Terrestrial Heat—Radiation—Transmission—Melloni’s experiments—Heat
in Solar Spectrum—Polarization of Heat—Nature of
Heat—Absorptions—Dew—Rain—Combustion—Expansion—Compensation
Pendulum—Transmission through Crystals—Propagation—Dynamic Theory
of Heat—Mechanical equivalent of Heat—Latent Heat is the Force of
Expansion—Steam—Work performed by Heat—Conservation of
Force—Mechanical Power in the Tides—Dynamical Power of
Light—Analogy between Light, Heat, and Sound.
THAT heat producing rays exist independently of those of light is a
matter of constant experience in the abundant emission of them from
boiling water. They dart in divergent straight lines from flame and from
each point in the surfaces of hot bodies, in the same manner as
diverging rays of light proceed from every point of those that are
luminous. According to the experiments of Sir John Leslie, radiation
proceeds not only from the surface of substances, but also from the
particles at a minute depth below it. He found that the emission is most
abundant in a direction perpendicular to the radiating surface, and that
it is more rapid from a rough than from a polished surface: radiation,
however, can only take place in air and in vacuo; it is altogether
imperceptible when the hot body is enclosed in a solid or liquid. Heated
substances, when exposed to the open air, continue to radiate heat till
they become nearly of the temperature of the surrounding medium. The
radiation is very rapid at first, but diminishes according to a known
law with the temperature of the heated body. It appears, also, that the
radiating power of a surface is inversely as its reflecting power; and
bodies that are most impermeable to heat radiate least. Substances,
however, have an elective power, only reflecting heat of a certain
refrangibility. Mr. Grove gives paper, snow, and lime as instances,
which, although all white, radiate heat of different refrangibilities,
while metals, whatever their colour may be, radiate all kinds alike.
Rays of heat, whether they proceed from the sun, from flame, or other
terrestrial sources, luminous or non-luminous, are instantaneously
transmitted through solid and liquid substances, there being no
appreciable difference in the time they take to pass through layers of
any nature or thickness whatever. They pass also with the same facility
whether the media be agitated or at rest; and in these respects the
analogy between light and heat is perfect. Radiant heat passes through
the gases with the same facility as light; but a remarkable difference
obtains in the transmission of light and heat through most solid and
liquid substances, the same body being often perfectly permeable to the
luminous, and altogether impermeable to the calorific rays. For example,
thin and perfectly transparent plates of alum and citric acid sensibly
transmit all the rays of light from an argand lamp, but stop eight or
nine tenths of the concomitant heat; whilst a large piece of brown
rock-crystal gives a free passage to the radiant heat, but intercepts
almost all the light. Alum united to green glass is also capable of
transmitting the brightest light, but it gives not the slightest
indication of heat; while rock-salt covered thickly over with soot, so
as to be perfectly opaque to light, transmits a considerable quantity of
heat. M. Melloni has established the general law in uncrystallized
substances such as glass and liquids, that the property of
instantaneously transmitting heat is in proportion to their refractive
powers. The law, however, is entirely at fault in bodies of a
crystalline texture. Carbonate of lead, for instance, which is
colourless, and possesses a very high refractive power with regard to
light, transmits less radiant heat than Iceland spar or rock-crystal,
which are very inferior to it in the order of refrangibility; whilst
rock-salt, which has the same transparency and refractive power with
alum and citric acid, transmits six or eight times as much heat. This
remarkable difference in the transmissive power of substances having the
same appearance is attributed by M. Melloni to their crystalline form,
and not to the chemical composition of their molecules, as the following
experiments prove. A block of common salt cut into plates entirely
excludes calorific radiation; yet, when dissolved in water, it increases
the transmissive power of that liquid: moreover, the transmissive power
of water is increased in nearly the same degree, whether salt or alum be
dissolved in it; yet these two substances transmit very different
quantities of heat in their solid state. Notwithstanding the influence
of crystallization on the transmissive power of bodies, no relation has
been traced between that power and the crystalline form.
The transmission of radiant heat is analogous to that of light through
coloured media. When common white light passes through a red liquid,
almost all the more refrangible rays, and a few of the red, are
intercepted by the first layer of the fluid; fewer are intercepted by
the second, still less by the third, and so on: till at last the losses
become very small and invariable, and those rays alone are transmitted
which give the red colour to the liquid. In a similar manner, when
plates of the same thickness of any substance, such as glass, are
exposed to an argand lamp, a considerable portion of the radiant heat is
arrested by the first plate, a less portion by the second, still less by
the third, and so on, the quantity of lost heat decreasing till at last
the loss becomes a constant quantity. The transmission of radiant heat
through a solid mass follows the same law. The losses are very
considerable on first entering it, but they rapidly diminish in
proportion as the heat penetrates deeper, and become constant at a
certain depth. Indeed, the only difference between the transmission of
radiant heat through a solid mass, or through the same mass when cut
into plates of equal thickness, arises from the small quantity of heat
that is reflected at the surface of the plates. It is evident,
therefore, that the heat gradually lost is not intercepted at the
surface, but absorbed in the interior of the substance, and that heat
which has passed through one stratum of air experiences a less
absorption in each of the succeeding strata, and may therefore be
propagated to a greater distance before it is extinguished. The
experiments of M. de Laroche show that glass, however thin, totally
intercepts the obscure rays of heat when they flow from a body whose
temperature is lower than that of boiling water; that, as the
temperature increases, the calorific rays are transmitted more and more
abundantly; and, when the body becomes highly luminous, that they
penetrate the glass with perfect ease. The extreme brilliancy of the sun
is probably the reason why his heat, when brought to a focus by a lens,
is more intense than any that has been produced artificially. It is
owing to the same cause that glass screens, which entirely exclude the
heat of a common fire, are permeable by the solar heat.
The results obtained by M. de Laroche have been confirmed by the
experiments of M. Melloni on heat radiated from sources of different
temperatures, whence it appears that the calorific rays pass less
abundantly not only through glass, but through rock-crystal, Iceland
spar, and other diaphanous bodies, both solid and liquid, according as
the temperature of their origin is diminished, and that they are
altogether intercepted when the temperature is about that of boiling
water.
