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in the solar spectrum beyond the red end. Their exist-
ence there demonstrates that they are emitted along
with radiations of shorter wave-length froni a source of
high temperature like the sun.

After Melloni had discovered that rock salt transmits
all kinds of non-luminous radiations with nearly equal
facility, while every other substance absorbs them with
avidity, he made use of rock-salt lenses and prisms to
demonstrate that the radiation from a non-luminous source
is capable of refraction. Glass is as opaque to radiation
from a non-luminous source as black glass is to the visual
rays. But by employing rock salt Melloni knew that the
radiation which he was studying was not stopped by the
substance of his prisms and lenses. He was thus able to
demonstrate that the radiation from a body at low tempera-
tures may be concentrated at the focus of a lens, and
may be refracted by a prism. The receiving apparatus



UAItlATinX A XT.) ABSORPTIOX. 109

employed in this investigation was a sensitive thermopile,
and the source for obscure rays a blackened copper cube
filled with water at 100 C.

Forbes subsequently measured the index of refraction
from several sources of varying temperature, and demon-
strated that the refrangibility for non-luminous rays is less
than for luminous rays. The following are the indices of
refraction of rock salt for the several sources :

Mean luminous rays 1.602

Heat from incandescent platinum 1.572

Heat from a lamp without a chimney 1.571

Heat from brass at 370 C ' 1.568

The refrangibility therefore decreases with the tempera-
ture of the source, and the obscure rays are of smaller
refrangibility or longer wave-length than the visual rays.

73. Polarization of Heat (S., 2OO). Another evi-
dence of the fundamental identity of radiant heat and
light is derived from experiments in polarization. Malus
and Berard first showed by rejection experiments, similar
to those applied to light (I., 228), that the radiant heat of
the sun is capable of polarization. Later Forbes showed
that whether the source were a lamp or brass heated below
luminosity, the radiation is polarized by transmission
through tourmaline, and suffers extinction in the same
manner as light when the two plates are crossed (I., 224).
By the use of mica plates split by heat and acting like a
bundle of plates, he demonstrated that dark heat is polar-
ized by reflection and refraction. Mica in this state is
nearly opaque to light, but transmits non-luminous radia-
tions quite freely. When two such plates are placed at
the proper angle with the beam, and with the one turned



110 HEAT.

90 around the beam with respect to the other, they were
found to stop a large portion of the incident heat, includ-
ing the radiation from a blackened vessel containing boiling
water.

From such facts as the foregoing it can be affirmed that
we have the most complete experimental evidence that
radiant heat and light are transmitted through the ether
by the same undulatory disturbance, whatever may be its
mechanism. Not only are Fraunhofer (absorption) lines
found in the visible solar spectrum, but the thermopile and
the bolometer l reveal their presence in the infra-red end.
Rowland's photographs of the solar spectrum, extending
beyond the visible limit at the violet end, exhibit no dis-
tinctions which mark the boundaries of the visible portion.
That limit is imposed by the structure and physiology of
the eye. Langley has measured the energy of the radiation
from his bolometer at 2 C. to a block of ice at 20 C.
The analogy between radiant heat and light does not need
the support of any additional evidence.

74. Heat the Measure of Radiant Energy (M., 238).
From all the facts at command we have reached the
conclusion that radiant heat, like light, is propagated as a
transverse undulation in the ether as a medium. If by
some means, such as transmission through a prism, the
radiations have been separated according to wave-length,
and if from them we select for examination those that will
excite vision when received into the eye, or initiate
chemical changes in the appropriate substance, or finally

1 The bolometer, invented by Professor S. P. Langley, is an instrument whose
operation depends on the change of electrical resistance with temperature. A
thin strip or grating of blackened metallic foil composes one arm of a Wheat-
stone's bridge. When it is exposed to radiation it is heated, and the heat-energv
can be measured by means of the deflection of a galvanometer.



RADIATION AND ABSORPTION. Ill

produce heat when absorbed by lampblack, then it will be
found that, as the intensity is changed, all of these effects
rise and fall together. It is therefore the same ethereal
disturbance which produces visual, actinic, or thermal
effects, according to the constitution of the absorbent
which determines its function.

