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If the thermometer placed in presence of the ice had been at
a lower temperature than the ice, it would, for like reasons,
have risen. The ice in that case would have warmed the ther-

1563. Transmission of heat. When rays of heat are inci-
dent on the surfaces of certain media, they penetrate them in
greater or less quantity, according to the nature and properties
of the medium, just as rays of light pass through bodies which
are more or less transparent or diaphanous.

Media which are pervious to heat are said to be diatherma-
nous, and those which are impervious are called athermanous.

Bodies are diathermanous in different degrees, or altogether
athermanous, according to their various physical characters,
their thickness, the state of their surface, the nature of the heat
which is incident upon them, and other conditions.

1564. Mellon? s thermoscopic apparatus. Nearly all the
knowledge we possess in this branch of the physics of heat is
the result oft the recent researches of M. Melloni. The ther-
moscopic apparatus contrived and applied with singular felicity



and success by him, consisted of a thermo-galvanic pile acting
upon a highly sensitive galvanometer. It will be explained
hereafter that if the thermal equilibrium be disturbed in certain
metallic combinations, an electric current will be produced, the
intensity of which will be proportional to the difference of tem-
perature produced, and that the force of such a current can be
measured by the deviation it produces in a magnetic needle,
round which it is conducted spirally upon a coil of metallic wire
coated with a non-conducting substance.

The general form and arrangement of this apparatus, and the
manner of applying it to
thermal researches, are re-
presented \nfigs. 451, 452,
453, and 454.

Upon the stand s is placed
the source of heat which is
submitted to experiment.
Those which M. Mellon i
selected were a lamp L,
with a concave reflector t ;
a spiral wire of platinum
Ti, fig. 452., rendered incan-
descent by the flame of a
spirit-lamp; a plate of copper
i, fig. 453., blackened with
smoke, and raised to the tem-
perature of 700 by a spirit-
lamp ; and, in fine, a cubical

Fig. 451.

canister K, fig. 454., similar
to those used by Leslie.

J-ig. 452.

Fig. 453.

Fig. 454.


On the stand T was placed the body a*, through which the
rays of heat were to be transmitted, and which was formed
into a thin plate. An athermanous screen / was interposed,
having in it an aperture to limit the pencil of rays transmitted
to x. Another athermanous screen was placed at c, movable
upon a joint by which the pencil proceeding from the lamp
could be intercepted or transmitted at pleasure.

The thermo-voltaic pile was placed at p, having one end
presented to the thermal pencil, and movable in a case fitting it,
in which it was capable of sliding. Its poles p and n were con-
nected by conducting wires with the galvanometer, the needle
of which indicated by its deflection the intensity of the heat by
which the pile p was affected.

1565. Results of Melloms researches. The series of expe-
riments made with this apparatus gave the remarkable, and in
many respects unexpected, results which we shall now briefly

The only substance found to be perfectly diathermanous was
rocksalt. Plates of this crystal transmit nearly all the heat
which enters them, no matter from what source. Of the inci-
dent rays 7*7 per cent, are reflected from both surfaces of the
plate, and the whole of the remaining 92*3 per cent, are trans-
mitted. There is no absorption.

Bodies in general are less athermanous the higher the tempe-
rature of the radiator.

1566. Transparent media not proportionally diathermanous.
Media are not diathermanous in proportion as they are trans-
parent. On the contrary, certain media which are nearly opaque
are highly diathermanous, while others which are highly trans-
parent are nearly athermanous. Thus, black glass and plates of
smoked quartz so opaque that the disk of the sun in the meri-
dian is barely visible through them, are much more diatherma-
nous than plates of alum, which are very transparent ; and
plates of quartz smoked to opacity are more diathermanous
than when clean and transparent. In like manner, black glass
is more diathermanous than colourless glass.

