After the conversation recorded in the foregoing chapter, the party in the carryall rode on for some time in silence. They were going towards the east, and the morning sun shone very pleasantly upon them, so that it was not cold riding. The air had been frosty early in the morning, but the rays of the sun brought so much warmth with them that they enjoyed their ride very much. After some time, Rollo’s father began to speak again of the conductive powers of different substances, thus:—

“I think it probable that the fibres of wool and of fur are better non-conductors of heat than the fibres of wood, and grass, and straw, and other vegetable substances.”

Rollo did not at first understand what his father meant by non-conductors. He had not heard him use that term before. So he asked him what it meant. His father told him that, as people often wanted to employ substances which would not easily conduct heat, they had frequent occasion to use a word adapted to denote such substances as were suitable to such a purpose, and that non-conductors was the term which was so used.

“There are two reasons,” said he, “why I think it probable that the fibres of the covering of animals are better non-conductors—that is, that they conduct heat more slowly—than vegetable fibres. I should like to have you tell me whether you think they are satisfactory.”

“Well, sir,” said Rollo, “if you will tell us what the reasons are.”

“First, tell me of all the substances that you can think of which are composed of animal fibres.”

“Fur,” said Rollo, “and wool.”

“And hair,” said his mother.

“Yes,” said his father, “and woollen cloth, and blankets, and carpets, and flannel, are made of such animal fibres. Now, can you tell me of any substances which are composed of vegetable fibres?”

“Hay,” said Rollo; and then he paused. He could not think of any others.

“Cotton,” said his father, “and flax. They are vegetable fibres; and cotton cloth, and linen, and paper, are fabrics made of vegetable fibres. Now, there are two reasons which make me think that the animal fibres are better non-conductors than the vegetable. One is, that they are expressly intended for non-conductors, and therefore would probably be so formed as most perfectly to answer their design.

“For you see,” continued Mr. Holiday, “that the wool, and the fur, and the hair, of animals is generally intended as a covering to protect them from the cold. That is, it is intended to keep the heat of the body from passing off, and being wasted in the air. Therefore they must be non-conductors of heat, in order to answer this design. Now, we find that everything in nature is admirably adapted to answer its design, and therefore we should expect that these substances, which are expressly intended to confine heat, would be the best non-conductors of heat in nature. That is one reason.”

“I think it is a very good reason,” said Rollo’s mother.

“So do I,” said Rollo.

“The other reason is this,” continued Mr. Holiday. “All the substances made of the vegetable fibres feel colder to the touch in cold weather than those made of animal fibres. For example, suppose you were to go, in a very cold night, into a room where there was no fire, and put one hand into a bed between the sheets, and the other between the blankets,—which would feel the coldest?”

“The sheets,” said Rollo, “a great deal.”

“And the reason why they would feel coldest must be,” said Mr. Holiday, “that they conduct away the heat from your hand the fastest, which shows that they are better conductors of heat. If you were to put your hand upon iron in such a room, it would feel colder still, because the iron is a better conductor of heat than the vegetable fibres which the sheet is manufactured from.

“In the same manner,” continued Mr. Holiday, “a bare wooden floor feels colder than one with a carpet upon it. Now, in the case of a bare floor, it is vegetable fibre which conveys away the heat, and in the carpet, if it is a woollen carpet, it is animal fibre. But a stone floor would feel colder than a wooden one.”

“A painted floor is very cold,” said Rollo’s mother.

“Yes,” said Mr. Holiday; “paint is made of some metallic preparation generally, which is a better conductor than wood. All the metals are much better conductors than wood. That is the reason why they have wooden handles to tea-pots and other such vessels, because a handle of metal would conduct so much heat from the hot water to the hand, that we could not hold it.”

“Yes, sir,” said Rollo; “and the handle to the little tea-kettle at the tavern was a wooden one. I did not know what it was for.”

“It was to keep the heat from coming to your hand when you pour out the water. If the handle had been of copper, it would have been necessary to have a non-conductor to put around it, to keep the heat from coming out of the handle.”

“I never heard of such a thing,” said Rollo.

“Haven’t you?” said his father. “They make them sometimes, and it is a very ingenious contrivance.”

“How do they make them?” said Rollo.

“They make them of some substance formed of animal fibre,” said his father, “because that makes the best non-conductor. The article, when ready, can be put upon any handle which is likely to get hot, and, being a non-conductor, it stops the heat from coming from the handle to your hand. You can take up any hot thing by means of it,—unless it is hot enough to burn the animal fibre of which the non-conductor is composed.”

“I wish that I could see one,” said Rollo.

“Well,” said his father, “you can, when you get home. Dorothy has got one, I dare say.”

“Dorothy!” repeated Rollo.

“Yes,” said his father. “She calls it a holder.”

“O, a holder,” said Rollo. “I did not know you meant a holder.”

“Why, a holder is a non-conductor made of animal fibre, and intended only to stop the passage of the heat from the iron handle to your hand.”

“I thought it was only to keep our hands from touching the handle.”

“No,” said his father, “for an iron holder would do that; but an iron holder would soon get hot, and be of no use. That is, an iron holder would allow the heat to pass across very soon to your hand. It must be a non-conducting holder.

“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.”

“Why, sir?” asked Rollo.

“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.”

Rollo learned nothing more on the subject of heat during this ride, for very soon after this they began to draw towards home. After they had arrived, and the horses had been taken care of, he and Jonas went to look about for a good place to build the proposed ice-house.

What two classes of non-conductors were the subjects of conversation on the ride home? Name some examples of animal fibres; of vegetable fibres. Name some examples of fabrics composed of animal fibres; of vegetable fibres. Which class of substances did Mr. Holiday suppose were best non-conductors? What was the first reason which he gave?—the second reason? What did Mr. Holiday say, further, in respect to the operation of the heater? What was the point of analogy between the operation of the heater and Rollo’s pumping? What was the objection to iron as the material for the heater? What other substance did Mr. Holiday suggest? What objection to soapstone did he think of?