In a former lesson you read a description of the thermometer, a useful
instrument which enables us to estimate the temperature, or sensible
heat, of substances.
All bodies, even the coldest, contain heat; and they have also a
tendency to part with their heat to colder substances around them,
until all have the same temperature. Thus, when you lay your hand on a
block of iron or marble, heat leaves your hand to enter the less warm
material and raise its temperature; and it is this abstraction of heat
that produces in you the sensation of cold.
There is, then, a constant communication or transmission of heat from
one body to another. This communication is effected chiefly in two
ways--by _conduction_ and by _radiation_. In conduction, the bodies are
in contact; in radiation, they are at some distance apart.
If you push one end of a cold poker into the fire, that end will soon
become warm, and the heat will be propagated from particle to particle
through the poker, until the end most distant from the fire becomes too
hot to be touched without injury. This mode of transmission is called
conduction. Different substances possess this power in very different
degrees. Thus, if instead of a poker you thrust into the fire a bar
of wood of equal length and thickness, you will find that, even when
the inserted end is in flames, the other remains comparatively cold,
and may be handled with impunity. Hence we say that iron is a _good_
conductor, and wood a _bad_ conductor of heat.
The conducting power of bodies depends in a great measure on the
closeness of their particles--dense, solid substances being much
better conductors than those which are light and porous. The metals
are the best conductors, but they differ very much among themselves.
The best is silver; the others stand in this respect in the following
order--copper, gold, brass, tin, iron, steel, lead.
You will now understand why metals feel cold to the touch: it is
because, being good conductors, they carry the heat rapidly away from
that part of our body with which they are in contact.
Among the bad conductors of heat are fur, wool, cotton, silk, and
linen; straw, paper, feathers, wood, earth, snow, water, and air; and
loose bodies, such as sawdust and shavings, which contain a large
amount of air in the spaces between their particles.
Our clothing, as you know, is made of wool, cotton, or linen. Can you
tell why such materials are selected for the purpose? It is not, as
many ignorant people suppose, because they are best adapted to _impart_
warmth. The true reason is that, being bad conductors, they prevent the
cold air and other objects around us from robbing us of the heat which
is produced within our bodies.
When once you understand what is meant by conduction of heat, and
can distinguish between substances which are good conductors and
those which are not, you will be able to give a reason for many facts
that must appear strange to every one who does not possess such
information. A little reflection, for instance, will enable you to
explain why a linen garment feels colder to the skin than one made of
cotton or wool; why a silver spoon becomes hot when the bowl is left
for a few minutes in a cup of hot liquid; why a metal tea-pot or kettle
is commonly furnished with a handle of wood or ivory; why ice may be
preserved by being wrapped in flannel or covered with sawdust; why a
pump, in frosty weather, should be encased in straw or matting; and why
the farmer welcomes the snow, and regards it as a protection to his
crops.
Every warm body has the power of sending out rays of heat, as a
luminous body gives out rays of light. This mode of communicating heat
is called _radiation_, and it serves, as we shall see, a very important
purpose in the economy of nature.
When you stand before a fire, heat-rays stream forth from the burning
fuel, and create in you the sensation of warmth. In this case, the heat
of the fire is communicated to you, not by conduction, but by radiation.
Everything in nature is constantly radiating heat from its surface. If
a body be surrounded by objects hotter than itself, it becomes heated
by radiation; if it be exposed to the influence of objects colder than
itself, it becomes cooled by radiation; and if the objects around it
are neither hotter nor colder than itself, its temperature remains
unaltered.
But though all bodies radiate heat, they have not all the same
radiating power. Some substances possess this power in a far greater
degree than others. The metals, though they are the best conductors,
are the worst radiators. This is particularly the case when they are
polished. Dull, dark substances, and especially those which have a
rough or scratched surface, are good radiators; light-colored and
smooth substances, on the other hand, are bad radiators; and this
explains why, as every good housewife knows, a polished metal tea-pot
keeps tea warmer than a black earthen one--it does not part with its
heat so readily.