But while these radiations produce three distinct effects,
only one of them can be taken as the measure of the
energy transmitted, viz., the heat generated when they
are completely absorbed. This is true, not only because
the visual or chemical impressions produced by different
kinds of radiations are not proportional to the energy in-
volved, but because they are specific effects depending
on wave-length. While the physiological effect of light
of a definite wave-length bears some relation to the energy
of the vibrations, yet neither in vision nor in photography
can the results be taken in any scientific sense as a
measure of the energy of the cause. Chemical changes
are doubtless initiated by light because of the co-vibra-
tional action, whereby the unstable molecular equilibrium
of certain chemical compounds is broken up and more
stable combinations follow as a result of molecular forces.
But the energy that topples over a brick at the top of a
building and initiates the downfall is not measured by the
effect produced by the brick in falling under the operation
of gravity.

On the other hand, when any radiation is completely
absorbed by lampblack, its energy has simply undergone a
transformation from the energy of ethereal vibrations into
the energy of molecular agitation, which is called heat.
An energy spectrum of the radiations from any source
may therefore be mapped out by means of appropriate
apparatus. This has been done by Professor Langley, not



112 HEAT.

only for the solar spectrum, but for the spectra of radia-
tions from blackened copper at several low temperatures.
One important conclusion reached by him is that when the
energy and wave-lengths are plotted as coordinates, the
maximum energy ordinate moves toward the shorter wave-
lengths as the temperature of the source rises.

75. Absorption of Radiation (S., 198; P., 464).-
We are familiar with what occurs when luminous radia-
tions are incident on a body. In general, one part is
reflected, another is transmitted, and a third is absorbed.
Thus, a piece of red glass reflects a portion of the incident
beam, transmits only light belonging near the red end of
the spectrum, and absorbs the rest, converting its energy
into heat. If the transmission is reduced to zero, the body
is opaque ; if the surface is composed of lampblack, the
reflected light is sensibly zero and the entire incident
beam is absorbed. The absorption which rejects the red
only is called selective absorption, while that of lamp-
black is general. Absorption may, however, be general
as contrasted with selective, without being total.

This division of incident radiation, either by general or
by selective absorption, is not peculiar to those radiations
that affect the eye. Bodies which transmit radiant heat are
said to be diathermanous, Avhile those which absorb it are
called athermanous. A body transparent to light is not
therefore transparent also to non-luminous radiations.
Common glass is transparent even to vibrations somewhat
beyond the violet of the solar spectrum ; but it is very
athermanous to long heat-waves. Melloni showed that a
sheet of glass 2.6 mms. thick stops all the radiation from
blackened copper at 100 C C., and all but 6 per cent from
copper at 390 C. If a sheet of glass be held between the



RADIATIOX -1.V7) ABSORPTION. 113

heated ball and the mirror in the experiment of Fig. 34,
little or no heat will be detected at the focus of the distant
mirror. All glass exhibits selective absorption, but col-
ored glass has its range of absorption extended to some
portions of the visible spectrum.

Hard rubber in thin sheets is opaque to light, but quite
transparent to long heat-waves. Carbon disulphide trans-
mits in almost equal degree the luminous and the non-
luminous rays ; but if iodine be dissolved in it, more and
more light will be cut off as iodine is added, till at length
the solution becomes opaque. But heat is still freely
transmitted, or the solution is diathermanous. Tyndall
demonstrated that, by enclosing it in a hollow lens with
rock-salt faces, it transmits enough heat from an electric
arc light to raise platinum to incandescence at the focus.
These facts lead to the conclusion that selective absorp-
tion extends throughout the entire spectrum, visible and
invisible.

76. Two Characteristics of Absorption. The radi-
ation from a hot body which has passed through one plate
is more easily able to pass through another of the same
substance. This is precisely similar to the fact that the
light which colored glass transmits is almost wholly trans-
mitted by a second piece of glass of the same kind.
Melloni found that a plate of alum which transmitted only
9 per cent of the radiation from a naked lamp transmitted
90 per cent of the heat coming through a plate of the same
material. A second plate of selenite transmits 91 per cent
of the radiation transmitted by a first one. It is possible
to find athermanous combinations, just as red and green
glass together are opaque to light. Thus alum and black
mica furm a nearlv athermanous combination.



