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obtuse to other impressions.

1 The Rede Lecture delivered in the Senate House before the University of
Cambridge, May 16, 1865.



Nor does the optic nerve embrace the entire range even
of radiation. Some rays, when they reach it, are incom-
petent to evoke its power, while others never reach it at
all, being absorbed by the humors of the eye. To all rays
which, whether they reach the retina or not, fail to excite
vision, we give the name of invisible or obscure rays. All
non- luminous bodies emit such rays. There is no body
in nature absolutely cold, and every body not absolutely
cold emits rays of heat. But to render radiant heat fit to
affect the optic nerve a certain temperature is necessary.
A cool poker thrust into a fire remains dark for a time,
but when its temperature has become equal to that of the
surrounding coals, it glows like them. In like manner, if
a current of electricity, of gradually increasing strength,
be sent through a wire of the refractory metal platinum,
the wire first becomes sensibly warm to the touch; for a
time its heat augments, still, however, remaining obscure;
at length we can no longer touch the metal with impu-
nity; and at a certain definite temperature it emits a fee-
ble red light. As the current augments in power the
light augments in brilliancy, until finally the wire ap-
pears of a dazzling white. The light which it now emits
is similar to that of the sun.

By means of a prism Sir Isaac Newton unravelled the
texture of solar light, and by the same simple instrument
we can investigate the luminous changes of our platinum
wire. In passing through the prism all its rays (and they
are infinite in variety) are bent or refracted from their
straight course; and, as different rays are differently re-
fracted by the prism, we are by it enabled to separate one
class of rays from another. By such prismatic analysis
Dr. Draper has shown, that when the platinum wire first


begins to glow the light emitted is sensibly red. As the
glow augments the red becomes more brilliant, but at the
same time orange rays are added to the emission. Aug-
menting the temperature still further, yellow rays appear
beside the orange; after the yellow, green rays are emitted;
and after the green come, in succession, blue, indigo, and
violet rays. To display all these colors at the same time
the platinjim wire must be white-hot: the impression of
whiteness being in fact produced by the simultaneous
action of all these colors on the optic nerve.

In the experiment just described we began with a pla-
tinum wire at an ordinary temperature, and gradually
raised it to a white heat. At the beginning, and even
before the electric current had acted at all upon the wire,
it emitted invisible rays. For some time after the action
of the current had commenced, and even for a time after
the wire had become intolerable to the touch, its radiation
was still invisible. The question now arises, What be-
comes of these invisible rays when the visible ones make
their appearance? It will be proved in the sequel that
they maintain themselves in the radiation; that a ray once
emitted continues to be emitted when the temperature is
increased, and hence the emission from our platinum wire,
even when it has attained its maximum brilliancy, consists
of a mixture of visible and invisible rays. If, instead of
the platinum wire, the earth itself were raised to incan-
descence, the obscure radiation which it now emits would
continue to be emitted. To reach incandescence the planet
would have to pass through all the stages of non-lumin v 'is
radiation, and the final emission would embrace the rays
of all these stages. There can hardly be a doubt that,
from the sun itself, rays proceed similar in kind to those


which the dark earth pours nightly into space. In fact,
the various kinds of obscure rays emitted by all the plan-
ets of our system are included in the present radiation of
the sun.

The great pioneer in this domain of science was Sir
William Herschel. Causing a beam of solar light to pass
through a prism, he resolved it into its colored constitu-
ents; he formed what is technically called the solar spec-
trum. Exposing thermometers to the successive colors, he
determined their heating power, and found it to augment
from the violet or most refracted end, to the red or least
refracted end of the spectrum. But he did not stop here.
Pushing his thermometers into the dark space beyond the
red he found that, though the light had disappeared, the
radiant heat falling on the instruments was more intense
than that at any visible part of the spectrum. In fact, Sir
William Herschel showed, and his results have been veri-
fied by various philosophers since his time, that, besides
its luminous rays, the sun pours forth a multitude of other
rays, more powerfully calorific than the luminous ones, but
entirely unsuited to the purposes of vision.

