nothing; and the law of conservation makes no statement regarding that
quality. It takes the facts of attraction as they stand, and affirms
only the constancy of working-power. That power may exist in the form
of MOTION; or it may exist in the form of FORCE, _with distance to act
through_. The former is dynamic energy, the latter is potential
energy, the constancy of the sum of both being affirmed by the law of
conservation. The convertibility of natural forces consists solely in
transformations of dynamic into potential, and of potential into
dynamic energy. In no other sense has the convertibility of force any
scientific meaning.
Grave errors have been entertained as to what is really intended to be
conserved by the doctrine of conservation. This exposition I hope
will tend to remove them.
********************
II. RADIATION.
[Footnote: The Rede Lecture delivered in the Senate House before the
University of Cambridge, May 16, 1865.]
*****
1. Visible and Invisible Radiation.
BETWEEN the mind of man and the outer world are interposed the nerves
of the human body, which translate, or enable the mind to translate,
the impressions of that world into facts of consciousness and thought.
Different nerves are suited to the perception of different
impressions. We do not see with the ear, nor hear with the eye, nor
are we rendered sensible of sound by the nerves of the tongue. Out of
the general assemblage of physical actions, each nerve, or group of
nerves, selects and responds to those for the perception of which it
is specially organised.
The optic nerve passes from the brain to the back of the eyeball and
there spreads out, to form the retina, a web of nerve filaments, on
which the images of external objects are projected by the optical
portion of the eye. This nerve is limited to the apprehension of the
phenomena of radiation, and, notwithstanding its marvellous
sensibility to certain impressions of this class, it is singularly
obtuse to other impressions.
Nor does the optic nerve embrace the entire range even of radiation.
Some rays, when they reach it, are incompetent to evoke its power,
while others never reach it at all, being absorbed by the humours 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 impunity; and at a certain definite temperature it emits a
feeble red light. As the current augments in power the light augments
in brilliancy, until finally the wire appears 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
refracted 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. Augmenting
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
colours at the same time the platinum wire must be _white-hot_: the
impression of whiteness being in fact produced by the simultaneous
action of all these colours on the optic nerve.
In the experiment just described we began with a platinum 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 becomes 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 incandescence, 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-luminous 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 kind of obscure rays emitted by all the planets 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 coloured constituents; he formed what is technically called
the solar spectrum. Exposing thermometers to the successive colours
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 verified 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 refrangible end. Ritter 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, therefore, 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 colours, 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 world.
********************
2. Origin and Character of Radiation. The Aether.
When we see a platinum wire raised gradually to a white heat, and
emitting in succession all the colours 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 colours originate,
but the mind irresistibly infers that the appearance of the colours
corresponds 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 agitation 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 nothing but
_motion_.' 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
generates new and more rapid ones, and when a certain definite
rapidity has been attained, the wire begins to glow. The colour first
exhibited is red, which corresponds to the lowest rate of vibration of
which the eye is able to take cognisance. By augmenting the strength
of the electric current 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 rapidity, we pass
through blue, indigo, and violet, to the extreme ultra-violet rays.
Such are the changes recognised 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 insensible 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
aether_. Every vibration of every atom of our platinum wire raises in
this aether a wave, which speeds through it at the rate of 186,000
miles a second.
The aether suffers no rupture of continuity at the surface of the eye,
the inter-molecular spaces of the various humours are filled with it;
hence the waves generated by the glowing platinum can cross these
humours and impinge on the optic nerve at the back of the eye.
[Footnote: The action here described is analogous to the passage of
sound-waves through thick felt whose interstices are occupied by air.]
Thus the sensation of light reduces itself to the acceptance of
motion. Up to this point we deal with pure mechanics; but the
subsequent translation of the shock of the aethereal waves into
consciousness eludes mechanical science. As an oar dipping into the
Cam generates systems of waves, which, speeding from the centre of
disturbance, finally stir the sedges on the river's bank, so do the
vibrating atoms generate in the surrounding aether 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 process 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 aether at rest; light as aether in
motion. But in reality the aether 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
ultimate extinction. Every star is seen across the entanglement of
wave-motions produced by all other stars. It is the ceaseless thrill
caused by those distant orbs collectively in the aether, that
constitutes what we call the 'temperature 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
inter-stellar aether 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.
All vagueness with regard to the use of the terms 'radiation' and
'absorption' will now disappear. Radiation is the communication of
vibratory motion to the aether; and when a body is said to be chilled
by radiation, 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 medium in which they
vibrate. On the other hand, the waves of aether may so strike against
the molecules of a body exposed to their action as to yield up their
motion to the latter; and in this transfer of the motion from the
aether 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 Aether.
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 mutual attraction, and under suitable
circumstances they coalesce 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 condition 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. Picturing 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 aether pervading space,
and constituting the vehicle of atomic motion, 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 aether, and the acceptance of motion
from it, the deportment of the uncombined atoms will be different from
that of the combined.
********************
4. Absorption of Radiant 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
defined 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. Gases and vapours 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
liberating the atoms and molecules entirely from the bonds of
cohesion, and employing them in the gaseous or vaporous form.
Let us endeavour to obtain a perfectly clear mental image of the
problem now before us. Limiting in the first place our enquiries to
the phenomena of absorption, we have to picture a succession of waves
issuing from a radiant source and passing through a gas; some of them
striking against the gaseous molecules and yielding up their motion to
the latter; others gliding round the molecules, or passing through the
intermolecular spaces without apparent 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 molecules
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 emerging 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 sensible 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, [Footnote: In
the Appendix to the first chapter of 'Heat as a Mode of Motion,' the
construction of the thermo-electric pile is fully explained.] 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 falling upon the pile. This
famous instrument, and not an ordinary thermometer, is what we shall
use in these enquiries, 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 anterior 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 neutralise the former. When the neutralisation 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 acting at the same time on the two
opposite faces of the pile. When by means of an adjusting screen,
perfectly equal 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 intercepting 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 intercept.
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 mercury 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 respective gases, the quantity
intercepted by air being taken as unity.
Radiation through Gases.
Name of gas Relative absorption
Air 1
Oxygen 1
Nitrogen 1
Hydrogen 1
Carbonic oxide 750
Carbonic acid 972
Hydrochloric acid. 1,005
Nitric oxide 1,590
Nitrous oxide 1,860
Sulphide of hydrogen 2,100
Ammonia 5,460
Olefiant gas 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
unexpected manner the influence of chemical combination. Thus the
elementary gases, oxygen, hydrogen, and nitrogen, and the mixture
atmospheric air, prove to be practical 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 sulphurous
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, hydrogen, and nitrogen, the waves of aether pass
without absorption, and these gases are not sensibly changed in
temperature by the most powerful calorific rays. The position of
nitrous oxide in the foregoing table is worthy of particular notice.
In this gas we have the same atoms in a state of chemical union, that
exist uncombined in the atmosphere; 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 attained 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
carbon) were to a considerable extent transparent to calorific rays of
low refrangibility. These facts, harmonising so strikingly with the
deportment of the simple gases, suggested further enquiry. Sulphur
dissolved in bisulphide of carbon was found almost perfectly
diathermic. The dense and deeply-coloured element bromine was
examined, and found competent to cut off the light of our most
brilliant 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 impracticable to examine the
substance in its usual solid condition. It however dissolves freely
in bisulphide of carbon. 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 radiant 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. [Footnote:
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.]