114 HEAT.

The hypothesis to account for this fact applies the prin-
ciple of sympathetic vibration in Sound. Any resonant
body absorbs those vibrations which correspond with its
own vibration-rate (I., 151). So the molecules of every
substance are assumed to have vibration-rates of their own ;
and when the disturbances transmitted to them by the
associated ether have corresponding rates, the vibrations
are taken up by the body. Periodic disturbances of other
frequencies are rejected and pass through.

The second important general fact is that most sub-
stances, including those transparent to light, are nearly
opaque to radiations of long wave-length. It is much
easier to find transparent substances than diathermanous
ones. Rock salt is diathermanous in a remarkable degree,
but Balfour Stewart has shown that it absorbs those vibra-
tions of great wave-length which it radiates when heated ;
and Forbes has shown that the general index of refraction
of a beam of radiant heat is increased by transmission, indi-
cating that the percentage loss is the greater on the less
refrangible side. This rule is not without exceptions.
The solution of iodine in carbon disulphide is a case in
point ; and a piece of smoked rock salt stops most of the
light, but transmits heat.

77. Diathermancy of Liquids. - The diathermancy
of liquids was investigated by Mellon i by enclosing them
in a glass cell, while the source of heat was an Argand
lamp with a glass chimney. For such radiations water is
exceedingly opaque. The solution of a salt rather in-
creases its diathermancy. A solution of alum is slightly
more diathermanous than pure water. This conclusion is
contrary to the common opinion, but it has lately been
confirmed by Shelford Bidwell. The old notion that a



AXD ABSORPTION. 115

strong solution of alum is more athermanous than water
was probably derived from the fact that a plate of alum is
highly athermanous ; but it is less so than rock candy or
ice, though the thermopile will readily reveal the heat
transmitted through a block of the latter substance. Water
and ice appear to be pervious and impervious to the same
radiations, so that one may be used as a sieve to secure
radiations that will pass through the other.

In Tyndall's experiments the liquids were contained in
a cell with rock-salt faces, and the source of heat was an
incandescent platinum spiral. The results are in substan-
tial agreement with those of Melloni.

78. Diathermancy of Gases (P., 47O; Tyn., 274). l
Experiments on the most elaborate scale by Tyndall failed
to show any appreciable absorption of heat by dry air.
They were conducted by passing radiant heat through a
tube filled with pure air and closed at both ends with
plates of rock salt.

The old opinion that other gases and vapors are equally
diathermanous proved not to be true. Ammonia, olefiant
gas, sulphur dioxide, marsh gas, hydrogen disulphide, and
nitrous oxide were shown to absorb very perceptible por-
tions of the thermal flux through the tube.

When the temperature of the source is raised, the per-
centage of absorption diminishes. The diathermancy of
volatile liquids and that of their vapors appear to follow
nearly the same relative order. In the main, the molecules
retain their power as absorbers independently of the state
of aggregation. Since ice and water are very athermanous,
aqueous vapor may l>e expected to show marked absorption
of radiant heat. Tyndall's experiments lead to the con-

Tymlall'3 Contributions to Molecular Physics in the Domain of Radiant Heat.



116 HEAT.

elusion that this anticipation in regard to the opacity of
aqueous vapor is justified. But it has been contested by
Magnus, who found the effect of dry air to be precisely
the same as that of moist air, and " that the water present
in the atmosphere at 16 C. exercises no perceptible influ-
ence on the radiation."

79. Prevost's Theory of Exchanges (M., 240; S.,
204). If a warm body, such as a thermometer, be hung
within an enclosure cooler than itself, it will lose heat by
radiation and convection till thermal equilibrium ensues.
Even in a vacuum the equilibrium will be attained by
radiation alone. The question arises, Does all radiation
eease when the body and the enclosure are at the same
temperature, and does it radiate no heat when surrounded
by bodies warmer than itself ? If a cold body were intro-
duced into the enclosure it would immediately begin to
receive heat by radiation ; but it can have no direct effect
on the radiation of other bodies within the enclosure.
Prevost therefore came to the conclusion that the radia-
tion continues all the time, and that its intensity has
no relation to the temperature of other bodies, but is a
function of the nature of its surface and of its temper-
ature. If the body radiates more than it receives, its tem-
perature falls ; but if it receives more than it radiates, its
temperature rises. " If two bodies have the same tempera-
ture, the radiation emitted by the first and absorbed by the
second is equal in amount to the radiation emitted by the
second and absorbed by the first during the same time."