At the less refrangible end of the solar spectrum, then,
the range of the sun's radiation is not limited by that of
the eye. The same statement applies to the more refran-
gible end. Bitter discovered the extension of the spectrum
into the invisible region beyond the violet; and, in recent
times, this ultra-violet emission has had peculiar interest
conferred upon it by the admirable researches of Professor
Stokes. The complete spectrum of the sun consists, there-
fore, of three distinct parts: first, of ultra-red rays of high
heating power, but unsuited to the purposes of vision;
secondly, of luminous rays which display the succession


of colors, red, orange, yellow, green, blue, indigo, violet;
thirdly, of ultra-violet rays which, like the ultra-red ones,
are incompetent to excite vision, but which, unlike the
ultra- red rays, possess a very feeble heating power. In
consequence, however, of their chemical energy these ultra-
violet rays are of the utmost importance to the organic

2. Origin and Character of Radiation. The Ether

When we see a platinum wire raised gradually to a
white heat, and emitting in succession all the colors of the
spectrum, we are simply conscious of a series of changes in
the condition of our own eyes. "We do not see the actions
in which these successive colors originate, but the mind
irresistibly infers that the appearance of the colors corre-
sponds to certain contemporaneous changes in the wire.
What is the nature of these changes? In virtue of what
condition does the wire radiate at all ? We must now
look from the wire, as a whole, to its constituent atoms.
Could we see those atoms, even before the electric current
has begun to act upon them, we should find them in a
state of vibration. In this vibration, indeed, consists such
warmth as the wire then possesses. Locke enunciated this
idea with great precision, and it has been placed beyond
the pale of doubt by the excellent quantitative researches
of Mr. Joule. "Heat," says Locke, "is a very brisk agi-
tation of the insensible parts of the object, which produce
in us that sensation from which we denominate the object
hot: so what in our sensations is heat in the object is noth-
ing but motion. 11 When the electric current, still feeble,
begins to pass through the wire, its first act is to intensify
the vibrations already existing, by causing the atoms to


swing through wider ranges. Technically speaking, the
amplitudes of the oscillations are increased. The current
does this, however, without altering the periods of the old
vibrations, or the times in which they were executed. But
besides intensifying the old vibrations the current gener-
ates new and more rapid ones, and when a certain definite
rapidity has been attained, the wire begins to glow. The
color first exhibited is red, which corresponds to the low-
est rate of vibration of which the eye is able to take cog-
nizance. By augmenting the strength of the electric cur-
rent more rapid vibrations are introduced, and orange rays
appear. A quicker rate of vibration produces yellow, a
still quicker green; and by further augmenting the rapid-
ity, we pass through blue, indigo, and violet, to the ex-
treme ultra-violet rays.

Such are the changes recognized by the mind in the
wire itself, as concurrent with the visual changes taking
place in the eye. But what connects the wire with this
organ ? By what means does it send such intelligence of
its varying condition to the optic nerve? Heat being, as
defined by Locke, "a very brisk agitation of the insen-
sible parts of an object," it is readily conceivable that
on touching a heated body the agitation may communicate
itself to the adjacent nerves, and announce itself to them
as light or heat. But the optic nerve does not touch the
hot platinum, and hence the pertinence of the question,
By what agency are the vibrations of the wire transmitted
to the eye ?

The answer to this question involves one of the most
important physical conceptions that the mind of man has
yet achieved : the conception of a medium filling space and
fitted mechanically for the transmission of the vibrations


of light and heat, as air is fitted for the transmission of
sound. This medium is called the luminiferous ether.
Every vibration of every atom of our platinum wire raises
in this ether a wave, which speeds through it at the rate
of 186,000 miles a second. The ether suffers no rupture
of continuity at the surface of the eye, the inter-molecular
spaces of the various humors are filled with it; hence the
waves generated by the glowing platinum can cross these
humors and impinge on the optic nerve at the back of the
eye. ' Thus the sensation of light reduces itself to the ac-
ceptance of motion. Up to this point we deal with pure
mechanics; but the subsequent translation of the shock of
the ethereal waves into consciousness eludes mechanical
science. As an oar dipping into the Cam generates sys-
tems of waves, which, speeding from the centre of dis-
turbance, finally stir the sedges on the river's bank, so
do the vibrating atoms generate in the surrounding ether
undulations, which finally stir the filaments of the retina.
The motion thus imparted is transmitted with measurable,
and not very great, velocity to the brain, where, by a proc-
ess which the science of mechanics does not even tend to
unravel, the tremor of the nervous matter is converted into
the conscious impression of light.

