the non-luminous rays of the sun, and he also sought to render the
obscure rays visible by concentration. Intercepting the luminous
portion of his spectrum 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 renders
attainable far more powerful foci of invisible rays than could
possibly be obtained by the method of Sir William 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, luminous and non-luminous, we can
intercept the former by the iodine, and do what we please with the
latter. Experiments of this character, not only with the iodine
solution, but also with black glass and layers of lampblack, 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 previously.
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 introduced, and, as anticipated, the
invisible foci have been thereby rendered more intense. The mode of
operating remains however the same, in principle, as that made known
in 1862. It was then found that an instant's exposure of the face of
the thermoelectric pile to the focus of invisible rays, dashed the
needles of a coarse galvanometer violently aside. It is now found
that on substituting for the face of the thermo-electric pile a
combustible body, the invisible rays are competent to set that body on
fire.
********************
6. Visible and Invisible Rays of the Electric Light.
We have next to examine what proportion the non-luminous rays of the
electric light bear to the luminous ones. This the opaque solution of
iodine enables us to do with an extremely close approximation to the
truth.
The pure bisulphide of carbon, which is the solvent of the iodine, is
perfectly transparent to the luminous, and almost perfectly
transparent to the dark, rays of the electric lamp. Supposing the
total radiation of the lamp to pass through the transparent
bisulphide, while through the solution of iodine only the dark rays
are transmitted. If we determine, by means of a thermoelectric pile,
the total radiation, and deduct from it the purely obscure, we obtain
the value of the purely luminous emission. Experiments, performed in
this way, prove that if all the visible rays of the electric light
were converged to a focus of dazzling brilliancy, its heat would only
be one-eighth of that produced at the unseen focus of the invisible
rays.
Exposing his thermometers to the successive colours of the solar
spectrum, Sir William Herschel determined the heating power of each,
and also that of the region beyond the extreme red. Then drawing a
straight line to represent the length of the spectrum, he erected, at
various points, perpendiculars to represent the calorific intensity
existing at those points. Uniting the ends of all his perpendiculars,
he obtained a curve which showed at a glance the manner in which the
heat was distributed in the solar spectrum. Professor Müller of
Freiburg, with improved instruments, afterwards made similar
experiments, and constructed a more accurate diagram of the same kind.
We have now to examine the distribution of heat in the spectrum of the
electric light; and for this purpose we shall employ a particular form
of the thermo-electric pile, devised by Melloni. Its face is a
rectangle, which by means of movable side-pieces can be rendered as
narrow as desired. We can, for example, have the face of the pile the
tenth, the hundredth, or even the thousandth of an inch in breadth. By
means of an endless screw, this linear thermo-electric pile may be
moved through the entire spectrum, from the violet to the red, the
amount of heat falling upon the pile at every point of its march,
being declared by a magnetic needle associated with the pile.
When this instrument is brought up to the violet end of the spectrum
of the electric light, the heat is found to be insensible. As the
pile is gradually moved from the violet end towards the red, heat soon
manifests itself, augmenting as we approach the red. Of all the
colours of the visible spectrum the red possesses the highest heating
power. On pushing the pile into the dark region beyond the red, the
heat, instead of vanishing, rises suddenly and enormously in
intensity, until at some distance beyond the red it attains a maximum.
Moving the pile still forward, the thermal power falls, somewhat more
rapidly than it rose. It then gradually shades away, but, for a
distance beyond the red greater than the length of the whole visible
spectrum, signs of heat may be detected.
Drawing a datum line, and erecting along it perpendiculars,
proportional in length to the thermal intensity at the respective
points, we obtain the extraordinary curve, shown on the opposite page,
which exhibits the distribution of heat in the spectrum of the
electric light. In the region of dark rays, beyond the red, the curve
shoots up to B, in a steep and massive peak--a kind of Matterhorn of
heat, which dwarfs the portion of the diagram C D E, representing the
luminous radiation. Indeed the idea forced upon the mind by this
diagram is that the light rays are a mere insignificant appendage to
the heat-rays represented by the area A B C D, thrown in as it were by
nature for the purpose of vision.
Figure 1. Spectrum of Electric Light
The diagram drawn by Professor Müller to represent the distribution of
heat in the solar spectrum is not by any means so striking as that
just described, and the reason, doubtless, is that prior to reaching
the earth the solar rays have to traverse our atmosphere. By the
aqueous vapour there diffused, the summit of the peak representing the
sun's invisible radiation is cut off. A similar lowering of the
mountain of invisible heat is observed when the rays from the electric
light are permitted to pass through a film of water, which acts upon
them as the atmospheric vapour acts upon the rays of the sun.