Prevost was probably led to this theory of exchanges, or
of a movable equilibrium of temperature, by the experi-
ment described in Art. 70, where the piece of ice at the
focus of one mirror caused a fall of temperature of the




KADI A TION^NI^^^S i PTIOy. 117



thermopile at the focus of the other. Since cold is only
the absence of heat, it is inadmissible to suppose that cold
is radiated. Such a supposition is not only unscientific,
but unnecessary. The thermopile radiates toward the ice
exactly as it radiates toward the hot ball, but it receives
from the ice less than it expends by radiation, and its
temperature therefore falls.

The two processes of radiation and absorption are then
going on simultaneously and continuously, and a stationary
temperature is maintained only so long as the emission
and the absorption are exactly equal to each other. Pre-
vost's theory has been greatly extended at various times
by Leslie, Stewart, Kirchhoff, and others. It has not only
been verified by subsequent investigations, but it has
suggested new theories which have also received experi-
mental verification. It is necessary to prepare the way
before proceeding to the extension of Prevost's theory, by
a brief account of Leslie's experiment on radiation and
by some definitions.

8O. Leslie's Experiment. Leslie examined the radi-
ating power of different surfaces by means of a hollow
metal cube ; one side was polished, a second was roughened,
a third was covered with varnish or with white lead, and
the fourth with lampblack. When the cube was filled with
boiling water the relative radiations from the several sur-
faces were compared ; the roughened surface was found to
radiate more freely than the polished one, while it was sur-
passed by the third and fourth, which exhibited nearly
equal radiating power.

In a similar way Leslie investigated the reflection of
heat from surfaces of different character, and found that
the best reflectors are the poorest radiators. Taking a



118 HEAT.

polished brass surface as a standard of comparison, he
found the following relative reflecting powers :

Brass 100 Lead . 60

Silver 90 Amalgamated tin ... 10

Tin 80 Glass 10

Steel 70 Lampblack

The absolute reflecting power, that is, the percentage
of incident radiation which is reflected, has since been
measured for several substances, with the following re-
sults :

Silver 0.97 Steel 0.82

Gold 0.95 Zinc 0.81

Brass ....... 0.93 Iron 0.77

Platinum 0.83 Cast iron 0.74

81. Definitions. Lampblack is taken as the standard
with which to compare the absorption and radiation of other
surfaces because it reflects no sensible part of the radiation
incident on it, and because it radiates more freely than any
other substance. Emissive power and absorbing power
may then be defined with respect to lampblack as follows :

The emissive power, or emissivity, of a surface is the
ratio of the quantity of radiation which it emits to the
quantity which a lampblack surface of equal area emits at
the same temperature in the same time.

The absorbing power of a surface is the ratio of the
quantity of radiation which it absorbs to the amount which
a lampblack surface of equal area would absorb in the
same time.

Since a lampblack surface is assumed to absorb all the
radiation which falls on it, the absorbing power of a body
under given conditions may be more simply defined as the



IfADIATION AND ABSORPTION.



119



fraction of the whole incident radiation which it absorbs
under those conditions.

These two quantities are connected by the simple re-
lation that the emissivity and absorbing power of any
surface at a given temperature are equal.

Tyndall found the following values by coating the faces
of a Leslie cube with powders of the different materials :

Substance. Absorbing power. Emissive power.

Rock salt 0.319 0.307

Fluor spar 0.577 0.589

Red oxide of lead 0.741 0.707

Oxide of cobalt 0.732 0.752

^ulphate of iron 0.824 0.808

These numbers do not differ greatly, considering the diffi-
culty of an exact numerical determination.

A simple experi-
ment demonstrates
the equality be-
tween the absorbing
power and the emis-
sivity. Let AB and
CD (Fig. 36) be two
tin plates with the
front of one polished
and that of the other
covered with lamp-
black. To the back
of each is soldered a
piece of bismuth E

to form a thermo- Fig 36

electric couple. The

polished and lampblack sides are arranged to face each
other, and between them is placed a Leslie cube L. The




120 HEAT.

side of the cube facing the polished plate is covered with
lampblack, while the side facing the lampblack is polished.
The wires at Or lead to a galvanometer. If one of the
thermoelectric junctions be heated more than the other,
the differential electromotive force generated will produce
a current.

