surface of our screen, but upon a sheet of paper which has been wetted
with a saturated solution of the sulphate of quinine and afterwards
dried, an obvious extension of the spectrum is revealed. We have, in
the first instance, a portion of the violet rendered whiter and more
brilliant; but, besides this, we have the gleaming of the colour
where, in the case of unprepared paper, nothing is seen. Other
substances produce a similar effect. A substance, for example,
recently discovered by President Morton, and named by him _Thallene_,
produces a very striking elongation of the spectrum, the new light
generated being of peculiar brilliancy.
Fluor spar, and some other substances, when raised to a temperature
still under redness, emit light. During the ages which have elapsed
since their formation, this capacity of shaking the ether into visual
tremors appears to have been enjoyed by these substances. Light has
been potential within them all this time; and, as well explained by
Draper, the heat, though not itself of visual intensity, can unlock
the molecules so as to enable them to exert their long-latent power of
vibration. This deportment of fluor spar determined Stokes in his
choice of a name for his great discovery: he called this rendering
visible of the ultra-violet rays _Fluorescence_.
By means of a deeply coloured violet glass, we cut off almost the
whole of the light of our electric beam; but this glass is peculiarly
transparent to the violet and ultra-violet rays. The violet beam now
crosses a large jar filled with water, into which I pour a solution of
sulphate of quinine. Clouds, to all appearance opaque, instantly
tumble downwards. Fragments of horse-chestnut bark thrown upon the
water also send down beautiful cloud-like strife. But these are not
clouds: there is nothing precipitated here: the observed action is an
action of _molecules_, not of _particles_. The medium before you is
not a turbid medium, for when you look through it at a luminous
surface it is perfectly clear.
If we paint upon a piece of paper a flower or a bouquet with the
sulphate of quinine, and expose it to the full beam, scarcely anything
is seen. But on interposing the violet glass, the design instantly
flashes forth in strong contrast with the deep surrounding violet.
President Morton has prepared for me a most beautiful example of such
a design which, when placed in the violet light, exhibits a peculiarly
brilliant fluorescence. From the experiments of Drs. Bence Jones and
Dupré, it would seem that there is some substance in the human body
resembling the sulphate of quinine, which causes all the tissues of
the body to be more or less fluorescent. All animal infusions show
this fluorescence. The crystalline lens of the eye exhibits the effect
in a very striking manner. When, for example, I plunge my eye into
this violet beam, I am conscious of a whitish-blue shimmer filling the
space before me. This is caused by fluorescent light generated in the
eye itself. Looked at from without, the crystalline lens at the same
time is seen to gleam vividly.
Long before its physical origin was understood this fluorescent light
attracted attention. Boyle describes it with great fulness and
exactness. 'We have sometimes,' he says, 'found in the shops of our
druggists certain wood which is there called _Lignum Nephriticum,_
because the inhabitants of the country where it grows are wont to use
the infusion of it, made in fair water, against the stone in the
kidneys. This wood may afford us an experiment which, besides the
singularity of it, may give no small assistance to an attentive
considerer towards the detection of the nature of colours. Take
_Lignum, Nephriticum_, and with a knife cut it into thin slices: put
about a handful of these slices into two or three or four pounds of
the purest spring water. Decant this impregnated water into a glass
phial; and if you hold it directly between the light and your eye, you
shall see it wholly tinted with an almost golden colour. But if you
hold this phial from the light, so that your eye be placed betwixt the
window and the phial, the liquid will appear of a deep and lovely
ceruleous colour.'
