produces an electric current; the current, passing through the coil,
deflects the needles, and the magnitude of the deflection may be made
a measure of the heat. The upper needle moves over a graduated dial
far too small to be directly seen. It is now, however, strongly
illuminated; and above it is a lens which, if permitted, would form an
image of the needle and dial upon the ceiling. There, however, it
could not be conveniently viewed. The beam is therefore received upon
a looking-glass, placed at the proper angle, which throws the image
upon a screen. In this way the motions of this small needle may be
made visible to you all.
The delicacy of this apparatus is such that in a room filled, as this
room now is, with an audience physically warm, it is exceedingly
difficult to work with it. My assistant stands several feet off. I
turn the pile towards him: the heat radiated from his face, even at
this distance, produces a deflection of 90°. I turn the instrument
towards a distant wall, a little below the average temperature of the
room. The needle descends and passes to the other side of zero,
declaring by this negative deflection that the pile has lost its
warmth by radiation against the cold wall. Possessed of this
instrument, of our ray-filter, and of our large Nicol prisms, we are
in a condition to investigate a subject of great philosophical
interest; one which long engaged the attention of some of our foremost
scientific workers--the substantial _identity of light and radiant
heat_.
That they are identical in _all_ respects cannot of course be the
case, for if they were they would act in the same manner upon all
instruments, the _eye_ included. The identity meant is such as
subsists between one colour and another, causing them to behave alike
as regards reflection, refraction, double refraction, and
polarization. Let us here run rapidly over the resemblances of light
and heat. As regards reflection from plane surfaces, we may employ a
looking-glass to reflect the light. Marking any point in the track of
the reflected beam, cutting off the light by the dissolved iodine,
and placing the pile at the marked point, the needle immediately
starts aside, showing that the heat is reflected in the same direction
as the light. This is true for every position of the mirror.
Recurring, for example, to the simple apparatus employed in our first
lecture (fig. 3, p. 11); moving the index attached to the mirror along
the divisions of our graduated arc (_m_ _n_), and determining by the
pile the positions of the invisible reflected beam, we prove that the
angular velocity of the heat-beam, like that of the light-beam, is
twice that of the mirror.
[Illustration: Fig. 49.]
As regards reflection from curved surfaces, the identity also holds
good. Receiving the beam from our electric lamp on a concave mirror
(_m_ _m_, fig. 49), it is gathered up into a cone of reflected light
rendered visible by the floating dust of the air; marking the apex of
the cone by a pointer, and cutting off the light by the iodine
solution (T), a moment's exposure of the pile (P) at the marked point
produces a violent deflection of the needle.
The common reflection and the total reflection of a beam of radiant
heat may be simultaneously demonstrated. From the nozzle of the lamp
(L, fig. 50) a beam impinges upon a plane mirror (M N), is reflected
upwards, and enters a right-angled prism, of which _a_ _b_ _c_ is the
section. It meets the hypothenuse at an obliquity greater than the
limiting angle,[23] and is therefore totally reflected. Quenching the
light by the ray-filter at F, and placing the pile at P, the totally
reflected heat-beam is immediately felt by the pile, and declared by
the galvanometric deflection.
[Illustration: Fig. 50.]
§ 7. _Invisible Images formed by Radiant Heat._
Perhaps no experiment proves more conclusively the substantial
identity of light and radiant heat, than the formation of invisible
heat-images. Employing the mirror already used to raise the beam to
its highest state of concentration, we obtain, as is well known, an
inverted image of the carbon points, formed by the light rays at the
focus. Cutting off the light by the ray-filter, and placing at the
focus a thin sheet of platinized platinum, the invisible rays declare
their presence and distribution, by stamping upon the platinum a
white-hot image of the carbons. (See fig. 51.)
[Illustration: Fig. 51.]
§ 8. _Polarization of Heat_.
Whether radiant heat be capable of polarization or not was for a long
time a subject of discussion. Bérard had announced affirmative
results, but Powell and Lloyd failed to verify them. The doubts thus
thrown upon the question were removed by the experiments of Forbes,
who first established the polarization and 'depolarization' of heat.