In fact, he has proved that the heat emanating from the sun or from a
bright flame consists of rays which differ from each other as much as
the coloured rays do which constitute white light. This explains the
reason of the loss of heat as it penetrates deeper and deeper into a
solid mass, or in passing through a series of plates; for, of the
different kinds of rays which dart from a vivid flame, all are
successively extinguished by the absorbing nature of the substance
through which they pass, till those homogeneous rays alone remain which
have the greatest facility in passing through that particular substance;
exactly as in a red liquid the violet, blue, green, orange, and yellow
rays are extinguished, and the red are transmitted.
M. Melloni employed four sources of heat, two of which were luminous and
two obscure; namely, an oil-lamp without a glass, incandescent platina,
copper heated to 696°, and a copper vessel filled with water at the
temperature of 178-1/2° of Fahrenheit. Rock-salt transmitted heat in the
proportion of 92 rays out of 100 from each of these sources; but all
other substances pervious to radiant heat, whether solid or liquid,
transmitted more heat from sources of high temperature than from such as
are low. For instance, limpid and colourless fluate of lime transmitted
in the proportion of 78 rays out of 100 from the lamp, 69 from the
platina, 42 from the copper, and 33 from the hot water; while
transparent rock-crystal transmitted 38 rays in 100 from the lamp, 28
from the platina, 6 from the copper, and 9 from the hot water. Pure ice
transmitted only in the proportion of 6 rays in the 100 from the lamp,
and entirely excluded those from the other three sources. Out of 39
different substances, 34 were pervious to the calorific rays from hot
water, 14 excluded those from the hot copper, and 4 did not transmit
those from the platinum.
Thus it appears that heat proceeding from these four sources is of
different kinds: this difference in the nature of the calorific rays is
also proved by another experiment, which will be more easily understood
from the analogy of light. Red light, emanating from red glass, will
pass in abundance through another piece of red glass, but it will be
absorbed by green glass; green rays will more readily pass through a
green medium than through one of any other colour. This holds with
regard to all colours; so in heat. Rays of heat of the same intensity,
which have passed through different substances, are transmitted in
different quantities by the same piece of alum, and are sometimes
stopped altogether; showing that rays which emanate from different
substances possess different qualities. It appears that a bright flame
furnishes rays of heat of all kinds, in the same manner as it gives
light of all colours; and, as coloured media transmit some coloured rays
and absorb the rest, so bodies transmit some rays of heat and exclude
the others. Rock-salt alone resembles colourless transparent media in
transmitting all kinds of heat, even that of the hand, just as they
transmit white light, consisting of rays of all colours. Radiant heat is
unequally refracted by a prism of rock-salt like light, and the rays of
heat thus dispersed are found to possess properties analogous to the
rays of the coloured spectrum.
The property of transmitting the calorific rays diminishes to a certain
degree with the thickness of the body they have to traverse, but not so
much as might be expected. A piece of very transparent alum transmitted
three or four times less radiant heat from the flame of a lamp than a
piece of nearly opaque quartz about a hundred times as thick. However,
the influence of thickness upon the phenomena of transmission increases
with the decrease of temperature in the origin of the rays, and becomes
very great when that temperature is low. This is a circumstance
intimately connected with the law established by M. de Laroche; for M.
Melloni observed that the difference between the quantities of heat
transmitted by the same plate of glass, exposed successively to several
sources of heat, diminished with the thinness of the plate, and vanished
altogether at a certain limit; and that a film of mica transmitted the
same quantity of heat, whether it was exposed to incandescent platinum
or to a mass of iron heated to 360°.
Coloured glasses transmit rays of light of certain degrees of
refrangibility, and absorb those of other degrees. For example, red
glass absorbs the more refrangible rays, and transmits the red, which
are the least refrangible. On the contrary, violet glass absorbs the
least refrangible, and transmits the violet, which are the most
refrangible. Now M. Melloni has found, that, although the colouring
matter of glass diminishes its power of transmitting heat, yet red,
orange, yellow, blue, violet, and white glass transmit calorific rays of
all degrees of refrangibility; whereas green glass possesses the
peculiar property of transmitting the least refrangible calorific rays,
and stopping those that are most refrangible. It has therefore the same
elective action for heat that coloured glass has for light, and its
action on heat is analogous to that of red glass on light. Alum and
sulphate of lime are exactly opposed to green glass in their action on
heat, by transmitting the most refrangible rays with the greatest
facility.
The heat which has already passed through green or opaque black glass
will not pass through alum, whilst that which has been transmitted
through glasses of other colours traverses it readily.
By reversing the experiment, and exposing different substances to heat
that had already passed through alum, M. Melloni found that the heat
emerging from alum is almost totally intercepted by opaque substances,
and is abundantly transmitted by all such as are transparent and
colourless, and that it suffers no appreciable loss when the thickness
of the plate is varied within certain limits. The properties of the heat
therefore which issues from alum nearly approach to those of light and
solar heat.
Radiant heat in traversing various media is not only rendered more or
less capable of being transmitted a second time, but, according to the
experiments of Professor Powell, it becomes more or less susceptible of
being absorbed in different quantities by black or white surfaces.
M. Melloni has proved that solar heat contains rays which are affected
by different substances in the same way as if the heat proceeded from a
terrestrial source; whence he concludes that the difference observed
between the transmission of terrestrial and solar heat arises from the
circumstance of solar heat containing all kinds of heat, whilst in other
sources some of the kinds are wanting.
Radiant heat, from sources of any temperature whatever, is subject to
the same laws of reflection and refraction as rays of light. The index
of refraction from a prism of rock-salt, determined experimentally, is
nearly the same for light and heat.