1567- Decomposition of heat by absorption. The thermal
pencil is composed of rays, some of which are absorbed, and
others transmitted by certain media. This effect is altogether
analogous to that which is produced by coloured media on
light. If a pencil of solar light be incident upon red glass, the

126 HEAT.

red rays alone will be transmitted, those of the other colours
being absorbed ; but if the red light transmitted through such a
plate be received upon a second red plate, there will be no
further absorption, at least so far as depends on the colour of
the light. In like manner, when a thermal pencil enters cer-
tain diathermanous media, a part of its rays are intercepted,
others being transmitted. If these last be received upon
another plate of the same diathermanous substance, they will
pass freely through it without further absorption.

It is therefore inferred that such a medium decomposes by
absorption the thermal pencil in the same manner as a coloured
transparent medium decomposes by absorption a pencil of white
light. This inference is confirmed by the fact that different
partially diathermanous media absorb different constituents of
the thermal pencil. Thus we may cause its entire absorption
by causing it successively to pass through two media, each of
which absorbs the rays transmissible by the other.

This is also analogous to the effects of coloured transparent
media upon luminous pencils. If a pencil of solar light be
successively incident upon two plates, one of red and the other
of the complementary tint of bluish-green, it will be wholly
absorbed, the second plate absorbing all the rays transmitted by
the first.

1568. Absorption not superficial, but limited to a certain
depth. The partial absorption produced by such imperfectly
diathermanous media is not effected at the surface. The rays
are absorbed gradually as they pass through the medium. This,
however, is not continual. All absorption ceases after they
have passed through a certain thickness, and the rays trans-
mitted by a plate of that thickness would, in passing through a
second plate of the same substance, undergo no further absorp-

Glass and rock crystal are each partially diathermanous, the
thermal rays transmitted and absorbed however being different.
If a thermal pencil pass through a plate of glass of a certain
thickness, a part of the rays composing it will be absorbed. If
the rays transmitted be received on another similar plate of
glass, they will be all or nearly all transmitted, no further ab-
sorption taking place. But if these rays thus transmitted by the
glass be received upon a plate of rock crystal of sufficient thick-
ness, a portion of them will be absorbed. Now if the glass and



the rock crystal had each the power of absorbing the rays
transmitted by the other, their combination would be absolutely
athermanous, just as two plates of coloured glass would be
opaque, if each transmitted only the colours complementary to
those transmitted by the other.

1569. Physical conditions of diathermanism. It appears
from the researches of Melloni, that the physical conditions
which render bodies more or less diathermanous have no con-
nection with those which affect their transparency. Water is
one of the least diathermanous substances, although its trans-
parency is so nearly perfect. If, therefore, it be desired to
transmit light without heat, or with greatly diminished heat,
it is only necessary to let the rays pass through water, by
which they will be strained of a great part of their heat.

If the quantity of radiant heat transmitted through air be
expressed by 100, the following numbers will express the
quantity transmitted through an equal thickness of the sub-
stances named below.

- 30

- 27

- 21

- 20

- 17

- 15

- 15

- 12

- 11

It appears, therefore, that of all solid bodies rock salt is the
most diathermanous, and alum the least so. Of all liquids,
bisulphuret of carbon is the most, and water the least, dia-

It is evident from this table, that bodies are not diather-
manous and transparent in the same degree. Rocksalt is less
transparent but more diathermanous than glass.

It has been found that the power of thermal rays to penetrate
an imperfectly diathermanous body is augmented by raising
the temperature of the radiator. This is rendered very appa-
rent in the case of glass, which is much more diathermanous to
heat radiated by a body at a very high than by one at a mode-
rate temperature. This may explain the fact that bodies in
general are more diathermanous to solar light than to light
proceeding from artificial sources.

G 4

Rocksalt (transparent)
Flint glass
Bisulphuret of carbon
Calcareous spar (transp
Rock crystal -
Topaz, brown -
Crown glass -
Oil of turpentine

- 100
- 92
- 67
- 63
arent) - 62
- 62
- 57
- 49
- 31

Rape oil
Tourmaline (green)
Sulphuric ether
Sulphuric acid
Nitric acid
Alum, crystals

128 HEAT.

It is found that heat radiated by bodies which are in a state
of ignition or incandescence penetrate diathermanous media
more freely than those radiated by bodies which are not lumi-
nous. This is in accordance with the general principle already
stated, that thermal rays penetrate diathermanous bodies more
easily the higher is the temperature of the radiator.