If now the cube be filled with boiling water and be
placed exactly midway between the plates, the galvanom-
eter will show no current. Hence the amount of heat
absorbed by the two plates must be the same. The black-
ened face of the cube radiates more than the polished face ;
but the polished plate absorbs only a fraction of the inci-
dent radiation, while the blackened one absorbs all' the
radiation coming from the polished face of the cube pre-
sented to it. It follows that the fraction which the polished
plate absorbs is just equal to the fraction which the pol-
ished face radiates, both compared with lampblack, or the
emissive and absorbing powers of the polished plate are
the same.

82. Extension of Prevost's Theory (M., 243 ; S., 2O7 ;
P., 442). The theory of exchanges may be shown to
be applicable to every distinction in the
quality of the radiation as well as to the
total amount of it. By quality of radia-
tion is meant any specific difference, such
as wave-length or plane of polarization,
which may affect absorption.

Imagine a thermometer T suspended
in a blackened chamber with which it is
in thermal equilibrium (Fig. 37). It will]
Fig. 37. ^ j n equilibrium with the enclosure and

with everything in it in whatever part of the chamber it




liADIATIOy AXD ABSORPTION. 121

may be placed. Suppose its bulb to be covered with lamp-
black ; it then radiates and absorbs a maximum quantity
of heat, and its radiation equals its absorption because
its temperature remains constant. Another thermometer,
whose bulb is silvered, will indicate the same temperature ;
but it absorbs only about three per cent of the incident
radiation ; therefore to maintain its temperature unchanged,
it must radiate only the same small per cent as compared
with the blackened one. The same relation will hold
true for another thermometer covered with any other
substance.

If any part of the walls of the enclosure exhibits some
selective absorbing power, then the stream of radiation
from this part of the enclosure must remain unaltered
because the blackened thermometer maintains a constant
temperature ; therefore the wall must radiate specifically
what it absorbs, both in quantity and quality, so that the
emitted radiation added to the reflected radiation shall
equal lampblack radiation.

Suppose further that a thin plate of some substance,
which transmits radiations of certain definite wave-lengths
only, be suspended within the enclosure. This plate will
radiate just as much as it absorbs because its temperature
remains constant. But since the blackened thermometer
continues to receive the same radiation in amount and
quality from the direction of the plate, the latter must
emit on one side exactly the same quality of radiation
which it absorbs on the other, so that the transmitted plus
the emitted radiation shall remain equal both in quantity
and quality to the stream of radiant heat from that direc-
tion before the introduction of the plate.

It may thus be seen that the stream of radiation in such
an enclosure must be the same throughout in quantity and



122 HE A T.

m

quality, depending only on the temperature and not at all
on the materials and shape of the enclosure. In such an
enclosure the absorption is equal to the emission for every
kind of radiation.

The result may be generalized by saying that the
radiation reflected plus the radiation emitted by any sub-
stance in an enclosure of constant temperature equals the
total lampblack radiation at that temperature.

83. Illustrations of the Preceding Principle. - The
generalization of the last section has numerous conse-
quences, only a few of which can be alluded to here.

Balfour Stewart demonstrated by experiment that a
cold piece of rock salt absorbs in large part the radiation
emitted by a heated piece.

Air and other transparent gases are poor absorbers, and
when raised to a temperature at which opaque bodies
become incandescent, they emit so little light as to be
scarcely luminous in the dark.

The equality in the kind of the radiation and absorp-
tion is exhibited in a remarkable way by sodium vapor.
This substance when heated emits radiations of two wave-
lengths differing only slightly from each other. Now,
if light from a white-hot solid be transmitted through
sodium vapor of a lower temperature than the source, the
spectroscope reveals two dark absorption lines identical
in position with the bright-line spectrum of the sodium
vapor. If the temperature of the sodium vapor is raised
till it corresponds with that of the incandescent solid the
dark lines disappear, indicating that the absorption and



Online LibraryHenry S. (Henry Smith) CarhartPhysics for university students (Volume 2) → online text (page 8 of 28)