Rollo's Philosophy [Fire] — Part 50

“The heater, you observe,” said he to Rollo, “furnishes heat to the tea-kettle by conduction, to supply the waste by radiation.”

“What, sir?” said Rollo. He did not understand his father very well.

“Why, the tea-kettle loses its heat by radiation principally, and the heater supplies it by conduction, and so one restores what is lost by the other.”

“I don’t understand it, exactly,” said Rollo.

“Well,” said his father, “I’ll explain it to you presently, alter we have taken our seats at the table.”

Accordingly, when they were all seated at the table, Rollo asked his father about the operation of the heater and the tea-kettle.

“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.”

“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.”

“Its being so bright prevents its radiating the heat of the water within it very fast away, and so the water keeps hot longer. But a stove pipe, which is of dark color, and not polished, is a good radiator. They make them so on purpose. A stove pipe made of some bright metal would look a great deal better, but it would not warm the room so well.”