'These,' he continues, 'and other phenomena which I have observed in
this delightful experiment, divers of my friends have looked upon, not
without some wonder; and I remember an excellent oculist, finding by
accident in a friend's chamber a phial full of this liquor, which I
had given that friend, and having never heard anything of the
experiment, nor having anybody near him who could tell him what this
strange liquor might be, was a great while apprehensive, as he
presently afterwards told me, that some strange new distemper was
invading his eyes. And I confess that the unusualness of the
phenomenon made me very solicitous to find out the cause of this
experiment; and though I am far from pretending to have found it, yet
my enquiries have, I suppose, enabled me to give such hints as may
lead your greater sagacity to the discovery of the cause of this
wonder.'[21]
Goethe in his 'Farbenlehre' thus describes the fluorescence of
horse-chestnut bark:--'Let a strip of fresh horse-chestnut bark be
taken and clipped into a glass of water; the most perfect sky-blue
will be immediately produced.'[22] Sir John Herschel first noticed and
described the fluorescence of the sulphate of quinine, and showed that
the light proceeded from a thin stratum of the solution adjacent to
the surface where the light enters it. He showed, moreover, that the
incident beam, although not sensibly weakened in luminous intensity,
lost, in its transmission through the solution of sulphate of quinine,
the power of producing the blue fluorescent light. Sir David Brewster
also worked at the subject; but to Professor Stokes we are indebted
not only for its expansion, but for its full and final explanation.
§ 3. _The Heat of the Electric Beam. Ignition through a Lens of Ice.
Possible Cometary Temperature_.
But the waves from our incandescent carbon-points appeal to another
sense than that of vision. They not only produce light, but heat, as a
sensation. The magnified image of the carbon-points is now upon the
screen; and with a suitable instrument the heating power of the rays
which form that image might be readily demonstrated. In this case,
however, the heat is spread over too large an area to be very intense.
Drawing out the camera lens, and causing a movable screen to approach
the lamp, the image is seen to become smaller and smaller; the rays at
the same time becoming more and more concentrated, until finally they
are able to pierce black paper with a burning ring. Pushing back the
lens so as to render the rays parallel, and receiving them upon a
concave mirror, they are brought to a focus; paper placed at that
focus is caused to smoke and burn. Heat of this intensity may be
obtained with our ordinary camera and lens, and a concave mirror of
very moderate power.
[Illustration: Fig. 48.]
We will now adopt stronger measures with the radiation. In this larger
camera of blackened tin is placed a lamp, in all particulars similar
to those already employed. But instead of gathering up the rays from
the carbon-points by a condensing lens, we gather them up by a concave
mirror (_m_ _m'_, fig. 48), silvered in front and placed behind the
carbons (P). By this mirror we can cause the rays to issue through the
orifice in front of the camera, either parallel or convergent. They
are now parallel, and therefore to a certain extent diffused. We place
a convex lens (L) in the path of the beam; the light is converged to a
focus (C), and at that focus paper is not only pierced, but it is
instantly set ablaze.
Many metals may be burned up in the same way. In our first lecture
the combustibility of zinc was mentioned. Placing a strip of
sheet-zinc at this focus, it is instantly ignited, burning with its
characteristic purple flame. And now I will substitute for our glass
lens (L) one of a more novel character. In a smooth iron mould a lens
of pellucid ice has been formed. Placing it in the position occupied a
moment ago by the glass lens, I can see the beam brought to a sharp
focus. At the focus I place, a bit of black paper, with a little
gun-cotton folded up within it. The paper immediately ignites and the
cotton explodes. Strange, is it not, that the beam should possess such
heating power after having passed through so cold a substance? In his
arctic expeditions Dr. Scoresby succeeded in exploding gunpowder by
the sun's rays, converged by large lenses of ice; here we have
succeeded in producing the effect with a small lens, and with a
terrestrial source of heat.
In this experiment, you observe that, before the beam reaches the
ice-lens, it has passed through a glass cell containing water. The
beam is thus sifted of constituents, which, if permitted to fall upon
the lens, would injure its surface, and blur the focus. And this leads
me to say an anticipatory word regarding transparency. In our first
lecture we entered fully into the production of colours by absorption,
and we spoke repeatedly of the quenching of the rays of light. Did
this mean that the light was altogether annihilated? By no means. It
was simply so lowered in refrangibility as to escape the visual range.
It was converted into heat. Our red ribbon in the green of the
spectrum quenched the green, but if suitably examined its temperature
would have been found raised. Our green ribbon in the red of the
spectrum quenched the red, but its temperature at the same time was
augmented to a degree exactly equivalent to the light extinguished.
