THE RIDE HOME.
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.
QUESTIONS.
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?