CONDUCTION.
Very soon after Jonas had driven out of the village, Mr. Holiday went on
with his explanation. Rollo sat, as before, upon the front seat; but, by
turning a little in his seat, he could hear what his father said very
well.
“Let us see,” said his father; “where did we leave off?”
“About the chips,” said Rollo’s mother.
“And the hay,” said Rollo.
“Yes,” said his father. “We’ll take the case of the hay. Should you
think, from what I told you, that hay would be a good or bad radiator?”
“Good,” said Rollo.
“Why?” asked his father.
“Because,” replied Rollo, “it is not bright and polished.”
“Right,” said his father. “And should you think that it would be a good
or a bad receiver of radiation?”
“Good,” replied Rollo again.
“Yes,” added his father, “because all those surfaces which are good
radiators, are good receivers of radiation.”
“Only,” said Rollo, “the snow doesn’t melt under it. It all turns to
ice.”
“True,” said his father. “How thick was the hay in the place that you
refer to? that is, how much was there, and how thick a covering did it
make upon the ground?”
“Not very thick,” replied Rollo.
“An inch?” asked his father.
“O, yes, sir, more than an inch.” Here Rollo held his two hands at a
little distance apart, in order to show his father how thick the
covering of hay was. It might have been three or four inches, though it
is not probable that Rollo’s recollection of it was very exact.
“Very well,” said his father; “we will call it three inches. Now, you
must remember that the sun shines only upon the upper surface of the
hay, and, if it is ever so good a receiver of radiation, it is only the
upper surface that can be warmed by the direct rays. In order to melt
the snow, the hay has not only got to receive the heat from the sun by
radiation on the upper surface, but it has also got to convey it down
through this space of three inches by conduction. Now a substance may be
a very good radiator, and a very good receiver of radiation, while it is
a very bad conductor. And this is the case exactly with hay. It receives
radiation very well, and so the top gets warm under the rays of the sun;
but it _conducts_ very slowly, and so the bottom keeps cool, although
the top is warm. So, you see, a single blade of hay, laid upon the snow
under the rays of the sun, would make the snow melt the faster; but a
layer of it three inches thick would keep it from melting. A layer of
two feet thick would perhaps keep it from melting all summer.”
“Why, father,” said Rollo, “I should think it would make it warmer.”
“No,” said his father; “it would keep the warmth from getting to it,
because it is a slow conductor. If any thing already warm is covered up
in hay, it keeps it from getting cool; if any thing already cool is
covered up, it keeps it from getting warm.”
“What things are good conductors?” asked Rollo’s mother.
“The metals,” said his father, “are the best.”
“If they are polished?” asked Rollo.
“Whether they are polished or not,” replied his father. “I do not know
that that makes any difference. Because, you see, in being conducted,
the heat moves through the middle of the body, and not along its surface
so much; and I don’t know that the character of the surface is of much
importance. Some metals conduct better than others. The philosophers
have a curious kind of an instrument to show the different conducting
powers of different substances.”
“What is it, father?” said Rollo.
“Why, they have a small iron plate——”
“Round?” asked Rollo.
“No, square,” replied his father.
“I thought plates were always round,” said Rollo.
“No,” replied his father; “a plate, in philosophical language, is any
flat and comparatively thin piece of metal, or other such substance,
whatever its shape may be. This plate that I was speaking of is square,
or oblong,—I believe it is oblong,—and it is about as large as my hand,
and perhaps half an inch thick.
“In this plate,” continued his father, “there are drilled a number of
little holes.”
“How many, sir?” asked Rollo.
“Why, perhaps eight,” said his father. “They are in a row, running along
the middle of the plate. The holes are about a quarter of an inch in
diameter.”
“How much is that?” asked Rollo.
“Why, I should think a common lead pencil might be a quarter of an inch
in diameter. The holes are of such a size that you could just put a lead
pencil in before it is cut.”
“Are the holes bored entirely through the plate?” asked Rollo’s mother.
“No,” replied his father, “a little more than half through.”
“Then,” continued he, “they have a number of little rods made of
different substances,—such substances as they wish to try, in respect to
their conducting power. One is made of brass, one of iron, one of glass,
one of wood, &c., and these are fitted into the holes. That is, one end
of each rod is made just large enough to go into its hole and take out,
and yet not be loose in it.
“The upper ends of these rods are hollowed out a little, so as to form
small cavities or cups, to put some phosphorus into.”
“What is that for?” asked Rollo.
“I’ll tell you,” said his father. “When they are going to use the
instrument, they put all the rods into their sockets, and a little bit
of phosphorus into the top of each one, in the cavity. Each piece of
phosphorus is about as big as the head of a pin. Now, when these rods
are all arranged in this way, the whole apparatus is put upon some hot
place, where the whole plate will be heated alike, as, for example, upon
the top of a stove. The heat from the stove is conducted into the iron
plate; from the iron plate it is conducted into the ends of the rods
which are in the sockets; thence it is conducted up the rods towards the
phosphorus at the tops of them, only it goes up much more slowly through
some than through others. And we can tell which rod conducts the heat
the most rapidly, for the phosphorus upon the top of that rod will be
the first to take fire.”