Our black ribbon, when passed through the spectrum, was found
competent to quench all its colours; but at every stage of its
progress an amount of heat was generated in the ribbon exactly
equivalent to the light lost. It is only when _absorption_ takes place
that heat is thus produced: and heat is always a result of absorption.
Examine the water, then, in front of the lamp after the beam has
passed through it: it is sensibly warm, and, if permitted to remain
there long enough, it might be made to boil. This is due to the
absorption, by the water, of a certain portion of the electric beam.
But a portion passes through unabsorbed, and does not at all
contribute to the heating of the water. Now, ice is also in great part
transparent to these latter rays, and therefore is but little melted
by them. Hence, by employing the portion of the beam transmitted by
water, we are able to keep our lens intact, and to produce by means of
it a sharply defined focus. Placed at that focus, white paper is not
ignited, because it fails to absorb the rays emergent from the
ice-lens. At the same place, however, black paper instantly burns,
because it absorbs the transmitted light.
And here it may be useful to refer to an estimate by Newton, based
upon doubtful data, but repeated by various astronomers of eminence
since his time. The comet of 1680, when nearest to the sun, was only a
sixth of the sun's diameter from his surface. Newton estimated its
temperature, in this position, to be more than two thousand times that
of molted iron. Now it is clear from the foregoing experiments that
the temperature of the comet could not be inferred from its nearness
to the sun. If its power of absorption were sufficiently low, the
comet might carry into the sun's neighbourhood the chill of stellar
space.
§ 4. _Combustion of a Diamond by Radiant Heat_.
The experiment of burning a diamond in oxygen by the concentrated rays
of the sun was repeated at Florence, in presence of Sir Humphry Davy,
on Tuesday, the 27th of March, 1814. It is thus described by
Faraday:--'To-day we made the grand experiment of burning the diamond,
and certainly the phenomena presented were extremely beautiful and
interesting. A glass globe containing about 22 cubical inches was
exhausted of air, and filled with pure oxygen. The diamond was
supported in the centre of this globe. The Duke's burning-glass was
the instrument used to apply heat to the diamond. It consists of two
double convex lenses, distant from each other about 3½ feet; the large
lens is about 14 or 15 inches in diameter, the smaller one about 3
inches in diameter. By means of the second lens the focus is very much
reduced, and the heat, when the sun shines brightly, rendered very
intense. The diamond was placed in the focus and anxiously watched. On
a sudden Sir H. Davy observed the diamond to burn visibly, and when
removed from the focus it was found to be in a state of active and
rapid combustion.'
The combustion of the diamond had never been effected by radiant heat
from a terrestrial source. I tried to accomplish this before crossing
the Atlantic, and succeeded in doing so. The small diamond now in my
hand is held by a loop of platinum wire. To protect it as far as
possible from air currents, and also to concentrate the heat upon it,
it is surrounded by a hood of sheet platinum. Bringing a jar of oxygen
underneath, I cause the focus of the electric beam to fall upon the
diamond. A small fraction of the time expended in the experiment
described by Faraday suffices to raise the diamond to a brilliant red.
Plunging it then into the oxygen, it glows like a little white star;
and it would continue to burn and glow until wholly consumed. The
focus can also be made to fall upon the diamond in oxygen, as in the
Florentine experiment: the result is the same. It was simply to secure
more complete mastery over the position of the focus, so as to cause
it to fall accurately upon the diamond, that the mode of experiment
here described was resorted to.
§ 5. _Ultra-red Rays: Calorescence_.
In the path of the beam issuing from our lamp I now place a cell with
glass sides containing a solution of alum. All the _light_ of the beam
passes through this solution. This light is received on a powerfully
converging mirror silvered in front, and brought to a focus by the
mirror. You can see the conical beam of reflected light tracking
itself through the dust of the room. A scrap of white paper placed at
the focus shines there with dazzling brightness, but it is not even
charred. On removing the alum cell, however, the paper instantly
inflames. There must, therefore, be something in this beam besides its
light. The _light_ is not absorbed by the white paper, and therefore
does not burn the paper; but there is something over and above the
light which _is_ absorbed, and which provokes combustion. What is this
something?