Darkness might then be defined as ether at rest; light
as ether in motion. But in reality the ether is never at
rest, for in the absence of light-waves we have heat-waves
always speeding through it. In the spaces of the universe
both classes of undulations incessantly commingle. Here
the waves issuing from uncounted centres cross, coincide,
oppose, and pass through each other, without confusion or

1 The action here described is analogous to the passage of sound-waves
through thick felt whose interstices are occupied by air.


ultimate extinction. Every star is seen across the entan-
glement of wave -motions produced by all other stars. It
is the ceaseless thrill caused by those distant orbs collec-
tively in the ether that constitutes what we call the "tem-
perature of space." As the air of a room accommodates
itself to the requirements of an orchestra, transmitting each
vibration of every pipe and string, so does the interstellar
ether accommodate itself to the requirements of light and
heat. Its waves mingle in space without disorder, each
being endowed with an individuality as indestructible as
if it alone had disturbed the universal repose.

AIL vagueness with regard to the use of the terms
* 'radiation" and * 'absorption" will now disappear. Kadi-
ation is the communication of vibratory motion to the
ether; and when a body is said to be chilled by radia-
tion, as for example the grass of a meadow on a starlight
night, the meaning is, that the molecules of the grass have
lost a portion of their motion, by imparting it to the me-
dium in which they vibrate. On the other hand, the waves
of ether may so strike against the molecules of a body ex-
posed to their action as to yield up their motion to the
latter; and in this transfer of the motion from the ether
to the molecules consists the absorption of radiant heat.
All the phenomena of heat are in this way reducible to
interchanges of motion; and it is purely as the recipients
or the donors of this motion that we ourselves become
conscious of the action of heat and cold.

3. The Atomic Theory in reference to the Ether

The word "atoms" has been more than once employed
in this discourse. Chemists have taught us that all matter
is reducible to certain elementary forms to which they give


this name. These atoms are endowed with powers of mu-
tual attraction, and under suitable circumstances they coa-
lesce to form compounds. Thus oxygen and hydrogen are
elements when separate, or merely mixed, but they may be
made to combine so as to form molecules, each consisting
of two atoms of hydrogen and one of oxygen. In this con-
dition they constitute water. So also chlorine and sodium
are elements, the former a pungent gas, the latter a soft
metal; and they unite together to form chloride of sodium
or common salt. In the same way the element nitrogen
combines with hydrogen, in the proportion of one atom of
the former to three of the latter, to form ammonia. Pict-
uring in imagination the atoms of elementary bodies as
little spheres, the molecules of compound bodies must be
pictured as groups of such spheres. This is the atomic
theory as Dalton conceived it. Now, if this theory have
any foundation in fact, and if the theory of an ether per-
vading space, and constituting the vehicle of atomic mo-
tion, be founded in fact, it is surely of interest to examine
whether the vibrations of elementary bodies are modified
by the act of combination whether as regards radiation
and absorption, or, in other words, whether as regards the
communication of motion to the ether, and the acceptance
of motion from it, the deportment of the uncombined atoms
will be different from that of the combined.

4. Absorption of Itadiant Heat by Gases

We have now to submit these considerations to the
only test by which they can be tried, namely, that of
experiment. An experiment is well denned as a question
put to Nature; but, to avoid the risk of asking amiss, we
ought to purify the question from all adjuncts which do


not necessarily belong to it. Matter has been shown to be
composed of elementary constituents, by the compounding
of which all its varieties are produced. But, besides the
chemical unions which they form, both elementary and
compound bodies can unite in another and less intimate
way. Grases and vapors aggregate to liquids and solids,
without any change of their chemical nature. We do not
yet know how the transmission of radiant heat may be
affected by the entanglement due to cohesion; and, as our
object now is to examine the influence of chemical union
alone, we shall render our experiments more pure by lib-
erating the atoms and molecules entirely from the bonds
of cohesion, and employing them in the gaseous or vapor-
ous form.