“Will the phosphorus take fire?” said Rollo.
“Yes,” replied his father. “If the stove is pretty hot, enough heat will
be conducted up the rods to set the phosphorus on fire. It will look as
if the tops of the rods were getting on fire. First one will blaze up,
and then another, and then another, according as the rods convey the
heat up more or less readily. Perhaps the phosphorus upon the top of the
glass or wooden rod will not take fire at all.”
“I should think,” said Rollo, “that the heat would get up some time or
other, if it went up ever so slowly.”
“No,” said his father, “for some of the heat is conveyed off into the
air, up and down the sides of the rods; and so, if the rods are very
long, or if they are very bad conductors, they would radiate the heat
from their sides so fast that not enough would get up to the top to set
the phosphorus on fire.”
“What is the slowest conductor of heat?” said Rollo.
“The air is a very slow conductor,” replied his father, “if we can only
keep it still. So is wood. A heap of shavings is a very slow conductor,
because it consists of wood and air together.”
“How is that?” said Rollo.
“Why, the little interstices between the shavings are all filled with
air; and the heat, in passing through, has to go through the thickness
of a shaving first, and then through a small space of air; then through
another shaving, and then a little more air; and so it works its way
along very slowly, and with great difficulty. Therefore, if any thing is
covered with shavings, heat neither gets into it nor out of it very
easily. It is the same with hay.”
“Only the blades of hay are not wood,” said Rollo.
“Not exactly,” replied his father, “and yet they are very near it. The
fibres of the stems of grass are very similar to the fibres of wood. At
any rate, they are both slow conductors; so that hay, like shavings,
will keep the heat from coming away from any thing that is hot, and from
getting into any thing that is cool.”
“It is very strange,” said Rollo, “that the same thing should be good to
keep things hot, and to keep them cool too.”
“I don’t know,” said his father, “that it is very strange, if we
consider how it operates. It keeps the heat from passing either way. Hay
has no warmth in itself. If you put hay about your feet when you are
riding, it will keep them warm, because the warmth is in your feet
already, and the hay keeps it from going away. On the other hand, if you
put hay around a lump of ice, it will keep it cool, because the ice
cannot melt unless some heat gets into it from the sun and air, and the
hay impedes the progress of the heat in going in, as well as in coming
out. It is just so with furs or blankets. They don’t make things warm.
They only keep them from getting cold.”
“But, father,” said Rollo, “if my feet were very cold, and I were to
wrap them up in blankets, wouldn’t it make them grow warm?”
“Yes,” said his father, “they would grow warm, but not because of any
warmth in the blankets. The warmth would be produced in your body by the
blood, and the blankets would keep it from escaping. That is all.
Blankets and furs would be the best things to put up ice in, to keep it
cool. They keep the heat from going off from our bodies, and so, in the
same way, they would keep heat from going in from the sun and air to the
ice.”
“Why don’t they keep ice so, then?” said Rollo.
“Because it would be very expensive. Shavings, and tan, and such things,
will do nearly as well, and they are a great deal cheaper. Branches of
trees would do very well, if the leaves were on. Once I heard of a boy
who made an ice-nest, that kept the ice all summer.”
“How did he do it, sir?” said Jonas.
“Why, he found a little hollow on the north side of a hill, and he
covered the bottom of it with hemlock branches a foot thick. Then he
piled up square blocks of ice upon the branches till he made a pyramid
of ice as high as his head, and as large on the bottom as a large table.
Then he heaped up straw and shavings all around it till the ice was
covered very thick. He also contrived to put some sheets of bark over
it, to keep the rain off; and thus he kept his ice nearly all summer.”
“I should like to try it,” said Jonas.
“It would be better for you to make a little building,” said Mr.
Holiday; “that would be much more convenient to get into. You would only
have to make the walls double, and fill the space between with tan, or
shavings, or straw, or any other substance like that, to keep the heat
out.”
“Let us do it, Jonas,” said Rollo. “I should like a little ice-house
very much.”
Rollo’s mother said that she wished Jonas would; so it was settled that
he was to try and see if he could make an ice-house.
QUESTIONS.
What phenomenon that Rollo had observed was it that Mr. Holiday was
going to explain when they were seated in the carryall? Did they
conclude that hay would be a good or bad radiator?—a good or bad
receiver of radiation? How, then, did Mr. Holiday account for the
snow and ice melting so slowly under it? What was the object of the
philosophical instrument constructed? What are some of the best
conductors of heat? What substances are slow conductors? Describe
the plan which the boy devised for keeping ice all summer.