CONVEYANCE AND VARIETIES OF HEAT.
"To-day we review the modes in which heat passes or is conveyed from place
to place. It is evident that if heat were confined to the very place or
point where it is generated, it could subserve none of those uses to which
it is now applied in the economy of Nature or in the works and arts of
man. But heat passes from place to place with great facility, and by one
method, with the speed of light, it tends to diffuse itself evenly through
all; it seeks an equilibrium. The modes of its diffusion, or conveyance,
are three in number. Ansel may name them."
"Heat passes from place to place and from body to body by 'conduction,' by
'radiation,' and by 'convection.'"
"What is meant, Ansel, by the 'conduction' of heat?"
"The passing of heat from atom to atom and from particle to particle
through a body is called conduction."
"That is right. I will call upon Peter to give some illustrations of the
conduction of heat."
"The examples are so many," Peter answered, "that I hardly know what to
mention first. If I hold a pin in the flame of a lamp, the part of the pin
that touches the flame is first heated, but soon the heat runs along the
whole length of the pin and burns my fingers. The parts of a stove which
touch the fire are first heated, and from them the heat spreads through
the whole stove. A pine-wood shaving, kindled at one end, is heated by
conduction, but the heat passes through it very little faster than the
flame follows. Heat escapes from our bodies by being slowly conducted
through our clothing. There is no end to the examples of conduction which
one might give."
"We must not think of the conduction of heat," said Mr. Wilton, "as if it
were a fluid slowly absorbed by a porous body, as water poured upon the
ground soaks into it, or as water percolates through a lump of sugar and
moistens the whole of it. We must remember that the transfer of heat is
not a transfer of any substance, but a transfer of motion. One atom is set
in motion, and strikes against another atom and sets that in motion, and
thus motion is communicated from atom to atom and from molecule to
molecule through the whole mass of matter till every atom is agitated with
the heat vibrations. Do all bodies conduct heat with equal rapidity?"
"No, sir," replied Ansel; "there is the greatest possible difference. Some
substances are called good conductors, because heat permeates them so
readily and rapidly; others conduct heat very slowly, and are called poor
conductors or bad conductors."
"That is right. Every child soon learns by experience to make a practical
distinction of this kind. He very soon understands that he can hold a
stick of wood without burning his hand, even though it be blazing at the
other end, but that when a piece of iron is red hot at one end he must not
take hold of it at the other. The child very soon learns to know the
different feeling of a cotton night-gown from one of flannel, and the
difference in apparent warmth between a linen pillow-case and a woolen
blanket. After a room has been heated for a considerable time the various
objects in it all become of the same temperature, and the same is true in
a cold room; but how great the difference in the sensations produced by
touching the oil-cloth and a woolen carpet in a cold room! Good conductors
of heat, if hot, feel very hot; or if cold, feel very cold; while poor
conductors make a much less decided impression. Why is this, Samuel?"
"The good conductors receive heat or part with it very readily. If the
good conductor be hotter than our bodies, it imparts its heat rapidly to
our hand, and because we receive heat rapidly from it, it feels to us very
hot. Or if it be colder than our bodies, it takes heat from our hands very
rapidly, and gives the impression of being very cold. Poor conductors
impart heat to the skin or take it away more slowly, and hence feel as if
their temperature were more nearly like that of the body."
"The conducting qualities of bodies," said Mr. Wilton, "seem to depend
chiefly upon their structure or the arrangement of their atoms. Bodies
which are compact and solid in their structure convey heat more rapidly
than those which are loose and porous. Hence solids are better conductors
than fluids, and fluids are better conductors than gases, and among solids
the metals are better conductors than organized bodies, like wood or
flesh, and better than the loose and porous minerals. In bodies of loose,
porous, or fibrous texture, the continuity of the conductory substance is
constantly broken. The particles in a mass of sawdust touch only at a few
points, leaving frequent spaces. In woolen and cotton fabrics the points
of junction of the fibres are very few, comparatively. For this reason the
motion is not readily communicated from atom to atom.
"The crystalline arrangement of atoms has an influence upon conduction of
heat. Heat is conducted more rapidly in a direction parallel with the axis
of crystallization than across that axis. Wood conducts heat more rapidly
in the direction of the grain. This arrangement seems to be well adapted
for keeping trees warm in winter. Their roots reach down into the earth,
which remains warm in the coldest weather. This heat of the earth travels
along the fibres up through the tree, while the heat conducted across the
fibres escapes much more slowly into the open air. The bark also, being a
very bad conductor, hinders the escape of heat. Of metals, silver is the
best conductor. I will give you a brief table which will show the great
difference in the conducting qualities of some of the metals. Counting the
conducting qualities of silver as 100, the table is: 'Silver, 100; Gold,
53; Copper, 74; Iron, 12; Platinum, 8; German Silver, 6; Bismuth,
2.'--_Youmans._
"What is the second method by which heat passes from place to place?"
