SPECIFIC HEAT—FUSION—LATENT HEAT—CONDUCTION AND CONVECTION OF
HEAT—CALORESCENCE.
We have considered the effects of heat upon water, and touched upon one
or two kindred experiments. But we have some other subjects to discuss,
two in particular; viz., _Specific Heat_, and _Latent Heat_.
The specific heat of any substance is “the number of units of heat
required to raise one pound of such substance one degree.” We can
explain this farther. When heat is communicated to a body it has two or
three functions to perform. Some of it has to overcome the resistance
of the air in expanding the body, more of it expands, and the remainder
increases the temperature of the body. So some heat disappears as
heat, and is turned into energy,—“molecular potential energy,”—as it
is called, and the rest remains. Of course in objects the molecules
vary very much in weight and in their mutual attraction, and the heat
requisite to raise equal weights of different substances through
the same number of degrees of temperature will vary. This is called
capacity for heat, or specific heat. The capacity of different metals
for heat can easily be shown. The specific heat of water is very high,
because its capacity for heat is great. We can cool a hot iron in very
little water, and it takes thirty times as much heat to raise a given
weight of water a certain number of degrees, as it would to raise the
same weight of mercury to the same temperature. Water has greater
specific heat, generally speaking, than other bodies, and it is owing
to this circumstance that the climate is so affected by ocean currents.
Nearly all substances can be melted by heat, if we go far enough, or
frozen, if we could take the heat away. Solid can be made liquid, and
these liquids can be made gases and fly off in vapour. Similarly, if
we could only get heat away sufficiently from the atoms of a substance
we could freeze it. We cannot freeze alcohol, nor make ice from
air, nor can we liquify it, for we are unable to take away its heat
sufficiently. But we can turn water into steam, and into ice; or ice
into water, and then into steam. But there is one body we cannot melt
by heat, that is carbon. In the hottest fire coal will not melt, it
becomes soft. We call this melting _fusion_, and every body has its
melting point, or fusing point, which is the same at all times if the
air pressure be the same.
It is a curious fact that when a body is melting it rises to a certain
temperature (its fusing point), and then gets no hotter, no matter
whether or not the fire be increased;—all the extra heat goes to melt
the remainder of the substance. The heat only produces _changes of
state_. So this heat above fusing point disappears apparently, and is
called _Latent Heat_. This can easily be proved by melting ice. Ice
melts at 32° Fahr., or 0° Cent., and at that temperature it will remain
so long as any ice is left; but the water at 32°, into which the ice
has melted, contains a great deal of _latent_ heat, for it has melted
the ice quickly, and yet the thermometer does not show it. It is just
the same with boiling water.
When substances are fused they expand as a rule, but ice contracts;
so does antimony. On the other hand, when water solidifies it does
not contract as most things do. It expands, as many of us are aware,
by finding our water pipes burst in the winter; and the geologist
will tell us how the tiny trickling rills of water fall in between
the cracks of rocks and there freeze. In freezing the drops expand
and split the granite blocks. Type-metal expands also when it becomes
solid, and leaves us a clear type; but copper contracts, and won’t do
for moulding, so we have to stamp it when we want an impression on it.
There is no doubt that chemical combinations produce heat, as we can
see every day in house-building operations, when water is poured upon
lime; but there are also chemical combinations which produce cold.
Fahrenheit produced his greatest cold by combining snow and salt, for
in the act of combining, a great quantity of heat is swallowed up by
reason of the heat becoming latent, as it will do when solid bodies
become liquid. Such mixtures or combinations are used as _Freezing
Mixtures_ when it is necessary to produce intense cold artificially.
Sulphate of Sodium and Hydrochloric acid will also produce great cold,
and there are many other combinations equally or even more efficacious.
Heat is communicated to surrounding objects in three well-known
ways—by conduction, by radiation, and by convection. Conduction of
heat is easily understood, and is the propagation of heat through
any body, and it varies very much according to the substance through
which it passes. Some substances are better conductors of heat than
others. Silver has a far greater conductivity than gold, and copper
is a better heat-conductor than tin. Flannel is a non-conductor, or
rather a bad conductor, for no substance can be termed actually a
non-conductor. Flannel, we know, will keep ice from melting, and a
sheep’s wool or a bird’s feathers are also bad conductors of heat; so
Nature has provided these coverings to keep in the animal heat of the
body. A good conductor of heat feels cold to the touch of our fingers,
because it takes the heat from our hands. This can be tried by touching
silver, lead, marble, wood, and wool. Each in turn will feel cold and
less cold, because they respectively draw away, or conduct less and
less heat from our bodies. So our clothes are made of bad-conducting
substances. The bark of a tree is a bad conductor, and if you strip off
this clothing the tree will die.
Solids conduct heat the better the more compact they are. Air being a
bad conductor it follows that the less tightly the molecules are packed
the less conductibility there will be; and even a substance powdered
will be a worse conductor than the same substance in solid form; and
also more readily in the direction of the fibres than crossways.
Liquids do not possess great conductivity, but they, as well as gases,
are influenced by _convection_, or the transport of heat from the
bottom layers to the top (_conveho_, to carry up). We have already
mentioned that the heated particles of water rise to the top because
they expand, and so become lighter. This is convection of heat; and
by it liquids and gases, though actually bad conductors, may become
heated throughout to a uniform temperature. Of course the more easily
expansible the body is the more rapidly will convection take place—so
gases are more readily affected than liquids. Solids are not affected,
because convection of heat depends upon molecular movement or mobility,
and it is obvious that the particles of solid bodies are not mobile.