Liquids, the various kinds of glass, and probably all substances,
whether solid or liquid, that do not crystallize regularly, are more
pervious to the calorific rays according as they possess a greater
refractive power. For example, the chloride of sulphur, which has a high
refractive power, transmits more of the calorific rays than the oils,
which have a less refractive power: oils transmit more radiant heat than
the acids; the acids more than aqueous solutions; and the latter more
than pure water, which of all the series has the least refractive power,
and is the least pervious to heat. M. Melloni observed also that each
ray of the solar spectrum follows the same law of action with that of
terrestrial rays having their origin in sources of different
temperatures; so that the very refrangible rays may be compared to the
heat emanating from a focus of high temperature, and the least
refrangible to the heat which comes from a source of low temperature.
Thus, if the calorific rays emerging from a prism be made to pass
through a layer of water contained between two plates of glass, it will
be found that these rays suffer a loss in passing through the liquid as
much greater as their refrangibility is less. The rays of heat that are
mixed with the blue or violet light pass in great abundance, while those
in the obscure part which follows the red light are almost totally
intercepted. The first, therefore, act like the heat of a lamp, and the
last like that of boiling water.
These circumstances explain the phenomena observed by several
philosophers with regard to the point of greatest heat in the solar
spectrum, which varies with the substance of the prism. Sir William
Herschel, who employed a prism of flint glass, found that point to be a
little beyond the red extremity of the spectrum; but, according to M.
Seebeck, it is found to be upon the yellow, upon the orange, on the red,
or at the dark limit of the red, according as the prism consists of
water, sulphuric acid, crown or flint glass. If it be recollected that,
in the spectrum from crown glass, the maximum heat is in the red part,
and that the solar rays, in traversing a mass of water, suffer losses
inversely as their refrangibility, it will be easy to understand the
reason of the phenomenon in question. The solar heat which comes to the
anterior face of the prism of water consists of rays of all degrees of
refrangibility. Now, the rays possessing the same index of refraction
with the red light suffer a greater loss in passing through the prism
than the rays possessing the refrangibility of the orange light, and the
latter lose less in their passage than the heat of the yellow. Thus the
losses, being inversely proportional to the degree of refrangibility of
each ray, cause the point of maximum heat to tend from the red towards
the violet, and therefore it rests upon the yellow part. The prism of
sulphuric acid, acting similarly, but with less energy than that of
water, throws the point of greatest heat on the orange; for the same
reason, the crown and flint glass prisms transfer that point
respectively to the red and to its limit. M. Melloni, observing that the
maximum point of heat is transferred farther and farther towards the red
end of the spectrum, according as the substance of the prism is more and
more permeable to heat, inferred that a prism of rock-salt, which
possesses a greater power of transmitting the calorific rays than any
known body, ought to throw the point of greatest heat to a considerable
distance beyond the visible part of the spectrum,—an anticipation which
experiment fully confirmed, by placing it as much beyond the dark limits
of the red rays as the red part is distant from the blueish green band
of the spectrum.
In all these experiments M. Melloni employed a thermomultiplier,—an
instrument that measures the intensity of the transmitted heat with an
accuracy far beyond what any thermometer ever attained. It is a very
elegant application of M. Seebeck’s discovery of thermo-electricity; but
the description of this instrument is reserved for a future occasion,
because the principle on which it is constructed has not yet been
explained.
In the beginning of the present century, not long after M. Malus had
discovered the polarization of light, he and M. Berard proved that the
heat which accompanies the sun’s light is capable of being polarized;
but their attempts totally failed with heat derived from terrestrial,
and especially from non-luminous sources. M. Berard, indeed, imagined
that he had succeeded; but, when his experiments were repeated by Mr.
Lloyd and Professor Powell, no satisfactory result could be obtained. M.
Melloni resumed the subject, and endeavoured to effect the polarization
of heat by tourmaline, as in the case of light. It was already shown
that two slices of tourmaline, cut parallel to the axis of the crystal,
transmit a great portion of the incident light when looked through with
their axes parallel, and almost entirely exclude it when they are
perpendicular to one another. Should radiant heat be capable of
polarization, the quantity transmitted by the slices of tourmaline in
their former position ought greatly to exceed that which passes through
them in the latter, yet M. Melloni found that the quantity of heat was
the same in both cases: whence he inferred that heat from a terrestrial
source is incapable of being polarized. Professor Forbes of Edinburgh,
who prosecuted this subject with great acuteness and success, came to
the same conclusion in the first instance; but it occurred to him, that,
as the pieces of tourmaline became heated by being very near the lamp,
the secondary radiation from them rendered the very small difference in
the heat that was transmitted in the two positions of the pieces of
tourmaline imperceptible. Nevertheless he succeeded in proving, by
numerous observations, that heat from various sources is polarized by
the tourmaline; but that the effect with non-luminous heat is very
minute and difficult to perceive, on account of the secondary radiation.
Though light is almost entirely excluded in one position of the pieces
of tourmaline, and transmitted in the other, a vast quantity of radiant
heat passes through them in all positions. Eighty-four per cent. of the
heat from an argand lamp passed through them in the case where light was
altogether stopped. It is only the difference in the quantity of
transmitted heat that gives evidence of its polarization. The second
slice of tourmaline, when perpendicular to the first, stops all the
light, but transmits a great proportion of heat; alum, on the contrary,
stops almost all the heat, and transmits the light; whence it may be
concluded that heat, though intimately partaking the nature of light,
and accompanying it under certain circumstances, as in reflection and
refraction, is capable of almost complete separation from it under
others. The separation has since been perfectly effected by M. Melloni,
by passing a beam of light through a combination of water and green
glass, coloured by the oxide of copper. Even when the transmitted light
was concentrated by lenses, so as to render it almost as brilliant as
the direct light of the sun, it showed no sensible heat.
Professor Forbes next employed two bundles of laminæ of mica, placed at
the polarizing angle, and so cut that the plane of incidence of the heat
corresponded with one of the optic axes of this mineral. The heat
transmitted through this apparatus was polarized from a source whose
temperature was even as low as 200°; heat was also polarized by
reflection; but the experiments, though perfectly successful, are more
difficult to conduct.