In the year 1800 Sir William Herschel passed a thermometer through
the various colours of the solar spectrum, and marked the rise of
temperature corresponding to each colour. He found the heating effect
to augment from the violet to the red; he did not, however, stop at
the red, but pushed his thermometer into the dark space beyond it.
Here he found the temperature actually higher than in any part of the
visible spectrum. By this important observation, he proved that the
sun emitted heat-rays which are entirely unfit for the purposes of
vision. The subject was subsequently taken up by Seebeck, Melloni,
Müller, and others, and within the last few years it has been found
capable of unexpected expansions and applications. I have devised a
method whereby the solar or electric beam can be so _filtered_ as to
detach from it, and preserve intact, this invisible ultra-red
emission, while the visible and ultra-violet emissions are wholly
intercepted. We are thus enabled to operate at will upon the purely
ultra-red waves.
In the heating of solid bodies to incandescence, this non-visual
emission is the necessary basis of the visual. A platinum wire is
stretched in front of the table, and through it an electric current
flows. It is warmed by the current, and may be felt to be warm by the
hand. It emits waves of heat, but no light. Augmenting the strength of
the current, the wire becomes hotter; it finally glows with a sober
red light. At this point Dr. Draper many years ago began an
interesting investigation. He employed a voltaic current to heat his
platinum, and he studied, by means of a prism, the successive
introduction of the colours of the spectrum. His first colour, as
here, was red; then came orange, then yellow, then green, and lastly
all the shades of blue. As the temperature of the platinum was
gradually augmented, the atoms were caused to vibrate more rapidly;
shorter waves were thus introduced, until finally waves were obtained
corresponding to the entire spectrum. As each successive colour was
introduced, the colours preceding it became more vivid. Now the
vividness or intensity of light, like that of sound, depends not upon
the length of the wave, but on the amplitude of the vibration. Hence,
as the less refrangible colours grew more intense when the more
refrangible ones were introduced, we are forced to conclude that side
by side with the introduction of the shorter waves we had an
augmentation of the amplitude of the longer ones.
These remarks apply not only to the visible emission examined by Dr.
Draper, but to the invisible emission which precedes the appearance of
any light. In the emission from the white-hot platinum wire now before
you, the lightless waves exist with which we started, only their
intensity has been increased a thousand-fold by the augmentation of
temperature necessary to the production of this white light. Both
effects are bound up together: in an incandescent solid, or in a
molten solid, you cannot have the shorter waves without this
intensification of the longer ones. A sun is possible only on these
conditions; hence Sir William Herschel's discovery of the invisible
ultra-red solar emission.
The invisible heat, emitted both by dark bodies and by luminous ones,
flies through space with the velosity of light, and is called _radiant
heat_. Now, radiant heat may be made a subtle and powerful explorer of
molecular condition, and, of late years, it has given a new
significance to the act of chemical combination. Take, for example,
the air we breathe. It is a mixture of oxygen and nitrogen; and it
behaves towards radiant heat like a vacuum, being incompetent to
absorb it in any sensible degree. But permit the same two gases to
unite chemically; then, without any augmentation of the quantity of
matter, without altering the gaseous condition, without interfering in
any way with the transparency of the gas, the act of chemical union is
accompanied by an enormous diminution of its _diathermancy_, or
perviousness to radiant heat.
The researches which established this result also proved the
elementary gases, generally, to be highly transparent to radiant heat.
This, again, led to the proof of the diathermancy of elementary
liquids, like bromine, and of solutions of the solid elements sulphur,
phosphorus, and iodine. A spectrum is now before you, and you notice
that the transparent bisulphide of carbon has no effect upon the
colours. Dropping into the liquid a few flakes of iodine, you see the
middle of the spectrum cut away. By augmenting the quantity of iodine,
we invade the entire spectrum, and finally cut it off altogether. Now,
the iodine, which proves itself thus hostile to the light, is
perfectly transparent to the ultra-red emission with which we have now
to deal. It, therefore, is to be our ray-filter.
Placing the alum-cell again in front of the electric lamp, we assure
ourselves, as before, of the utter inability of the concentrated light
to fire white paper-Introducing a cell containing the solution of
iodine, the light is entirely cut off; and then, on removing the
alum-cell, the white paper at the dark focus is instantly set on fire.