********************
7. Combustion by Invisible Rays.
The sun's invisible rays far transcend the visible ones in heating
power, so that if the alleged performances of Archimedes during the
siege of Syracuse had any foundation in fact, the dark solar rays
would have been the philosopher's chief agents of combustion. On a
small scale we can readily produce, with the purely invisible rays of
the electric light, all that Archimedes is said to have performed with
the sun's total radiation. Placing behind the electric light a small
concave mirror, the rays are converged, the cone of reflected rays and
their point of convergence being rendered clearly visible by the dust
always floating in the air. Placing between the luminous focus and
the source of rays our solution of iodine, the light of the cone is
entirely cut away; but the intolerable heat experienced when the band
is placed, even for a moment, at the dark focus, shows that the
calorific rays pass unimpeded through the opaque solution.
Almost anything that ordinary fire can effect may be accomplished at
the focus of invisible rays; the air at the focus remaining at the
same time perfectly cold, on account of its transparency to the
heat-rays. An air thermometer, with a hollow rack-salt bulb, would be
unaffected by the heat of the focus: there would be no expansion, and
in the open air there is no convection. The aether at the focus, and
not the air, is the substance in which the heat is embodied. A block
of wood, placed at the focus, absorbs the heat, and dense volumes of
smoke rise swiftly upwards, showing the manner in which the air itself
would rise, if the invisible rays were competent to heat it. At the
perfectly dark focus dry paper is instantly inflamed: chips of wood
are speedily burnt up: lead, tin, and zinc are fused: and disks of
charred paper are raised to vivid incandescence. It might be supposed
that the obscure rays would show no preference for black over white;
but they do show a preference, and to obtain rapid combustion, the
body, if not already black, ought to be blackened. When metals are to
be burned, it is necessary to blacken or otherwise tarnish them, so as
to diminish their reflective power. Blackened zinc foil, when brought
into the focus of invisible rays, is instantly caused to blaze, and
burns with its peculiar purple light. Magnesium wire flattened, or
tarnished magnesium ribbon, also bursts into flame. Pieces of
charcoal suspended in a receiver full of oxygen are also set on fire
when the invisible focus falls upon them; the dark rays after having
passed through the receiver, still possessing sufficient power to
ignite the charcoal, and thus initiate the attack of the oxygen. If,
instead of being plunged in oxygen, the charcoal be suspended in
vacuo, it immediately glows at the place where the focus falls.
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8. Transmutation of Rays: Calorescence.
[Footnote: I borrow this term from Professor Challis, 'Philosophical
Magazine,' vol. xii. P. 521]
Eminent experimenters were long occupied in demonstrating the
substantial identity of light and radiant heat, and we have now the
means of offering a new and striking proof of this identity. A
concave mirror produces, beyond the object which it reflects, an
inverted and magnified image of the object. Withdrawing, for example,
our iodine solution, an intensely luminous inverted image of the
carbon points of the electric light is formed at the focus of the
mirror employed in the foregoing experiments. When the solution is
interposed, and the light is cut away, what becomes of this image? It
disappears from sight; but an invisible thermograph remains, and it is
only the peculiar constitution of our eyes that disqualifies us from
seeing the picture formed by the calorific rays. Falling on white
paper, the image chars itself out: falling on black paper, two holes
are pierced in it, corresponding to the images of the two coke points:
but falling on a thin plate of carbon in vacuo, or upon a thin sheet
of platinised platinum, either in vacuo or in air, radiant heat is
converted into light, and the image stamps itself in vivid
incandescence upon both the carbon and the metal. Results similar to
those obtained with the electric light have also been obtained with
the invisible rays of the lime-light and of the sun.
Before a Cambridge audience it is hardly necessary to refer to the
excellent researches of Professor Stokes at the opposite end of the
spectrum. The above results constitute a kind of complement to his
discoveries. Professor Stokes named the phenomena which he has
discovered and investigated _Fluorescence_; for the new phenomena here
described I have proposed the term _Calorescence_. He, by the
interposition of a proper medium, so lowered the refrangibility of the
ultraviolet rays of the spectrum as to render them visible. Here, by
the interposition of the platinum foil, the refrangibility of the
ultra-red rays is so exalted as to render them visible. Looking
through a prism at the incandescent image of the carbon points, the
light of the image is decomposed, and a complete spectrum is obtained.