It appears, from the various experiments of M. Melloni and Professor
Forbes, that all the calorific rays emanating from the sun and
terrestrial sources are equally capable of being polarized by reflection
and by refraction, whether double or single, and that they are also
capable of circular polarization by all the methods employed in the
circular polarization of light. Plates of quartz cut at right angles to
the axis of the prism possess the property of turning the calorific rays
in one direction, while other plates of the same substance from a
differently modified prism cause the rays to rotate in the contrary
direction; and two plates combined, when of different affection, and of
equal thickness, counteract each other’s effects as in the case of
light. Tourmaline separates the heat into two parts, one of which it
absorbs, while it transmits the other; in short, the transmission of
radiant heat is precisely similar to that of light.
Since heat is polarized in the same manner as light, it may be expected
that polarized heat transmitted through doubly refracting substances
should be separated into two pencils, polarized in planes at right
angles to each other; and that when received on an analyzing plate they
should interfere and produce invisible phenomena, perfectly analogous to
those described in Section XXII. with regard to light (N. 221).
It was shown, in the same section, that if light polarized by reflection
from a pane of glass be viewed through a plate of tourmaline, with its
longitudinal section vertical, an obscure cloud, with its centre wholly
dark, is seen on the glass. When, however, a plate of mica uniformly
about the thirteenth of an inch in thickness is interposed between the
tourmaline and the glass, the dark spot vanishes, and a succession of
very splendid colours are seen; and, as the mica is turned round in a
plane perpendicular to the polarized ray, the light is stopped when the
plane containing the optic axis of the mica is parallel or perpendicular
to the plane of polarization. Now, instead of light, if heat from a
non-luminous source be polarized in the manner described, it ought to be
transmitted and stopped by the interposed mica under the same
circumstances under which polarized light would be transmitted or
stopped. Professor Forbes found that this is really the case, whether he
employed heat from luminous or non-luminous sources: and he had
evidence, also, of circular and elliptical polarization of heat. It
therefore follows, that if heat were visible, under similar
circumstances we should see figures perfectly similar to those given in
Note 213, and those following; and, as these figures are formed by the
interference of undulations of light, it may be inferred that heat, like
light, is propagated by undulations of the ethereal medium, which
interfere under certain conditions, and produce figures analogous to
those of light. It appears also, from Mr. Forbes’s experiments, that the
undulations of heat are longer than the undulations of light; and it has
already been mentioned that Professor Draper considers them to be
normal, like those of sound.
That light and heat are both vibrations of the ethereal medium is not
the less true on account of the rays of heat being unseen, for the
condition of visibility or invisibility may only depend upon the
construction of our eyes, and not upon the nature of the motion which
produces these sensations in us. The sense of seeing may be confined
within certain limits. The chemical rays beyond the violet end of the
spectrum may be too rapid, or not sufficiently excursive, in their
vibrations, to be visible to the human eye; and the calorific rays
beyond the other end of the spectrum may not be sufficiently rapid, or
too extensive, in their undulations, to affect our optic nerves, though
both may be visible to certain animals or insects. We are altogether
ignorant of the perceptions which direct the carrier-pigeon to his home,
or of those in the antennæ of insects which warn them of the approach of
danger; nor can we understand the telescopic vision which directs the
vulture to his prey before he himself is visible even as a speck in the
heavens. So, likewise, beings may exist on earth, in the air, or in the
waters, which hear sounds our ears are incapable of hearing, and which
see rays of light and heat of which we are unconscious. Our perceptions
and faculties are limited to a very small portion of that immense chain
of existence which extends from the Creator to evanescence.
The identity of action under similar circumstances is one of the
strongest arguments in favour of the common nature of the chemical,
visible, and calorific rays. They are all capable of reflection from
polished surfaces, of refraction through diaphanous substances, of
polarization by reflection and by doubly refracting crystals; their
velocity is prodigious; they may be concentrated and dispersed by convex
and concave mirrors; they pass with equal facility through rock-salt and
are capable of radiation; and they are subject to the same law of
interference with those of light: hence there can be no doubt that the
whole assemblage of rays visible and invisible which constitute a solar
beam are propagated by the undulations of the ethereal medium, and
consequently as motions they come under the same laws of analysis.
When radiant heat falls upon a surface, part of it is reflected and part
of it is absorbed; consequently, the best reflectors possess the least
absorbing powers. The temperature of very transparent fluids is not
raised by the passage of the sun’s rays, because they do not absorb any
of them; and, as his heat is very intense, transparent solids arrest a
very small portion of it. The absorption of the sun’s rays is the cause
both of the colour and temperature of solid bodies. A black substance
absorbs all the rays of light, and reflects none; and since it absorbs,
at the same time, all the calorific rays, it becomes sooner warm, and
rises to a higher temperature, than bodies of any other colour. Blue
bodies come next to black in their power of absorption. And, since
substances of a blue tint absorb all the other colours of the spectrum,
they absorb by far the greatest part of the calorific rays, and reflect
the blue where they are least abundant. Next in order come the green,
yellow, red, and, last of all, white bodies, which reflect nearly all
the rays both of light and heat. However, there are certain limpid and
colourless media, which in some cases intercept calorific radiations and
become heated, while in other cases they transmit them and undergo no
change of temperature.
All substances may be considered to radiate heat, whatever their
temperature may be, though with different intensities, according to
their nature, the state of their surfaces, and the temperature of the
medium into which they are brought. But every surface absorbs as well as
radiates heat; and the power of absorption is always equal to that of
radiation; for, under the same circumstances, matter which becomes soon
warm also cools rapidly. There is a constant tendency to an equal
diffusion of heat, since every body in nature is giving and receiving it
at the same instant; each will be of uniform temperature when the
quantities of heat given and received during the same time are
equal—that is, when a perfect compensation takes place between each and
all the rest. Our sensations only measure comparative degrees of heat:
when a body, such as ice, appears to be cold, it imparts fewer calorific
rays than it receives; and when a substance seems to be warm—for
example, a fire—it gives more heat than it takes. The phenomena of dew
and hoar-frost are owing to this inequality of exchange; the heat
radiated during the night by substances on the surface of the earth,
into a clear expanse of sky, is lost to us, and no return is made from
the blue vault, so that their temperature sinks below that of the air,
whence they abstract a part of that heat which holds the atmospheric
humidity in solution, and a deposition of dew takes place. If the
radiation be great, the dew is frozen and becomes hoar-frost, which is
the ice of dew. Cloudy weather is unfavourable to the formation of dew,
by preventing the free radiation of heat; and actual contact is
requisite for its deposition, since it is never suspended in the air
like fog. Plants derive a great part of their nourishment from this
source; and, as each possesses a power of radiation peculiar to itself,
they are capable of procuring a sufficient supply for their wants. The
action of the chemical rays imparts to all substances more or less the
power of condensing vapour on those parts on which they fall, and must
therefore have a considerable influence on the deposition of dew. There
may be a low degree of humidity in the air which may yet contain a great
quantity of aqueous vapour, for vapour while it exists as gas is dry.
