“And now,” continued Mr. Holiday, “you understand fully the philosophy of the copper tea-kettle. The iron heater was the reservoir of heat. As it was a good conductor, it conveyed the heat rapidly into the bottom of the tea-kettle, and so into the water. The water, being a good conductor, conveyed the heat to the sides of the tea-kettle, but they were made bright and polished, so that they should not radiate it fast into the air. Still, however, they radiated the heat considerably; but the waste was supplied by fresh heat from the heater. At first, the heat came faster from the heater than it was radiated from the sides of the kettle, and so the water was made to boil. But gradually the heater became less hot, and so did not conduct heat so fast into the bottom of the tea-kettle,—until, at length, there was a time when the supply was just equal to the waste; that is, the heat was conducted in just as fast as it was dissipated by radiation, and the temperature was kept at the same point.”
“That was something like my pumping, Jonas,” said Rollo.
Jonas nodded his head, but did not speak, as he did not wish to interrupt Mr. Holiday.
“At length,” he continued, “the supply by conduction became less than the radiation, and so the water began to cool, and probably, when we left the table, the water had cooled considerably below the boiling point. I think it likely it would have been better to have made the heater of some other substance than iron,—something that is not so good a conductor.”
“Because,” said Mr. Holiday, “in that case it would not have communicated its heat so fast, and so the supply would have lasted longer. The iron is so good a conductor, that at first it poured its heat up through the bottom of the tea-kettle in such quantities as to make the water boil. This was more than was wanted, and the heat at first was wasted. In consequence of this, after a time it became too cool, and did not heat the water enough. Now, if it had been made of something which was not a good conductor, it would have parted with its heat more slowly at first, and so the heat would have lasted longer.”
“Yes, sir,” said Rollo; “and what would have been good?”
“Soapstone is a very good non-conductor,” said Mr. Holiday. “But then, perhaps soapstone would have got bruised and broken by constant use, and by heating and cooling. It isn’t a very hard kind of stone. I think it probable that iron is, on the whole, the best.”