"It is radiated," replied Ansel.
"And what is radiation?"
"It is motion in straight lines or rays diverging from a centre. From a
hot body heat is passing off in straight lines in every direction. As a
lamp radiates light, so does a hot body radiate heat."
"Radiant heat," said Mr. Wilton, "moves with the same velocity as light,
that is, one hundred and ninety-two thousand miles per second. It also
follows the same general principles as light in all its motions. It is
absorbed, reflected, or transmitted in the same manner as light. And this
is true of either luminous heat--that is, heat radiated from a body which
is red hot--or obscure, or dark heat.
"As there are good and poor conductors, so there are good and bad
radiators of heat. The radiation of heat depends upon three conditions:
"1. Upon the temperature of the body. The higher the temperature, the more
rapid and energetic is its radiation.
"2. Upon the surface of the radiating body. A dull, rough surface radiates
heat more rapidly than a surface bright and polished.
"3. Upon the substance of the radiating surface. With surfaces equally
smooth and bright, some substances radiate heat much better than others. A
surface of varnish radiates heat much more powerfully than a surface of
gold or silver.
"Ansel, you may, if you can, explain the radiation of heat."
"I can give no other explanation than that radiation is conduction through
that subtle ether which is supposed to pervade all space."
"Very well; perhaps that is as good an explanation as can be given. But it
seems rather like the propagation of an impulse than the spreading of
atomic vibrations in every direction. The motion is propagated in
straight lines. If it be conduction, it must be carried on by different
vibrations from those of ponderable substances. Heat, light, and
electricity are supposed to be all propagated through the same theoretical
ether. Sir Isaac Newton estimated the density of the ether as seventy
thousand times less than the density of our atmosphere, and its elasticity
in proportion to its density as four hundred and ninety millions times
greater. But the very existence of this universally-diffused ether is a
supposition made to account for the phenomena of light, heat, and
electricity; and, of course, all its qualities must be theoretical also.
Radiation is believed to be the propagation of a motion or impulse through
an inconceivably rare and elastic ether.
"Peter, what is the third method by which heat passes from place to
place?"
"Convection," was his reply.
"What is meant by convection of heat?"
"The conveyance of heat by carrying a heated body. If I remove a hot iron
or a kettle of hot water, I must of course carry the heat which it
contains."
"A very good illustration of the convection of heat," said Mr. Wilton,
"is seen in the common method of heating water. The heat is applied at the
bottom of the vessel containing the water; as fast as the water at the
bottom next the fire is heated, it rises and carries the heat to the top;
cold water comes to take its place, and this in turn is heated and rises
and carries heat to the top. This process is carried on till all the water
comes to the same temperature. Thus water is heated by convection of heat.
"A grander illustration is seen in winds and ocean currents. Warm winds
carry heat enough to warm a continent, and the mighty ocean currents are
still more efficient in transferring heat from one part of the earth to
another.
"Another point we need to understand. When radiant heat falls upon a body,
what becomes of it?"
"It is disposed of," answered Samuel, "in one of three ways: it may be
reflected according to the same principles by which light is reflected; or
it may be transmitted, that is, pass through the body; or it may be
absorbed, that is, stop in it."
"Very well stated, Samuel. In regard to reflection I need to say very
little. You know how light is reflected from a polished surface, such as
a lamp reflector: heat is reflected in the same manner. One fact you must
bear in mind touching reflected heat: it does not heat the reflecting
body.
"There is no need of telling you that light passes through certain
substances. It passes through gases and through some liquids and some
solids. The best of glass, though it is so solid, interposes very little
hindrance to the passage of light. Heat in like manner radiates through
certain solids. Luminous heat is radiated through glass. Rock-salt
transmits dark heat also. A plate of alum permits light to pass, but stops
both luminous heat and dark heat. Remember that transmitted heat, as was
said of reflected heat, does not heat the body through which it passes. I
have seen boys make burning-glasses of ice. The heat passes through them
and burns that upon which it is concentrated, while the ice itself through
which the heat passes is not melted.