The subject was subsequently followed up by Melloni, an investigator
of consummate ability, who sagaciously turned to account his own
discovery, that the obscure rays of luminous sources are in part
transmitted by black glass. Intercepting by a plate of this glass the
light from an oil flame, and operating upon the transmitted invisible
heat, he obtained effects of polarization, far exceeding in magnitude
those which could be obtained with non-luminous sources. At present
the possession of our more perfect ray-filter, and more powerful
source of heat, enables us to pursue this identity question to its
utmost practical limits.
[Illustration: Fig. 52.]
Mounting our two Nicols (B and C, fig. 52) in front of the electric
lamp, with their principal sections crossed, no light reaches the
screen. Placing our thermo-electric pile (D) behind the prisms, with
its face turned towards the source, no deflection of the galvanometer
is observed. Interposing between the lamp (A) and the first prism (B)
our ray-filter, the light previously transmitted through the first
Nicol is quenched; and now the slightest turning of either Nicol opens
a way for the transmission of the heat, a very small rotation
sufficing to send the needle up to 90°. When the Nicol is turned back
to its first position, the needle again sinks to zero, thus
demonstrating, in the plainest manner, the polarization of the heat.
When the Nicols are crossed and the field is dark, you have seen, in
the case of light, the effect of introducing a plate of mica between
the polarizer and analyzer. In two positions the mica exerts no
sensible influence; in all others it does. A precisely analogous
deportment is observed as regards radiant heat. Introducing our
ray-filter, the thermo-pile, playing the part of an eye as regards the
invisible radiation, receives no heat when the eye receives no light;
but when the mica is so turned as to make its planes of vibration
oblique to those of the polarizer and analyzer, the heat immediately
passes through. So strong does the action become, that the momentary
plunging of the film of mica into the dark space between the Nicols
suffices to send the needle up to 90°. This is the effect to which the
term 'depolarization' has been applied; the experiment really proving
that with both light and heat we have the same resolution by the plate
of mica, and recompounding by the analyzer, of the ethereal
vibrations.
Removing the mica and restoring the needle once more to 0°, I
introduce between the Nicols a plate of quartz cut perpendicular to
the axis; the immediate deflection of the needle declares the
transmission of the heat, and when the transmitted beam is properly
examined, it is found to be circularly polarized, exactly as a beam of
light is polarized under the same conditions.
§ 9. _Double Refraction of Heat_.
I will now abandon the Nicols, and send through the piece of Iceland
spar (B, fig. 53), already employed (in Lecture III.) to illustrate
the double refraction of light, our sifted beam of invisible heat. To
determine the positions of the two images, let us first operate upon
the luminous beam. Marking the places of the light-images, we
introduce between N and L our ray-filter (not in the figure) and
quench the light. Causing the pile to approach one of the marked
places, the needle remains unmoved until the place has been attained;
here the pile at once detects the heat. Pushing the pile across the
interval separating the two marks, the needle first falls to 0°, and
then rises again to 90° in the second position. This proves the double
refraction of the heat.
[Illustration: Fig. 53.]
I now turn the Iceland spar: the needle remains fixed; there is no
alteration of the deflection. Passing the pile rapidly across to the
other mark, the deflection is maintained. Once more I turn the spar,
but now the needle falls to 0°, rising, however, again to 90° after a
rotation of 360°. We know that in the case of light the extraordinary
beam rotates round the ordinary one; and we have here been operating
on the extraordinary heat-beam, which, as regards double refraction,
behaves exactly like a beam of light.
§ 10. _Magnetization of Heat_.
To render our series of comparisons complete, we must demonstrate the
magnetization of heat. But here a slight modification of our
arrangement will be necessary. In repeating Faraday's experiment on
the magnetization of light, we had, in the first instance, our Nicols
crossed and the field rendered dark, a flash of light appearing upon
the screen when the magnet was excited. Now the quantity of light
transmitted in this case is really very small, its effect being
rendered striking through contrast with the preceding darkness. When
we so place the Nicols that their principal sections enclose an angle
of 45°, the excitement of the magnet causes a far greater positive
augmentation of the light, though the augmentation is not so well
_seen_ through lack of contrast, because here, at starting, the field
is illuminated.