Professor Balfour Stewart says with reference to this that “were there
no gravity there would be no convection,” for the displacement of
the light warm particles by the heavier cold ones is due to gravity.
The instances of convection of heat in nature are numerous, and on a
gigantic scale. The ocean currents, trade winds, lake freezing, etc.,
while the chimney draught already referred to, is another example;
and in all these cases the particles of air or water are replaced by
convection. In the case of the lake freezing the cold particles at
the top sink, and the warmer ones ascend, until all the lake is at a
temperature of 36·2°, or say 4° above freezing. At this temperature
water assumes its maximum density, and then _expands_, as we have seen,
instead of contracting. Ice is formed, and being thus lighter than
water, floats; and so unites to cover in the water underneath, which is
never frozen solid, because the cold of the atmosphere cannot reach it
through the ice in time to solidify the whole mass.
[Illustration: Fig. 84.—Radiant heat.]
Radiant heat is the motion of heat transmitted to the ether, and
through it in the form of waves. The sun’s heat is radiant heat, and
radiation may be defined as “The communication of the motion of heat
from the articles of a heated substance to the ether.” The fire gives
out radiant heat, and so does heated metal, and it is transmitted by an
unseen medium. It is quite certain that the heat of a suspended red-hot
poker is not communicated to the air, because it will cool equally in
a vacuum. Sir Humphrey Davy proved that radiant heat could traverse a
vacuum, for by putting tin reflectors in an exhausted receiver he found
that a hot substance in the focus of one reflector caused an increase
in the heat of the other. If we put a red-hot or a hot substance in
one reflector, and tinder in the other, the latter will take fire.
The velocity of heat rays is equal to that of light, 186,000 miles in
a second, and indeed, radiant heat is identical with light. Heat is
reflected as is light, and is refracted in the same way as sound.
Some bodies allow the heat rays to pass through them, as air does, and
as rock salt will do. White clothing is preferable in summer (and also
in winter if we could only make people believe it). White garments
radiate less heat in winter, and absorb less heat in summer. An old
black kettle will boil water more quickly than a new bright one, but
the latter will keep the water hotter for the longer time when not on
the fire.
Heat, then, is movement of particles. Energy can be changed into heat,
as the savage finds when he rubs the bits of wood to produce heat and
fire. Friction causes heat, and chemical combination produces heat;
and, if “visible energy can be turned into heat, heat can be turned
back into visible energy.” For fire heats water, water expands into
steam, and steam produces motion and energy in the steam-engine.
If we heat water in Wollaston’s bulb,—the opening of which is
hermetically stopped by a piston,—the vapour will raise the piston. If
we cool the bulb we condense the steam, and the piston falls. Here we
have the principle of the steam-engine.
STEAM is the vapour of water educed by heat, and we may
give a few particulars concerning it. Its mechanical properties are
the same as those of other gases, and pure steam is colourless and
transparent—in fact, invisible. Its power when confined in boilers
and subjected to pressure is enormous, for the volume of the steam is
far greater than the water which gave rise to it. One cubic inch of
water will produce 1,700 cubic inches of steam—in other words, a cubic
inch of water produces a cubic foot of steam. When we obtain steam at
212°, we do so under the pressure of one atmosphere; but by increasing
the pressure we can raise the boiling point, and thus water at the
pressures of sixteen atmospheres will not steam till it reaches 398°.
It is thus we obtain pressure for locomotives, and other engines,
although a very small portion of the steam does work. Much the largest
portion is expended in overcoming cohesion, and one way and another,
taking into consideration defects in machinery, only about one-tenth of
the heat is employed in doing the work. The force exercised by steam
under atmospheric pressure is sufficient to raise a ton weight one foot.
To obtain very high temperatures we shall find the thermometer of no
use, for mercury boils at 662°, so an instrument called a Pyrometer
is used to ascertain the fusing point of metals. Mr. Wedgwood, the
celebrated china manufacturer, invented an instrument made of small
cylinders of clay moulded and backed, placed between two brass rods
as gauges divided into inches and tenths. But this instrument has
been long superseded by Professor Daniell’s Pyrometer, which consists
of a small bar of platina in an earthenware tube. The difference of
expansion between the platina and the tube is measured on a scale on
which one degree is equal to seven degrees of Fahrenheit. Thus the
melting temperatures of metals are ascertained.
The reflection and refraction of heat are ruled by the same laws as
the reflection and refraction of light. A convex lens will bring the
heat or light to a focus, and will act as a burning-glass if held in
the sunlight. Gunpowder has been ignited by a lens of ice, and more
than one house has been mysteriously set on fire at midday in summer
by the sun’s rays shining through a glass globe of water containing
gold fish, and falling upon some inflammable substance. Professor
Tyndall performed a series of experiments of a very interesting nature,
described in his book, “Heat considered as a Mode or Motion,” and
showed the transmutation of invisible heat rays into visible rays,
by passing a beam of electric light through an opaque solution, and
concentrating it upon a lens. The dark heat rays were thus brought to
a focus, all the light was cut off, and at the dark focus the heat was
found to be intense enough to melt copper and explode gunpowder. This
change of invisible heat into light is termed Calorescence.
It was Sir William Herschell who discovered that there were heat
rays beyond the red end of the spectrum. When light is split up into
its component rays, or decomposed, Sir William found that the heat
increased as the thermometer passed from violet to indigo, and so on
to blue, green, orange, and red, and the last were the hottest, while
beyond the spectrum there was heat even greater. A _Heat Spectrum_
was thus discovered, and by comparing, by means of the thermometer,
the various degrees of heat within certain limits, Professor Tyndall
found that the invisible Heat Spectrum is longer than the visible Light
Spectrum.