Let us endeavor to obtain a perfectly clear mental im-
age of the problem now before us. Limiting in the first
place our inquiries to the phenomena of absorption, we
have to picture a succession of waves issuing from a radi-
ant source and passing through a gas; some of them strik-
ing against the gaseous molecules and yielding up their
motion to the latter; others gliding round the molecules,
or passing through the inter- molecular spaces without ap-
parent hindrance. The problem before us is to determine
whether such free molecules have any power whatever to
stop the waves of heat; and if so, whether different mole-
cules possess this power in different degrees.

In examining the problem let us fall back upon an
actual piece of work, choosing as the source of our heat-
waves a plate of copper, against the back of which a
steady sheet of flame is permitted to play. On emerg-
ing from the copper, the waves, in the first instance, pass
through a space devoid of air, and then enter a hollow


glass cylinder, three feet long and three inches wide. The
two ends of this cylinder are stopped by two plates of
rock-salt, a solid substance which offers a scarcely sensi-
ble obstacle to the passage of the calorific waves. After
passing through the tube, the radiant heat falls upon the
anterior face of a thermo-electric pile, 1 which instantly
converts the heat into an electric current. This current
conducted round a magnetic needle deflects it, and the
magnitude of the deflection is a measure of the heat fall-
ing upon the pile. This famous instrument, and not an
ordinary thermometer, is what we shall use in these in-
quiries, but we shall use it in a somewhat novel way. As
long as the two opposite faces of the thermo-electric pile
are kept at the same temperature, no matter how high that
may be, there is no current generated. The current is a
consequence of a difference of temperature between the
two opposite faces of the pile. Hence, if after the ante-
rior face has received the heat from our radiating source,
a second source, which we may call the compensating
source, be permitted to radiate against the posterior face,
this latter radiation will tend to neutralize the former.
When the neutralization is perfect, the magnetic needle
connected with the pile is no longer deflected, but points
to the zero of the graduated circle over which it hangs.

And now let us suppose the glass tube, through which
the waves from the heated plate of copper are passing, to
be exhausted by an air-pump, the two sources of heat act-
ing at the same time on the two opposite faces of the pile.
When, by means of an adjusting screen, perfectly equal

1 In the Appendix to the first chapter of "Heat as a Mode of Motion," the
construction of the thermo-electric pile is fully explained.


quantities of heat are imparted to the two faces, the needle
points to zero. Let any gas be now permitted to enter the
exhausted tube; if its molecules possess any power of in-
tercepting the calorific waves, the equilibrium previously
existing will be destroyed, the compensating source will
triumph, and a deflection of the magnetic needle will be
the immediate consequence. From the deflections thus
produced by different gases we can readily deduce the
relative amounts of wave-motion which their molecules

In this way the substances mentioned in the following
table were examined, a small portion only of each being
admitted into the glass tube. The quantity admitted in
each case was just sufficient to depress a column of mer-
cury associated with the tube one inch: in other words,
the gases were examined at a pressure of one-thirtieth of
an atmosphere. The numbers in the table express the
relative amounts of wave- motion absorbed by the respec-
tive gases, the quantity intercepted by atmospheric air
being taken as unity.

Radiation through Gases

Name of gas


Air .


Oxygen .


Nitrogen .




Carbonic oxide .


Carbonic acid .


Hydrochloric acid .