The temperature at which the atmosphere can contain no more vapour
without precipitation is called the dew point, and is measured by the
hygrometer. In foretelling the changes of weather it is scarcely
inferior to the barometer.
Steam is formed throughout the whole mass of a boiling liquid, whereas
evaporation takes place only at the free surface of liquids, and that
under the ordinary temperature and pressure of the atmosphere. There is
a constant evaporation from the land and water all over the earth. The
rapidity of the formation does not depend altogether on the dryness of
the air; according to Dr. Dalton’s experiments, it depends also on the
difference between the tension of the vapour which is forming, and that
which is already in the atmosphere. In calm weather vapour accumulates
in the stratum of air immediately above the evaporating surface, and
retards the formation of more; whereas a strong wind accelerates the
process by carrying off the vapour as soon as it rises, and making way
for a succeeding portion of dry air.
Rain is formed by the mixing of two masses of air of different
temperatures; the colder part, by abstracting from the other the heat
which holds it in solution, occasions the particles to approach each
other and form drops of water, which, becoming too heavy to be sustained
by the atmosphere, sink to the earth by gravitation in the form of rain.
The contact of two strata of air of different temperatures, moving
rapidly in opposite directions, occasions an abundant precipitation of
rain. When the masses of air differ very much in temperature, and meet
suddenly, hail is formed. This happens frequently in hot plains near a
ridge of mountains, as in the south of France, from the sudden descent
of an intensely cold current of wind into a mass of air nearly saturated
with vapour. Such also is the cause of the severe hail-storms which
occasionally take place on extensive plains within the tropics.
An accumulation of heat invariably produces light: with the exception of
the gases, all bodies which can endure the requisite degree of heat
without decomposition begin to emit light at the same temperature; but,
when the quantity of heat is so great as to render the affinity of their
component particles less than their affinity for the oxygen of the
atmosphere, a chemical combination takes place with the oxygen, light
and heat are evolved, and fire is produced. Combustion—so essential for
our comfort, and even existence—takes place very easily from the small
affinity between the component parts of atmospheric air, the oxygen
being nearly in a free state; but, as the cohesive force of the
particles of different substances is very variable, different degrees of
heat are requisite to produce their combustion. The tendency of heat to
a state of equal diffusion or equilibrium, either by radiation or
contact, makes it necessary that the chemical combination which
occasions combustion should take place instantaneously; for, if the heat
were developed progressively, it would be dissipated by degrees, and
would never accumulate sufficiently to produce a temperature high enough
for the evolution of flame.
It is a general law that all bodies expand by heat and contract by cold.
The expansive force of heat has a constant tendency to overcome the
attraction of cohesion, and to separate the constituent particles of
solids and fluids; by this separation the attraction of aggregation is
more and more weakened, till at last it is entirely overcome, or even
changed into repulsion. By the continual addition of heat, solids may be
made to pass into liquids, and from liquids to the aëriform state, the
dilatation increasing with the temperature; and every substance expands
according to a law of its own. Gases expand more than liquids, and
liquids more than solids. The expansion of air is more than eight times
that of water, and the increase in the bulk of water is at least
forty-five times greater than that of iron. Metals dilate uniformly from
the freezing to the boiling points of the thermometer; the uniform
expansion of the gases extends between still wider limits; but, as
liquidity is a state of transition from the solid to the aëriform
condition, the equable dilatation of liquids has not so extensive a
range. This change of bulk, corresponding to the variation of heat, is
one of the most important of its effects, since it furnishes the means
of measuring relative temperature by the thermometer and pyrometer. The
rate of expansion of solids varies at their transition to liquidity, and
that of liquidity is no longer equable near their change to an aëriform
state. There are exceptions, however, to the general laws of expansion;
some liquids have a maximum density corresponding to a certain
temperature, and dilate whether that temperature be increased or
diminished. For example—water expands whether it be heated above or
cooled below 40°. The solidification of some liquids, and especially
their crystallization, is always accompanied by an increase of bulk.
Water dilates rapidly when converted into ice, and with a force
sufficient to split the hardest substances. The formation of ice is
therefore a powerful agent in the disintegration and decomposition of
rocks, operating as one of the most efficient causes of local changes in
the structure of the crust of the earth; of which we have experience in
the tremendous _éboulemens_ of mountains in Switzerland. But Professor
W. Thomson has proved experimentally that it requires a lower
temperature to freeze water under pressure than when free.
The dilatation of substances by heat, and their contraction by cold,
occasion such irregularities in the rate of clocks and watches as would
render them unfit for astronomical or nautical purposes, were it not for
a very beautiful application of the laws of unequal expansion. The
oscillations of a pendulum are the same as if its whole mass were united
in one dense particle, in a certain point of its length, called the
centre of oscillation. If the distance of this point from the point by
which the pendulum is suspended were invariable, the rate of the clock
would be invariable also. The difficulty is to neutralize the effects of
temperature, which is perpetually increasing or diminishing its length.