Experiments on the thermal analysis of solar light were made
by transmitting a pencil of solar light, either obtained directly
or by reflection, through the aperture in the screen f,Jiff- 451.

1570. Refraction, reflection, and polarization of heat. Ex-
periments on the refraction, reflection, and polarization of heat
were made, by placing on the stand t,fig. 451., prisms of various
materials, reflecting surfaces, polariscopes, or double refracting
crystals. The thermoscopic apparatus was in each case placed
in such a position as to receive the deflected thermal pencil.

In this manner pencils of heat proceeding from various
sources were submitted to the same effects of refraction, re-
flection, and polarization as have been already described in
Book IX. with respect to light, and analogous results were
obtained ; the thermal rays being subject to the same general
laws of reflection and refraction as prevail in relation to lumi-
nous rays.

1571. Application of these principles to explain various phe-
nomena. The general principles regulating the radiation,
absorption, reflection, and transmission of heat, which have
been here stated, serve to explain and illustrate various experi-
mental facts and natural phenomena, as will appear from what
follows :

If two concave parabolic reflectors be placed as described in
(946), any radiator of heat placed in the focus of either will
produce a corresponding effect upon a thermometer placed in
the focus of the other, the rays of heat issuing from the radi-
ating body being twice reflected and collected into the focus of
the second reflector, upon the principle explained in (946).

1572. Experiment of radiated and reflected heat with pair
of parabolic reflectors. Let E and K',fig. 455., be two such
reflectors. If lighted charcoal be placed in the focus F of one,
it will ignite amadou or any other easily inflammable substance
in the other, even though the distance between the reflectors
be twenty or thirty feet.

If a sensitive thermometer, such as the differential thermo-



meter (1349), be placed at F', it will show an increase or dimi-
nution of temperature, according as a hot or cold body is placed

Fig. 454.

at F. If a small globe filled with hot water be placed there,
an increase will be indicated ; and if the globe be filled with
snow or with a freezing mixture, a decrease will be mani-

1573. Materials fitted for vessels to keep liquids warm.
Vessels intended to hold liquids at a higher temperature than
that of the surrounding medium, should be constructed of ma-
terials which are bad radiators. Thus tea-urns, tea-pots, &c.,
are best adapted for their purpose when made of polished metal,
and worst when of black porcelain. A tea-kettle keeps water
hotter more effectually if clean and polished, than if covered
with the black of soot and smoke. Polished fire-irons remain
longer before a hot fire without being heated than rough un-
polished ones.

1574. Advantage of an unpolished stove. A polished stove
is a bad radiator ; one with a rough and blackened surface a
good radiator. The latter is therefore better adapted for warm-
ing an apartment than the former.

1575. Helmets and cuirasses should be polished. The helmet
and cuirass worn by cavalry is a cooler dress than might be
imagined, the polished metal being nearly a good radiator of
heat, and throwing off the solar rays,

1576. Deposition of moisture on window panes. When the
external air, which generally happens, is at a lower tempera-
ture than the air included in the room, it will be observed that
a deposition of moisture will be formed upon the inner surface
of the panes of glass in the windows. This is produced by the

G 5

130 HEAT.

vapor suspended in the atmosphere of the room being con-
densed by the cold surface of the glass. If the external air in
this case be at a temperature below 32, the deposition on the
inner surface of the glass will be congealed, and a rough coat-
ing of ice will be exhibited upon it.