The invisible rays of the electric light, remoulded by the atoms of
the platinum, shine thus visibly forth; ultra-red rays being converted
into red, orange, yellow, green, blue, indigo, violet, and ultraviolet
ones. Could we, moreover, raise the original source of rays to a
sufficiently high temperature, we might not only obtain from the dark
rays of such a source a single incandescent image, but from the dark
rays of this image we might obtain a second one, from the dark rays of
the second a third, and so on--a series of complete images and spectra
being thus extracted from the invisible emission of the primitive
source. [Footnote: On investigating the calorescence produced by rays
transmitted through glasses of various colours, it was found that in
the case of certain specimens of blue glass, the platinum foil glowed
with a pink or purplish light. The effect was not subjective, and
considerations of obvious interest are suggested by it. Different
kinds of black glass differ notably as to their power of transmitting
radiant heat. When thin, some descriptions tint the sun with a
greenish hue: others make it appear a glowing red without any trace of
green. The latter are far more diathermic than the former. In fact,
carbon when perfectly dissolved and incorporated with a good white
glass, is highly transparent to the calorific rays, and by employing
it as an absorbent the phenomena of 'calorescence' may be obtained,
though in a less striking form than with the iodine. The black glass
chosen for thermometers, and intended to absorb completely the solar
heat, may entirely fail in this object, if the glass in which the
carbon is incorporated be colourless. To render the bulb of a
thermometer a perfect absorbent, the glass ought in the first instance
to be green. Soon after the discovery of fluorescence the late Dr.
William Allen Miller pointed to the lime-light as an illustration of
exalted refrangibility. Direct experiments have since entirely
confirmed the view expressed at page 210 of his work on 'Chemistry,'
published in 1855.]
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9. Deadness of the Optic Nerve to the Calorific Rays.
The layer of iodine used in the foregoing experiments intercepted the
rays of the noonday sun. No trace of light from the electric lamp was
visible in the darkest room, even when a white screen was placed at
the focus of the mirror employed to concentrate the light. It was
thought, however, that if the retina itself were brought into the
focus the sensation of light might be experienced. The danger of this
experiment was twofold. If the dark rays were absorbed in a high
degree by the humours of the eye the albumen of the humours might
coagulate along the line of the rays. If, on the contrary, no such
high absorption took place, the rays might reach the retina with a
force sufficient to destroy it. To test the likelihood of these
results, experiments were made on water and on a solution of alum, and
they showed it to be very improbable that in the brief time requisite
for an experiment any serious damage could be done. The eye was
therefore caused to approach the dark focus, no defence, in the first
instance, being provided; but the heat, acting upon the parts
surrounding the pupil, could not be borne. An aperture was therefore
pierced in a plate of metal, and the eye, placed behind the aperture,
was caused to approach the point of convergence of invisible rays. The
focus was attained, first by the pupil and afterwards by the retina.
Removing the eye, but permitting the plate of metal to remain, a sheet
of platinum foil was placed in the position occupied by the retina a
moment before. The platinum became red-hot. No sensible damage was
done to the eye by this experiment; no impression of light was
produced; the optic nerve was not even conscious of heat.
But the humours of the eye are known to be highly impervious to the
invisible calorific rays, and the question therefore arises, 'Did the
radiation in the foregoing experiment reach the retina at all?' The
answer is, that the rays were in part transmitted to the retina, and
in part absorbed by the humours. Experiments on the eye of an ox
showed that the proportion of obscure rays which reached the retina
amounted to 18 per cent. of the total radiation; while the luminous
emission from the electric light amounts to no more than 10 per cent.
of the same total. Were the purely luminous rays of the electric lamp
converged by our mirror to a focus, there can be no doubt as to the
fate of a retina placed there. Its ruin would be inevitable; and yet
this would be accomplished by an amount of wave-motion but little more
than half of that which the retina, without exciting consciousness,
bears at the focus of invisible rays.
This subject will repay a moment's further attention. At a common
distance of a foot the visible radiation of the electric light
employed in these experiments is 800 times the light of a candle. At
the same distance, the portion of the radiation of the electric light
which reaches the retina, but fails to excite vision, is about 1,500
times the luminous radiation of the candle. [Footnote: It will be
borne in mind that the heat which any ray, luminous or non-luminous,
is competent to generate is the true measure of the energy of the
ray.] But a candle on a clear night can readily be seen at a distance
of a mile, its light at this distance being less than 1/20,000,000 of
its light at the distance of a foot.
Hence, to make the candle-light a mile off equal in power to the
non-luminous radiation received from the electric light at a foot
distance, its intensity would have to be multiplied by 1,500 x
20,000,000, or by thirty thousand millions. Thus the thirty thousand
millionth part of the invisible radiation from the electric light,
received by the retina at the distance of a foot, would, if slightly
changed in character, be amply sufficient to provoke vision. Nothing
could more forcibly illustrate that special relationship supposed by
Melloni and others to subsist between the optic nerve and the
oscillating periods of luminous bodies. The optic nerve responds, as
it were, to the waves with which it is in consonance, while it refuses
to be excited by others of almost infinitely greater energy, whose
periods of recurrence are not in unison with its own.