Among many contrivances, Graham’s compensation pendulum is the most
simple. He employed a glass tube containing mercury. When the tube
expands from the effects of heat, the mercury expands much more; so that
its surface rises a little more than the end of the pendulum is
depressed, and the centre of oscillation remains stationary. Harrison
invented a pendulum which consists of seven bars of steel and of brass,
joined in the shape of a gridiron, in such a manner that, if by change
of temperature the bars of brass raise the weight at the end of the
pendulum, the bars of steel depress it as much. In general, only five
bars are used; three being of steel, and two a mixture of silver and
zinc. The effects of temperature are neutralized in chronometers upon
the same principle; and to such perfection are they brought, that the
loss or gain of one second in twenty-four hours for two days running
would render one unfit for use. Accuracy in surveying depends upon the
compensation rods employed in measuring bases. Thus, the laws of the
unequal expansion of matter judiciously applied have an immediate
influence upon our estimation of time; of the motions of bodies in the
heavens, and of their fall upon the earth; on our determination of the
figure of the globe, and on our system of weights and measures; on our
commerce abroad, and the mensuration of our lands at home.
The expansion of the crystalline substances takes place under very
different circumstances from the dilatation of such as are not
crystallized. The latter become both longer and thicker by an accession
of heat, whereas M. Mitscherlich has found that the former expand
differently in different directions; and, in a particular instance,
extension in one direction is accompanied by contraction in another: for
example, Iceland spar is dilated in the direction of its axis of double
refraction (N. 205), but at right angles to that axis it is contracted,
which brings the crystal nearer to the form of the cube and diminishes
its double refractive power. When heat is applied to crystals of
sulphate of lime, the two optical axes (N. 207) gradually approach, and
at last coincide; when the heat is increased, the axes open again, but
in a direction at right angles to their former position. By experiment
M. Senarmont has concluded, that in media constituted like crystals of
the rhomboidal (N. 169) system the conducting power varies in such a
manner, that, supposing a centre of heat to exist within them, and the
medium to be indefinitely extended in all directions, the isothermal
surfaces are concentric ellipsoids of revolution round the axes of
symmetry, or at least surfaces differing but little from them. The
internal structure of crystallized matter must be very peculiar thus to
modify the expansive power of heat.
Heat applied to the surface of a fluid is propagated downwards very
slowly, the warmer, and consequently lighter strata, always remaining at
the top. This is the reason why the water at the bottom of lakes fed
from Alpine chains is so cold; for the heat of the sun is transfused but
a little way below the surface. When the heat is applied below a liquid,
the particles continually rise as they become specifically lighter, and
diffuse the heat through the mass, their place being perpetually
supplied by those that are more dense. The power of conducting heat
varies materially in different liquids. Mercury conducts twice as fast
as an equal bulk of water, and therefore it appears to be very cold. A
hot body diffuses its heat in the air by a double process: the air in
contact with it becoming lighter ascends and scatters its heat by
transmission, while at the same time another portion is discharged in
straight lines by the radiating power of the surface. Hence a substance
cools more rapidly in air than in vacuo, because in the latter case the
process is carried on by radiation alone. It is probable that the earth
having been originally of very high temperature has become cooler by
radiation alone, the ethereal medium being too rare to carry off much
heat by contact.
Heat is propagated with more or less rapidity through all bodies; air is
the worst conductor, and consequently mitigates the severity of cold
climates by preserving the heat imparted to the earth by the sun. On the
contrary, dense bodies, especially metals, possess the power of
conduction in the greatest degree, but the transmission requires time.
If a bar of iron twenty inches long be heated at one extremity, the heat
takes four minutes in passing to the other. The particle of the metal
that is first heated communicates the heat to the second, and the second
to the third: so that the temperature of the intermediate molecule at
any instant is increased by the excess of the temperature of the first
above its own, and diminished by the excess of its own temperature above
that of the third. That however will not be the temperature indicated by
the thermometer, because as soon as the particle is more heated than the
surrounding atmosphere it loses its heat by radiation, in proportion to
the excess of its actual temperature above that of the air. The velocity
of the discharge is directly proportional to the temperature, and
inversely as the length of the bar. As there are perpetual variations in
the temperature of all terrestrial substances, and of the atmosphere,
from the rotation of the earth, and its revolution round the sun, from
combustion, friction, fermentation, electricity, and an infinity of
other causes, the tendency to restore the equability of temperature by
the transmission of heat must maintain all the particles of matter in a
state of perpetual oscillation, which will be more or less rapid
according to the conducting powers of the substances. From the motion of
the heavenly bodies about their axes, and also round the sun, exposing
them to perpetual changes of temperature, it may be inferred that
similar causes will produce like effects in them too. The revolutions of
the double stars show that they are not at rest; and although we are
totally ignorant of the changes that may be going on in the nebulæ and
millions of other remote bodies, it is hardly possible that they should
be in absolute repose; so that, as far as our knowledge extends, motion
is a law of the universe and the immediate cause of heat, as in the
sunbeam so also in all terrestrial phenomena.
This is by no means hypothetical, but founded upon fact and experiment.
Heat is produced by motion and is equivalent to it, for we measure heat
by motion in the thermometer. The heat evolved by percussion is
proportional to the force of the blow; by repeated blows iron becomes
red hot; and the quantity of heat produced by friction, whether the
matter be solid or fluid, is always in proportion to the force employed:
in cold weather we rub our hands to make them warm, and the harder we
rub the warmer they become. The warmth of the sea after a storm is in
proportion to the force of the wind; and in Sir Humphry Davy’s
experiment of melting ice by friction in the receiver of an air-pump
kept at the freezing point, the heat which melted the ice was exactly
proportional to the force of friction. This experiment proves the
immateriality of heat, since the capacity of ice for heat is less than
that of water. Thus mechanical action and heat are equivalent to one
another. Mr. Joule of Manchester[13] has proved that the quantity of
heat requisite to raise the temperature of a pound of water one degree
of Fahrenheit’s thermometer, is equivalent to the mechanical force
developed by the fall of a body weighing 772·69 pounds through the
perpendicular height of one foot. This quantity is the mechanical
equivalent of heat. Thus heat is motion, and it is measured by force. In
fact, for every unit of force expended in friction or percussion, a
definite quantity of heat is generated; and conversely, when work is
performed by the consumption of heat, for each unit of force gained, a
unit of heat disappears. For since heat is a dynamical force of
mechanical effect, there must be an equivalent between mechanical work
and heat as between cause and effect. (N. 222.)
