
“The tea-kettle would conduct away the heat very fast, if there was any thing touching it all around, which would take heat easily by conduction.”
“How fast?” said Rollo.
“Why, if you were to put your hands to it, clasping it all around, the heat would be conducted very fast into your hand, and you would be burned. Your hands would receive the heat readily by conduction, but the air does not.
“And the tea-kettle,” continued his father, “does not radiate the heat very fast, because it is a bad radiator. It is made so on purpose.”
“What do you mean by a bad radiator, father?” asked Rollo.
“Why, all white, and bright, and polished surfaces radiate very little,—and all dark, and dull, and rough surfaces radiate very fast. So the bright surfaces are called bad radiators, and those that are of dark color and dull, are called good radiators. A bright tea-pot, made of metal, is a bad radiator, and that makes a good tea-pot.”
Rollo laughed.