Let two small pieces of tinfoil be fixed, one upon a part of
the external surface of one of the panes, and the other upon
the internal surface of another pane, in the evening ; it will be
found in the morning that that part of the internal surface of
the pane upon which is placed the external foil will be nearly
free from ice, while the surface of the internal foil will be more
thickly covered with ice than the parts of the inner surface of
the glass which are not covered with foil: these effects are
easily explained by radiation. When the tinfoil is placed on
the external surface, it reflects the heat which strikes on that
surface, and protects that part of the surface which is covered
from its action. The heat radiated from the objects in the
room striking on the inner surface of the glass penetrates it,
and encountering the foil attached to the exterior surface, is
reflected by it through the glass, and its escape into the external
atmosphere is intercepted ; the portion of glass, therefore, op-
posite to the tinfoil, is subject to the action of the heat radi-
ated from the chamber, but protected from the action of the
external heat. The temperature of that part of the glass is
therefore less depressed by the external atmosphere than the
temperature of those parts which are not covered by tinfoil.
Now glass being a bad conductor of heat, the temperature of
that part opposite to the external foil does not immediately
affect the remainder of the pane, and consequently we find
that, while the remainder of the interior surface of the pane is
thickly covered with ice, the portion opposite the tinfoil is
comparatively free from it. On the contrary, when the tin-
foil is applied on the internal surface, it reflects perfectly
the heat radiated from the objects in the room, while it admits
through the dimensions of the glass the heat proceeding from
the external atmosphere. The portion of the glass, therefore,
covered by the tinfoil, becomes colder than any other part
of the pane, and the tinfoil itself partakes of this tempera-
ture, which is not raised by the effect of the radiation of ob-
jects in the room, because the tinfoil itself is a good reflector
and a bad absorber. Hence the tinfoil presents a colder sur-


face to the atmosphere in the room, than any other part of the
surface of the pane, and consequently receives a more abundant
deposition of ice.

1577. Principles which explain the phenomena of dew and
hoarfrost. A clear unclouded sky in the absence of the sun
radiates but little heat towards the earth ; consequently, if good
radiators be exposed to such an aspect, they must suffer a fall
of temperature, since they lose more by radiation than they

Let a glass cup, for example, be placed in a silver basin, and
exposed during a cold night to a clear sky ; it will be found in
the morning that a copious deposition of moisture will have
been made on the glass, from which the silver vessel is per-
fectly free. Reversing the experiment, let a silver cup be
placed in a glass basin, and similar results will ensue, the basin
being perfectly covered with moisture, from which the cup
is free. This is easily explained: the metal, being a bad
radiator of heat, preserves its temperature ; the glass, being a
good radiator of heat, loses by radiation much more than it re-
ceives, and, consequently, its temperature falls, and it condenses
the vapour in the air around it.

The result of experiments of this kind supplied Dr. Wells
with his celebrated theory, by which he explained the pheno-
menon of dew.

According to what has been explained, it appears that the
objects which are good radiators, exposed to a clear sky at night,
will become colder than the surrounding atmosphere, and will
consequently condense the water suspended in the air around
them; while objects which are bad radiators will not do this.
Grass, foliage, and other products of vegetation are in general
good radiators. The vegetation, therefore, which covers the
surface of the ground in an open country on a clear night will
receive a deposition of moisture from the atmosphere ; while the
objects which are less perfect radiators, such as earth, stones,
See., do not in general receive such depositions. In the close
and sheltered streets of cities, the deposition of dew is rarely
observed, because there the objects are exposed to reciprocal
radiation, and an interchange of heat takes place which maintains
their temperature.

The effect of the radiation of foliage is strikingly manifested
by the following example. Of two thermometers, one laid

132 HEAT.

among leaves and grass, and the other suspended at some height
above them, the latter will be observed to fall at night many
degrees below the former.

1578. Dew not deposited under a clouded sky. In a cloudy
night, dew is not deposited, because in this case, although vege-
tation radiates as perfectly as before, the clouds also radiate,
and an interchange of heat takes place between them and the
surface of the earth, by which the fall of temperature pro-
ducing dew is prevented.

1579. Production of artificial ice by radiation in hot climates.
Artificial ice is sometimes produced in hot climates by the
following process. A position is selected, not exposed to the
radiation of surrounding objects, and a quantity of dry straw is
spread on the ground, on which pans of porous earthenware are
disposed in which the water to be cooled is placed. The water
radiates heat to the firmament, and receives no heat in return.
The straw upon which the vessels are placed, being a bad con-
ductor, intercepts the heat, which would otherwise be imparted
to the water in the vessels from the earth. The porous nature
of the pans also allowing a portion of the water to penetrate
them, produces a rapid evaporation, by which a considerable
quantity of the heat of the water is carried off in a latent state
by the vapour. Heat is thus dismissed at once by evaporation
and radiation, and the temperature of the water in the pans is
diminished until it attains the freezing point. In the morning
the water is found frozen, and is collected and placed in cellars
surrounded with straw or other bad conductor, which prevents
its liquefaction.