Besides the temperature indicated by the thermometer, bodies absorb
heat, and their capacity for heat is so various that very different
quantities of heat are required to raise different substances to the
same sensible temperature. It is evident, therefore, that much of the
heat is absorbed and becomes insensible to the thermometer. That portion
of heat requisite to raise a body to a given temperature is its specific
heat, but the latent or absorbed heat is an expansive force or energy,
which, acting upon the ether surrounding the ultimate particles of
bodies, changes them from solid to liquid, and from liquid to vapour or
gas. According to the law of absorption, the transfer of heat from a
warm body to one that is cold is a mere transfer of force, in which the
force of compression is exactly proportional to the force of expansion.
Ice remains at the temperature of 32° Fahrenheit till it has absorbed
140° of heat, and then it melts, but without raising the temperature of
the water above 32°. On the contrary, when a liquid is converted into a
solid, a quantity of heat leaves it without any diminution of
temperature. Thus water at 32° must part with 140° of heat before it
freezes. The slowness with which water freezes or ice thaws, is a
consequence of the time required for the ethereal atmospheres round the
particles of the water to contract or expand with a force equivalent to
140° of heat. A considerable degree of cold is felt during a thaw,
because the ice in its transition from a solid to a liquid state absorbs
sensible heat from the atmosphere and surrounding objects. The heat
absorbed and evolved by the rarefaction and condensation of air is
exactly proportional to the force evolved and absorbed in these
operations. In fact, the changes of temperature produced by these
rarefactions and condensations of air show that the heat of elastic
fluids is the mechanical force possessed by them; and since the
temperature of a gas determines its elastic force, it follows that the
elastic force or pressure must be the effect of the motion of the
constituent particles in any gas. Sir Humphry Davy, who first
demonstrated the immateriality of heat, assumed the hypothesis that the
motion we call heat is a rotation or vibration among the particles of
the fluid, which, according to Mr. Joule, agrees perfectly with the
observed phenomena, but he prefers the more simple view of Mr. Herapath,
that the elastic force or pressure is due to the impact of the particles
against any surface presented to them. Absorbed or latent heat may be
regarded as a quiescent energy ready to be restored to the form of
sensible heat when called forth: its vibrations as heat are extinguished
for the time by being transferred to the internal expansive force, and
are restored by compression. The absorbed heat of air and all elastic
fluids may be forced out by sudden compression like squeezing water out
of a sponge. The quantity of heat brought into action in this way is
well illustrated by the experiment of igniting tinder by the sudden
compression of air by a piston thrust into a cylinder closed at one end.
The development of heat on a stupendous scale is exhibited in lightning:
it is proportional to the square of the quantity of electricity
discharged, and is due to its excessive velocity and the violent
compression of the air in its transit through the atmosphere. Prodigious
quantities of heat are constantly absorbed or disengaged by the changes
to which substances are liable in passing from the solid to the liquid
and from the liquid to the gaseous form and the contrary, causing
endless vicissitudes of temperature over the globe, and endless
expansions and contractions, which are correlative terms for heat and
cold, while radiation of heat is merely a transfer of motion from the
particles on the surface of bodies to the adjacent particles of the
atmosphere.
By the continual application of heat, that is of the expansive force,
liquids are converted into steam or vapour, which is invisible and
highly elastic. Under the mean pressure of the atmosphere, that is when
the barometer stands at 30 inches, water in a boiler absorbs heat
continually till it attains the temperature of the boiling point, which
is 212° Fahrenheit. After that it ceases to show any increase of
sensible heat; but when it has absorbed an additional 1000° of heat or
expansive energy, that energy converts it into steam, and a condensing
force equivalent to 1000° of heat reduces it again to water. Water boils
at different temperatures under different degrees of pressure. It boils
at a lower temperature on the top of a mountain than on the plain below,
because the weight of the atmosphere is less at the higher station.
There is no limit to the temperature to which water might be raised: it
might even be made red hot, could a vessel be found strong enough to
resist the pressure, for the intensity of the expansive force prevented
from having effect by the extreme pressure of the boiler would be
converted into sensible heat which might eventually render the water red
hot. Thus, since the force of steam is in proportion to the temperature
at which the water boils, or to the pressure, it is under control, and,
perhaps with the exception of electricity, it is the greatest power that
has been made subservient to the wants of man.
It is found that the absolute quantity of heat consumed in the process
of converting water into steam is the same at whatever temperature water
may boil, but that the absolute heat of the steam is greater exactly in
proportion as its sensible heat is less. Thus, steam raised at 212°
Fahrenheit under the mean pressure of the atmosphere, and steam raised
at 180° under half the pressure, contain the same quantity of heat, with
this difference, that the one has more absorbed heat and less sensible
heat than the other. It is evident that, as the same quantity of heat is
requisite for converting a given weight of water into steam, at whatever
temperature or under whatever pressure the water may be boiled,
therefore, in the steam engine, equal weights of steam at a high
pressure and a low pressure are produced by the same quantity of fuel;
and whatever the pressure of the steam may be, the consumption of fuel
is proportional to the quantity of water converted into vapour. Steam of
whatever tension expands on being set free, but the expansion of high
pressure steam at the expense of its sensible heat is so great, that the
hand may be plunged into it without injury the instant it issues from
the orifice of a boiler. The steam becomes hotter by friction in issuing
through the orifice which maintains it in its dry form, for there is no
doubt that high-pressure steam is dry.
The elasticity or tension of steam, like that of common air, varies
inversely as its volume—that is, when the space it occupies is doubled,
its elastic force is reduced to one half. The expansion of steam is
indefinite; the smallest quantity of water expanded into vapour will
occupy many millions of cubic feet; a wonderful illustration of the
minuteness of the ultimate particles of matter.