"If a body have a good radiating surface, that is, if its surface be dull
and rough, the heat which falls upon it will be mostly absorbed. The
reflecting and absorbing qualities hold an inverse ratio to each other;
the better the reflecting qualities, the worse the absorbing, and the
worse the reflecting, the better the absorbing. Heat which is absorbed by
a body commonly raises its temperature, and remains in the body till it is
slowly radiated or is conducted away by the air or other bodies which come
in contact with it.
"What is that heat called, Ansel, which is absorbed by a body with no rise
of temperature?"
"It is called _latent_ heat."
"That is the old and common expression, but what is meant by latent heat?"
"The word _latent_ signifies _lying hidden_ or _concealed_. Latent heat,
as you suggested in your first question, is that heat which a body
receives without showing it by a change of temperature."
"That name 'latent heat,'" said Mr. Wilton, "expresses the opinion of
those who invented it; they supposed that heat was in some manner hidden
in certain bodies. We must not suppose, however, that this latent heat
continues to exist in bodies as heat; latent heat is that heat which is
converted into force or some other motion than the atomic heat vibrations,
and is employed otherwise than in raising the temperature. You will
understand this best by an illustration.
"Take one hundred pounds of ice at the temperature of thirty-two degrees,
that is, as warm as is possible without melting. That one hundred pounds
of ice will absorb heat which would raise one hundred pounds of ice water
through one hundred and forty degrees, and by receiving that heat it is
melted, but the water produced has the temperature of thirty-two degrees.
It has received one hundred and forty degrees of heat, but its temperature
is not raised a single degree. This one hundred and forty degrees of heat
has been transmuted into force and employed in overcoming the crystalline
attraction of the atoms of water.
"Let that ice water at thirty-two degrees of temperature receive one
hundred and eighty degrees of heat, and the water rises to two hundred and
twelve degrees, the temperature of boiling. But whatever additional heat
is absorbed brings no increase of temperature, but transforms the water
into steam. It is employed in overcoming the cohesive attraction of the
molecules of water and changing the liquid to a gas. About one thousand
degrees of heat is thus expended, but the steam which is produced has
only the temperature of two hundred and twelve degrees. If the process be
reversed, the steam gives up, as it is said, the one thousand degrees of
heat in returning to the condition of water and the one hundred and forty
degrees in resuming the crystalline structure of ice. The heat which was
employed as force in overcoming the atomic and molecular attractions is
transmuted again to heat, and shows itself in raising the temperature. And
that which is true of water is true of any other substance in changing its
form from a solid to a liquid or from a liquid to a gas, or the opposite.
In an amount different for each kind of matter, in all these changes of
condition, heat is transmuted to force or force to heat.
"These transmutations are going on ceaselessly in the operations of
Nature, and without understanding them we cannot appreciate the wonderful
operations of heat in the world. The heat of the sun beams upon the ocean;
the greater part of that heat is expended as force in overcoming the
molecular attraction of water, thus converting it to vapor, and in raising
that vapor to the higher regions of the atmosphere. This heat-force, or,
as we might call it, 'sunpower,' expended upon the earth, amounts to
thousands of millions of horse-power daily.
[Illustration: TRANSMUTATION of HEAT.
Page 113.]
"Examples of the transmutation of force into heat abound everywhere. A boy
strikes his heel upon the stone pavement; from the point of contact
between the stone and the steel points in his boot heel sparks of fire fly
out. Force is changed to heat so intense that particles of steel are set
on fire. Savages who have no better methods of kindling fire rub dry wood
together till the sticks ignite. The force expended in overcoming the
friction is changed to heat. In the combustion of coal beneath the steam
boiler we see both processes going on. The atoms of carbon dash against
the atoms of oxygen, and the force of the collision generates the heat of
the combustion. This heat, born thus of force, is again transmuted to
force, and drives the engine and the machinery attached. In our study of
God's management of heat we shall constantly meet with these changes. You
will need, therefore, to study carefully this subject of latent heat.
"Dr. Joule, of Manchester, England, has discovered the ratio between heat
and force, that is, the amount of force which by transmutation produces
any given amount of heat. The force of a one-pound weight which has fallen
one foot is taken as the unit of force, and the amount of heat which is
required to raise one pound of water one degree is taken as the unit of
heat. By many and various careful experiments, Dr. Joule demonstrated that
772 units of force are the equivalent of one unit of heat. A pound weight
falling 772 feet, or 772 pounds falling one foot, and then arrested,
produces heat sufficient to raise one pound of water one degree. The
result is the same whatever the method by which the force is expended. If
water be agitated or shaken, if sticks of wood or iron plates be rubbed
together, if an anvil be struck with a hammer, or if a bar of iron or
copper be moved back and forth between the poles of an electromagnet, the
force expended is changed to heat. You must remember, however, that force
becomes heat only so far as the force is actually expended, or used up so
that it no longer exists as force.