Bodies which radiate freely have the power in an equal degree of
_absorbing_ heat; that is, they are as ready to take it in, as they
are to throw it out again. Dark substances, therefore, must be good
absorbers, as they are good radiators of heat. A very simple experiment
will illustrate this fact very clearly. If you spread upon snow, in a
place exposed to the sunbeams, two pieces of cloth of the same texture,
one black and the other white, you will find, after some time, that
under the black cloth the snow has been melted, but under the white
cloth it remains as it was at first. The black material has been heated
quickly and intensely; the white has not been heated at all: the former
has _absorbed_ the sun’s rays; the latter has _reflected_ them.
The laws of absorption and radiation enable us to explain many curious
facts. Thus, dark-colored clothes are cold in the shade, because they
are then radiating heat from our bodies; but they are warm in the
sunshine, because they are then absorbing the heat that falls upon them
from the sun. On the other hand, light colored clothes are warm in the
shade, and cool in the sunshine. Again, a dish-cover or metal tea-pot
is kept as brightly polished as possible, in order to prevent the
escape of the heat by radiation; a black earthenware tea-pot, on the
contrary, has a dull and dark surface, so that it may be placed on the
hob and absorb the heat. So, too, if a kettle is to heat quickly, the
bottom and sides should be covered with soot, to absorb the heat; while
the upper part should be bright, to prevent radiation.
It is radiation, also, that accounts for the deposition of dew. It is
a common error to suppose that dew falls in the same manner as rain or
mist, only in much finer particles. Dew, in fact, does not fall, but is
formed on the surface of bodies by the condensation of the moisture of
the atmosphere.
The air around us contains at all times a quantity of moisture in the
form of vapor. Now this vapor has been formed from water by the action
of heat; and it may again be turned into water by being brought in
contact with objects that are cold. And this is just what takes place
in the formation of dew. When the sun has set, the trees and grass and
other objects on the earth’s surface immediately begin to radiate the
heat which they have absorbed from its beams during the day. The best
radiators, of course, become cool most rapidly, and quickly condense
the vapor that floats in the air around them; and in the morning we
find those objects which radiate freely, such as blades of grass,
leaves of plants, and floating cobwebs, covered with this condensed
vapor in the form of glittering dewdrops.
Clouds, in a great measure, prevent radiation, and hence the dew will
be most plentiful on a clear and cloudless night. If the radiation
continues till the temperature of the ground is very low, the dew
freezes as it is deposited and forms _hoar-frost_.
You may see a smooth road or gravel walk quite dry in the morning,
while the grass or box by its side is thickly coated with moisture. Why
is this? It is simply because the road or walk is a bad radiator and
cools slowly, while the grass and box, being good radiators, become
rapidly cold and condense the vapor of the passing air into dew.
Thus by a wise arrangement, the cultivated fields receive an abundance
of precious moisture, while not a drop is wasted on the bare rock, or
the sterile sands of the desert.
_Word Exercise._
ster´ĭle
precious (_prĕsh´us_)
ĭg´no-rant
rā-di-ā´tion
pō´rous
il-lŭs´trāte
ăt´mos-phēre (_ăt´mos-fēr_)
text´ure (_tekst´yure_)
par´ti-cle
sen-sā´tion
se-lĕct´ed
dep-o-sĭ´tion (_-zĭsh´un_)
trăns-mis´sion (_trăns-mish´un_)
e-cŏn´o-my
lū´mi-noŭs
ab-sorb´ing
in´flu-ence
cŏn-dŭc´tion
ar-rānge´ment
_Phrase Exercise._
1. Have a _tendency to part_ with their heat.--2. Constant
communication.--3. Heat will be _propagated_.--4. May be
_handled_ with _impunity_.--5. Adapted to impart warmth.--6.
Regards it as a protection.--7. Exposed to the influence.--8.
Remains unaltered.--9. A simple experiment.--10. Heated
_intensely_.--11. Immediately begin to radiate.--12. Thickly
coated with moisture.--13. Wise arrangement.--14. Cultivated
fields.--15. Precious moisture.--16. _Wasted_ on the _bare_
rock.