The force of steam, tremendous as the lightning itself when
uncontrolled, is merely the result of chemical affinity: it is the
chemical attraction between the particles of carbon, of coal or wood,
and the oxygen of the atmosphere. Mr. Joule has ascertained that a pound
of the best coal when burnt gives sufficient heat to raise the
temperature of 8086 pounds of water one degree of the Centigrade
thermometer, whence it has been computed by M. Helmholtz that the
chemical force arising from the combustion of that pound of coal is
capable of lifting a body of one hundred pounds weight to the height of
twenty miles. That is the _work_ performed by the heat arising from the
combustion of a pound of coal. In all cases where work is produced by
heat, a quantity of heat proportional to the work done is expended; and
conversely, by the expenditure of a like quantity of work, the same
amount of heat may be produced. The equivalence of heat and work is a
law of nature. The mechanical force exerted by the steam engine for
example is exactly proportional to the consumption of heat, nor more nor
less; if we could produce a greater quantity than its equivalent we
should have perpetual motion, which is impossible. Mechanical engines
generate no force. We cannot create force; we can only avail ourselves
of the inexhaustible stores of nature, the lightning, fire, water, wind,
chemical action, &c. The quantity of mechanical power in nature is ever
the same; it is never increased, it is never diminished, throughout the
whole circuit of natural powers. The conservation of force is as
permanent and unchangeable as matter. It may be dormant for a time, but
it ever exists. We are unconscious of the enormous dynamic power that is
either active or latent throughout the globe, because we do not attend
to it. By the ebb and flow of the tide alone a power is exerted by which
25,000 cubic miles of water is moved over a quarter of the globe every
twelve hours; and Professor W. Thomson has computed, by means of
Pouillet’s data of solar radiation and Mr. Joule’s mechanical equivalent
of heat, that the mechanical value of the whole energy active and
potential of the disturbances kept up in the ethereal medium by the
vibrations of the solar light within a cubic mile of our atmosphere is
equal to 12,050 times the unit of mechanical force, that is to say,
12,050 times the force that would raise a pound of matter to the height
of one foot, whence some idea may be formed of the vast amount of force
exerted by the sun’s light within the limits of the whole terrestrial
atmosphere. (N. 223.)
The dynamic energy of the undulations of the solar light gives the
leaves of plants the power of decomposing carbonic acid, and of
separating the particles of carbon and hydrogen from the oxygen for
which they have so strong an affinity. In this operation the undulations
of the sunbeam are extinguished as light and heat, and Professor W.
Thomson has proved that the quantity of these undulations thus
extinguished is precisely equal to the potential or quiescent energy
thus created, and that precisely that very quantity of light and heat is
restored when the plants are burned, whatever state they may be in; and
that thus, as Mr. George Stephenson[14] has truly and beautifully
observed, our coal fires and gas lamps restore to our use the light and
heat of the sun of the early geological epochs which have rested as
dormant powers under the seas and mountains for unnumbered ages. The sun
is therefore the source of the mechanical energy of all the heat and
motion of inanimate things, of all the motions of the heat and light of
fires and artificial flames, and of the heat of all living creatures.
For animal heat, and weights raised or resistance overcome, are
mechanical effects of the chemical combination of food with oxygen; and
food is either directly or indirectly vegetable, consequently dependent
upon the sun.
Professor Helmholtz of Bonn has put in a strong point of view the
enormous store of force possessed by our system by comparing it with its
equivalent of heat. The force with which the earth moves in its orbit is
such, that if brought to rest by a sudden shock, a quantity of heat
would be generated by the blow equal to that produced by the combustion
of fourteen such earths of solid coal; and supposing the capacity of the
earth for heat as low as that of water, the globe would be heated to
11,200° Cent. It would be quite fused and for the most part reduced to
vapour. If it should fall to the sun, which it would certainly do, the
quantity of heat developed by the shock would be four hundred times as
great.
The application of heat to the various branches of the mechanical and
chemical arts has within the present century effected a greater change
in the condition of man than had been accomplished in any equal period
of his existence. Armed by the expansion and condensation of fluids with
a power equal to that of the lightning itself, conquering time and
space, he flies over plains, and travels on paths cut by human industry
even through mountains with a velocity and smoothness more like
planetary than terrestrial motion; he crosses the deep in opposition to
wind and tide; by releasing the strain on the cable, he rides at anchor
fearless of the storm; he makes the lightning his messenger; and like a
magician he raises from the gloomy abyss of the mine the sunbeam of
former ages to dispel the midnight darkness.
The principal phenomena of heat may be illustrated by a comparison with
those of sound. Their excitation is not only similar but identical, as
in friction and percussion; they are both communicated by contact and
radiation; and Dr. Young observes that the effect of radiant heat in
raising the temperature of a body upon which it falls, resembles the
sympathetic agitation of a string when the sound of another string which
is in unison with it is transmitted through the air. Light, heat, sound,
and the waves of fluids are all subject to the same laws; their
undulatory theories are perfectly similar: hence the interference of two
hot rays must produce cold, that is, they must extinguish one another:
darkness results from the interference of two undulations of light,
silence ensues from the interference of two undulations of sound, and
still water or no tide is the consequence of the interference of two
tides. The propagation of sound, however, requires a much denser medium
than that of light and heat; its intensity diminishes as the rarity of
the air increases: so that, at a very small height above the surface of
the earth, the noise of the tempest ceases, and the thunder is heard no
more in those boundless regions where the heavenly bodies accomplish
their periods in eternal and sublime silence.
A consciousness of the fallacy of our senses is one of the most
important consequences of the study of nature. This study teaches us
that no object is seen by us in its true place, owing to aberration;
that the colours of substances are solely the effects of the action of
matter upon light; and that light itself as well as heat and sound are
not real beings, but mere motions communicated to our perceptions by the
nerves. The human frame may therefore be regarded as an elastic system,
the different parts of which are capable of receiving the tremors of
elastic media, and of vibrating in unison with any number of
superimposed undulations, all of which have their perfect and
independent effect. Here our knowledge ends: the mysterious influence of
matter on mind will in all probability be for ever hid from man.