1580. Heat developed or absorbed in chemical combination.
It has been already explained, that when two substances enter
into chemical combination, so as to form a new compound,
heat is generally either developed or absorbed, so that although
the components before their union have the same temperature,


the temperature of the compound which results will be gene-
rally above or below this common temperature, and sometimes
considerably so.

1581. This effect explained by specific heat of compound
being less or greater than that of components. If no change
in the state of aggregation of the constituents is produced by
their union, this phenomenon is explained by the specific heat
of the compound being less or greater than that of the com-
ponents, according as the temperature of the compound is
greater or less than that of the components. If greater, it is
because, the specific heat being less, the actual quantity of heat
contained in the compound gives it a higher temperature ; if
less, because it gives it a lower temperature.

1 582. Or by heat being developed or absorbed by change of
state. If the state of aggregation of either or both of the
components be changed, heat which was latent becomes sensible,
and raises the temperature of the compound ; or heat which was
sensible becomes latent, and lowers it. Thus when a solid
mixed with a liquid is dissolved in it, the solid in liquefying
absorbs and renders latent the same quantity of heat which
would have been necessary to melt it. This heat being ab-
stracted from the sensible heat of the compound lowers the tem-
perature. This phenomenon has been already noticed in the
case of freezing mixtures.

1583. Combustion. But of all the cases in which heat is
developed by chemical combination, the most important are those
in which combustion is produced.

When the quantity of heat suddenly developed by the chemical
combination of two bodies renders the compound luminous, the
bodies are said to burn, and the phenomenon is called combustion.
If the product of the combination be solid it is called fire ; if
gaseous, flame.

1584. Flame Flame, therefore, is gas rendered white hot
by the excessive heat developed in the combination which pro-
duces it.

1585. Agency of oxygen. It happens that, among the in-
finite variety of substances whose combination is productive of
this class of phenomena, one of the two combining bodies is
almost invariably oxygen gas. A few other substances, such as
chlorine, bromine, and iodine, produce similar effects ; but in all
ordinary cases of combustion, and universally where that effect

134 HEAT.

is resorted to as a source of artificial heat, one of the combining
substances is oxygen gas.

On this account this gas has been called a supporter of com-

1586. Combustibles The substances which combining with

it produce the phenomenon of combustion are called combus-

1587. Combustion explained. One of the circumstances
which render combustion so ordinary a phenomenon, is the fact
that the oxygen which forms one of the constituents of the at-
mosphere is either mechanically mixed in it, or, if chemically
united, is held in combination by the weakest possible affinity.
It therefore floats in the air in a state of almost complete freedom,
ready to combine with any body for which it has the least
affinity. When the temperature of a combustible, therefore, is
so elevated as to weaken sufficiently its cohesion, its molecules
enter into combination with the oxygen of the air, and heat and
light, and all the effects of combustion, are manifested.

1588. Temperature necessary to produce combustion. The
temperature necessary to produce this combination is different
for different substances ; phosphorus combines with oxygen, and
burns in the atmosphere if raised to 148. Hydrogen gas will
not burn till raised to incandescence. According to Sir H. Davy,
the temperatures necessary to the combustion of the several
combustibles here named are in the following order :

1. Phosphorus.

2. Phosphoretted hydrogen.

3. Hvdrogen and chlorine.

4. Sulphur.

5. Hydrogen and oxygen.

6. Olefiantgas.

7. Sulphuretted hydrogen.

8. Alcohol.

9. Wax.

10. Carbonic oxide.

1 1 . Carburetted hydrogen.

The heat developed iif the process of combustion is itself the
means of sustaining and rendering continuous the combustion.
If any source of heat of sufficient intensity be applied to the
wick of a candle, the matter of the wick will combine with the

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