Black paper is more absorbent than white for these rays; and the
consequence is, that with it the suddenness and vigour of the
combustion are augmented. Zinc is burnt up at the same place,
magnesium bursts into vivid combustion, while a sheet of platinized
platinum, placed at the focus, is heated to whiteness.
Looked at through a prism, the white-hot platinum yields all the
colours of the spectrum. Before impinging upon the platinum, the waves
were of too slow recurrence to awaken vision; by the atoms of the
platinum, these long and sluggish waves are broken up into shorter
ones, being thus brought within the visual range. At the other end of
the spectrum, by the interposition of suitable substances, Professor
Stokes _lowered_ the refrangibility, so as to render the non-visual
rays visual, and to this change he gave the name of _Fluorescence_.
Here, by the intervention of the platinum, the refrangibility is
_raised_, so as to render the non-visual visual, and to this change I
have given the name of _Calorescence_.
At the perfectly invisible focus where these effects are produced, the
air may be as cold as ice. Air, as already stated, does not absorb
radiant heat, and is therefore not warmed by it. Nothing could more
forcibly illustrate the isolation, if I may use the term, of the
luminiferous ether from the air. The wave-motion of the one is heaped
up to an extraordinary degree of intensity, without producing any
sensible effect upon the other. I may add that, with suitable
precautions, the eye may be placed in a focus competent to heat
platinum to vivid redness, without experiencing any damage, or the
slightest sensation either of light or heat.
The important part played by these ultra-red rays in Nature may be
thus illustrated: I remove the iodine filter, and concentrate the
total beam upon a test tube containing water. It immediately begins to
splutter, and in a minute or two it _boils_. What boils it? Placing
the alum solution in front of the lamp, the boiling instantly ceases.
Now, the alum is pervious to all the luminous rays; hence it cannot be
these rays that caused the boiling. I now introduce the iodine, and
remove the alum: vigorous ebullition immediately recommences at the
invisible focus. So that we here fix upon the invisible ultra-red rays
the heating of the water.
We are thus enabled to understand the momentous part played by these
rays in Nature. It is to them that we owe the warming and the
consequent evaporation of the tropical ocean; it is to them,
therefore, that we owe our rains and snows. They are absorbed close to
the surface of the ocean, and warm the superficial water, while the
luminous rays plunge to great depths without producing any sensible
effect. But we can proceed further than this. Here is a large flask
containing a freezing mixture, which has so chilled the flask, that
the aqueous vapour of the air of this room has been condensed and
frozen upon it to a white fur. Introducing the alum-cell, and placing
the coating of hoar-frost at the intensely luminous focus of the
electric lamp, not a spicula of the dazzling frost is melted.
Introducing the iodine-cell, and removing the alum, a broad space of
the frozen coating is instantly melted away. Hence we infer that the
snow and ice, which feed the Rhone, the Rhine, and other rivers with
glaciers for their sources, are released from their imprisonment upon
the mountains by the invisible ultra-red rays of the sun.
§ 6. _Identity of Light and Radiant Heat. Reflection from Plane and
Curved Surfaces. Total Reflection of Heat_.
The growth of science is organic. That which today is an _end_ becomes
to-morrow a _means_ to a remoter end. Every new discovery in science
is immediately made the basis of other discoveries, or of new methods
of investigation. Thus about fifty years ago OErsted, of Copenhagen,
discovered the deflection of a magnetic needle by an electric current;
and about the same time Thomas Seebeck, of Berlin, discovered
thermoelectricity. These great discoveries were soon afterwards turned
to account, by Nobili and Melloni, in the construction of an
instrument which has vastly augmented our knowledge of radiant heat.
This instrument, which is called a _thermo-electric pile_, or more
briefly a thermo-pile, consists of thin bars of bismuth and antimony,
soldered alternately together at their ends, but separated from each
other elsewhere. From the ends of this 'thermo-pile' wires pass to a
galvanometer, which consists of a coil of covered wire, within and
above which are suspended two magnetic needles, joined to a rigid
system, and carefully defended from currents of air.