Nitrous oxide 1,860

Sulphide of hydrogen 2,100

Ammonia * .... 5,460

Olefiantgas . .... 6,030

Sulphurous acid .*:.... 6,480


Every gas in this table is perfectly transparent to light,
that is to say, all waves within the limits of the visible
spectrum pass through it without obstruction; but for the
waves of slower period, emanating from our heated plate
of copper, enormous differences of absorptive power are
manifested. These differences illustrate in the most un-
expected manner the influence of chemical combination.
Thus the elementary gases, oxygen, hydrogen, and nitro-
gen, and the mixture atmospheric air, prove to be prac-
tical vacua to the rays of heat; for every ray, or, more
strictly speaking, for every unit of wave -motion, which
any one of them intercepts, perfectly transparent ammonia
intercepts 5,460 units, olefiant gas 6,030 units, while sul-
phurous acid gas absorbs 6,480 units. What becomes of
the wave-motion thus intercepted? It is applied to the
heating of the absorbing gas. Through air, oxygen, hy-
drogen, and nitrogen, the waves of ether pass without ab-
sorption, and these gases are not sensibly changed in tem-
perature by the most powerful calorific rays. The position
of nitrous oxide in the foregoing table is worthy of par-
ticular notice. In this gas we have the same atoms in a
state of chemical union that exist uncombined in the at-
mosphere; but the absorption of the compound is 1,800
times that of air.

5. Formation of Invisible Foci

This extraordinary deportment of the elementary gases
naturally directed attention to elementary bodies in other
states of aggregation. Some of Melloni's results now at-
tained a new significance. This celebrated experimenter
had found crystals of sulphur to be highly pervious to
radiant heat; he had also proved that lamp-black, and


black glass (which owes its blackness to the element car-
bon), were to a considerable extent transparent to calorific
rays of low refrangibility. These facts, harmonizing so
strikingly with the deportment of the simple gases, sug-
gested further inquiry. Sulphur dissolved in bisulphide
of carbon was found almost perfectly diathermic. The
dense and deeply -colored element bromine was examined,
and found competent to cut off the light of our most bril-
liant flames, while it transmitted the invisible calorific rays
with extreme freedom. Iodine, the companion element of
bromine, was next thought of, but it was found impracti-
cable to examine the substance in its usual solid condi-
tion. It, however, dissolves freely in bisulphide of car-
bon. There is no chemical union between the liquid and
the iodine; it is simply a case of solution, in which the
uncombined atoms of the element can act upon the radi-
ant heat. When permitted to do so, it was found that a
layer of dissolved iodine, sufficiently opaque to cut off the
light of the midday sun, was almost absolutely transparent
to the invisible calorific rays. 1

By prismatic analysis Sir William Herschel separated
the luminous from the non-luminous rays of the sun, and
he also sought to render the obscure rays visible by con-
centration. Intercepting the luminous portion of his spec-
trum, he brought, by a converging lens, the ultra-red rays
to a focus, but by this condensation he obtained no light.
The solution of iodine offers a means of filtering the solar
beam, or, failing it, the beam of the electric lamp, which

1 Professor Dewar has recently succeeded in producing a medium highly
opaque to light, and highly transparent to obscure heat, by fusing together
sulphur and iodine.


renders attainable far more powerful foci of invisible rays
than could possibly be obtained by the method of Sir Wil-
liam Herschel. For to form his spectrum he was obliged
to operate upon solar light which had passed through a
narrow slit or through a small aperture, the amount of the
obscure heat being limited by this circumstance. But with
our opaque solution we may employ the entire surface of
the largest lens, and having thus converged the rays, lumi-
nous and non-luminous, we can intercept the former by the
iodine, and do what we please with the latter. Experi-
ments of this character, not only with the iodine solution,
but also with black glass and layers of lamp-black, were
publicly performed at the Royal Institution in the early
part of 1862, and the effects at the foci of invisible rays,
then obtained, were such as had never been witnessed

In the experiments here referred to, glass lenses were
employed to concentrate the rays. But glass, though
highly transparent to the luminous, is in a high degree
opaque to the invisible, heat-rays of the electric lamp, and
hence a large portion of those rays was intercepted by the
glass. The obvious remedy here is to employ rock-salt
lenses instead of glass ones, or to abandon the use of
lenses wholly, and to concentrate the rays by a metallic
mirror. Both of these improvements have been intro-
duced, and, as anticipated, the invisible foci have been
thereby rendered more intense. The mode of operating

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