********************
10. Persistence of Rays.
At an early part of this lecture it was affirmed, that when a platinum
wire was, gradually raised to a state of high incandescence, new rays
were constantly added, while the intensity of the old ones was
increased. Thus, in Dr. Draper's experiments, the rise of temperature
that generated the orange, yellow, green, and blue augmented the
intensity of the red. What is true of the red is true of every other
ray of the spectrum, visible and invisible. We cannot indeed see the
augmentation of intensity in the region beyond the red, but we can
measure it and express it numerically. With this view the following
experiment was performed: A spiral of platinum wire was surrounded by
a small glass globe to protect it from currents of air; through an
orifice in the globe the rays could pass from the spiral and fall
afterwards upon a thermo-electric pile. Placing in front of the
orifice an opaque solution of iodine, the platinum was gradually
raised from a low dark heat to the fullest incandescence, with the
following results:
Appearance of spiral Energy of obscure radiation
Dark 1
Dark, but hotter 3
Dark, but still hotter 5
Dark, but still hotter 10
Feeble red 19
Dull red 25
Red 37
Full red. 62
Orange 89
Bright orange 144
Yellow 202
White 276
Intense white 440
Thus the augmentation of the electric current, which raises the wire
from its primitive dark condition to an intense white heat, exalts at
the same time the energy of the obscure radiation, until at the end it
is fully 440 times what it was at the beginning.
What has been here proved true of the totality of the ultra-red rays
is true for each of them singly. Placing our linear thermo-electric
pile in any part of the ultra-red spectrum, it may be proved that a
ray once emitted continues to be emitted with increased energy as the
temperature is augmented. The platinum spiral, so often referred to,
being raised to whiteness by an electric current, a brilliant spectrum
was formed from its light. A linear thermo-electric pile was placed
in the region of obscure rays beyond the red, and by diminishing the
current the spiral was reduced to a low temperature. It was then
caused to pass through various degrees of darkness and incandescence,
with the following results:
Appearance of spiral Energy of obscure rays
Dark 1
Dark 6
Faint red 10
Dull red 13
Red 18
Full red. 27
Orange 60
Yellow 93
White 122
Here, as in the former case, the dark and bright radiations reached
their maximum together; as the one augmented, the other augmented,
until at last the energy of the obscure rays of the particular
refrangibility here chosen, became 122 times what it was at first. To
reach a white heat the wire has to pass through all the stages of
invisible radiation, but in its most brilliant condition it embraces,
in an intensified form, the rays of all those stages.
And thus it is with all other kinds of matter, as far as they have
hitherto been examined. Coke, whether brought to a white heat by the
electric current, or by the oxyhydrogen jet, pours out invisible rays
with augmented energy, as its light is increased. The same is true of
lime, bricks, and other substances. It is true of all metals which
are capable of being heated to incandescence. It also holds good for
phosphorus burning in oxygen. Every gush of dazzling light has
associated with it a gush of invisible radiant heat, which far
transcends the light in energy. This condition of things applies to
all bodies capable of being raised to a white heat, either in the
solid or the molten condition. It would doubtless also apply to the
luminous fogs formed by the condensation of incandescent vapours. In
such cases when the curve representing the radiant energy of the body
is constructed, the obscure radiation towers upwards like a mountain,
the luminous radiation resembling a mere 'spur' at its base. From the
very brightness of the light of some of the fixed stars we may infer
the intensity of that dark radiation, which is the precursor and
inseparable associate of their luminous rays.
We thus find the luminous radiation appearing when the radiant body
has attained a certain temperature; or, in other words, when the
vibrating atoms of the body have attained a certain width of swing. In
solid and molten bodies a certain amplitude cannot be surpassed
without the introduction of periods of vibration, which provoke the
sense of vision. How are we to figure this? If permitted to
speculate, we might ask, are not these more rapid vibrations the
progeny of the slower? Is it not really the mutual action of the
atoms, when they swing through very wide spaces, and thus encroach
upon each other, that causes them to tremble in quicker periods? If
so, whatever be the agency by which the large swinging space is
obtained, we shall have light-giving vibrations associated with it. It
matters not whether the large amplitudes be produced by the strokes of
a hammer, or by the blows of the molecules of a non-luminous gas, like
air at some height above a gas-flame; or by the shock of the aether
particles when transmitting radiant heat. The result in all cases
will be incandescence. Thus, the invisible waves of our filtered
electric beam may be regarded as generating synchronous vibrations
among the atoms of the platinum on which they impinge; but, once these
vibrations have attained a certain amplitude, the mutual jostling of
the atoms produces quicker tremors, and the light-giving waves follow
as the necessary product of the heat-giving ones.
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11. Absorption of Radiant Heat by Vapours and Odours.