"These conclusions are supported by other beautiful experiments. 'An
electric current which, by resistance in passing through an imperfect
conductor, produces heat sufficient to raise one pound of water one
degree, sets free an amount of hydrogen which, when burned, raises exactly
one pound of water one degree. Again, the same amount of electricity will
produce an attractive magnetic force by which a weight of 772 pounds may
be raised one foot high.'--_Youmans._ We conclude from experiments like
these that heat, mechanical force, and electricity are interchangeable
forces; they may be transmuted the one into another.
"By this principle of the transmutation of heat and mechanical force we
explain the production of heat by compression and the loss of heat by
expansion. Samuel, you may state the fact upon this point."
"If any substance be suddenly compressed," answered Samuel, "heat appears;
if it be expanded, cold is produced. Since gases expand or yield to
pressure so readily, they furnish the best illustration of this
principle."
"The suddenness of the compression or expansion," said Mr. Wilton, "is a
matter of no consequence. The effect is the same whether the operation be
sudden or slow, but if the compression or expansion be slow, the heat or
cold generated is less apparent; the heat is dissipated as fast as
produced and the colder gas is warmed by the vessel which contains it.
Ansel, how shall we explain this?"
"I cannot explain it, sir."
"The explanation is very simple," said Mr. Wilton. "Mechanical force is
employed in the compression of the gas; the force is expended and used up
upon the gas, and appears again in the form of atomic heat motion. In the
expansion of gases the operation is just the reverse; the atomic heat
motion is expended in producing expansion, and hence disappears as heat.
The general principle is that no force can be expended in two ways at the
same time.
"One other point we must notice to-day, that is, _specific heat_. What is
understood, Ansel, by this term, specific heat?"
"The relative amount of heat which different substances require to raise
their temperature through any given number of degrees."
"That is right. I think that you all must have noticed that it requires
much more heat to raise the temperature of some bodies than others. What
an amount of heat is required to raise the temperature of water! That heat
which will raise one pound of water one degree will cause an equal
increase of temperature in five pounds of sulphur, or four pounds of air,
or nine pounds of iron, or eleven pounds of copper, or thirty pounds of
mercury, lead, or gold. This is what is meant by saying that one substance
has a greater capacity for heat than another. The specific heat of water
is greater than that of any other known substance except hydrogen gas.
This fact, taken in connection with its great specific latent heat and its
poor conducting qualities, renders it exceedingly important in regulating
climate and moderating extremes of temperature; of this you will be
reminded very often as our lessons go on.
"No law or principle determining the specific heat of the various elements
and explaining the different capacities for heat has as yet been
discovered. It has been suggested that specific heat depends upon the
number of atoms, that it holds an inverse ratio to their combining
numbers, or, what is the same thing, a direct ratio to the number of
atoms. This would harmonize well with the dynamic theory of heat, but the
harmony between the specific heat of substances and the number of atoms is
not sufficiently uniform to establish this supposition.
"This completes our review of first principles. I hope that this not very
entertaining review of your academic studies has not wearied you of the
very word _heat_ and worn out your interest in examining God's management
of heat before making a beginning."
"I think," said Samuel, "that we are not in the habit of becoming
disgusted with our studies."
"You may expect," continued Mr. Wilton, "if the past has been interesting
to you, that the lessons to come will prove more interesting still. Next
week we shall consider the abundant provision which the Creator has made
for warming the earth."
And let me say to you, patient reader, that if I had known that you were
as familiar with the laws and principles of heat as Ansel, Peter, and
Samuel seem to have been, this and the preceeding chapter would not have
been written. However dull this review may have seemed to you, it was
needful, perhaps, for others, that they might understand the wonderful
works of God which we shall now proceed to examine. And, reader, do not
forget that heat itself, that subtle motion and mighty force, with all its
laws and principles, is one of God's works. Already have we been looking
at the Creator's handiwork. Already have we been trying to trace out the
thoughts of God as they are written in the "Bible of Nature." The thoughts
of God are great and wonderful. It has been useful and interesting to read
thus far in this book written with the finger of the Creator of worlds and
of man, even if we turn not another page.