In trying to magnetize our beam of heat, we will adopt this
arrangement. Here, however, at the outset, a considerable amount of
heat falls upon one face of the pile. This it is necessary to
neutralize, by permitting rays from another source to fall upon the
opposite face of the pile. The needle is thus brought to zero. Cutting
off the light by our ray-filter, and exciting the magnet, the needle
is instantly deflected, proving that the magnet has opened a door for
the heat, exactly as in Faraday's experiment it opened a door for the
light. Thus, in every case brought under our notice, the substantial
identity of light and radiant heat has been demonstrated.
By the refined experiments of Knoblauch, who worked long and
successfully at this question, the double refraction of heat, by
Iceland spar, was first demonstrated; but, though he employed the
luminous heat of the sun, the observed deflections were exceedingly
small. So, likewise, those eminent investigators De la Povostaye and
Desains succeeded in magnetizing a beam of heat; but though, in their
case also, the luminous solar heat was employed, the deflection
obtained did not amount to more than two or three degrees. With
_obscure_ radiant heat the effect, prior to the experiments now
brought before you, had not been obtained; but, with the arrangement
here described, we obtain deflections from purely invisible heat,
equal to 150 of the lower degrees of the galvanometer.
§ 11. _Distribution of Heat in the Electric Spectrum_.
We have finally to determine the position and magnitude of the
invisible radiation which produces these results. For this purpose we
employ a particular form of the thermo-pile. Its face is a rectangle,
which by movable side-pieces can be rendered as narrow as desirable.
Throwing a small and concentrated spectrum upon a screen, by means of
an endless screw we move the rectangular pile through the entire
spectrum, and determine in succession the thermal power of all its
colours.
[Illustration: SPECTRUM OF ELECTRIC LIGHT.]
When this instrument is brought to the violet end of the spectrum,
the heat is found to be almost insensible. As the pile gradually moves
from the violet towards the red, it encounters a gradually augmenting
heat. The red itself possesses the highest heating power of all the
colours of the spectrum. Pushing the pile into the dark space beyond
the red, the heat rises suddenly in intensity, and at some distance
beyond the red it attains a maximum. From this point the heat falls
somewhat more rapidly than it rose, and afterwards gradually fades
away.
Drawing a horizontal line to represent the length of the spectrum, and
erecting along it, at various points, perpendiculars proportional in
length to the heat existing at those points, we obtain a curve which
exhibits the distribution of heat in the prismatic spectrum. It is
represented in the adjacent figure. Beginning at the blue, the curve
rises, at first very gradually; towards the red it rises more rapidly,
the line C D (fig. 54, opposite page) representing the strength of the
extreme red radiation. Beyond the red it shoots upwards in a steep and
massive peak to B; whence it falls, rapidly for a time, and afterwards
gradually fades from the perception of the pile. This figure is the
result of more than twelve careful series of measurements, from each
of which the curve was constructed. On superposing all these curves, a
satisfactory agreement was found to exist between them. So that it may
safely be concluded that the areas of the dark and white spaces,
respectively, represent the relative energies of the visible and
invisible radiation. The one is 7.7 times the other.
But in verification, as already stated, consists the strength of
science. Determining in the first place the total emission from the
electric lamp, and then, by means of the iodine filter, determining
the ultra-red emission; the difference between both gives the luminous
emission. In this way, it is found that the energy of the invisible
emission is eight times that of the visible. No two methods could be
more opposed to each other, and hardly any two results could better
harmonize. I think, therefore, you may rely upon the accuracy of the
distribution of heat here assigned to the prismatic spectrum of the
electric light. There is nothing vague in the mode of investigation,
or doubtful in its conclusions. Spectra are, however, formed by
_diffraction_, wherein the distribution of both heat and light is
different from that produced by the prism. These diffractive spectra
have been examined with great skill by Draper and Langley. In the
prismatic spectrum the less refrangible rays are compressed into a
much smaller space than in the diffraction spectrum.
LECTURE VI.
PRINCIPLES OF SPECTRUM ANALYSIS
PRISMATIC ANALYSIS OF THE LIGHT OF INCANDESCENT VAPOURS
DISCONTINUOUS SPECTRA
SPECTRUM BANDS PROVED BY BUNSEN AND KIRCHHOFF TO BE CHARACTERISTIC
OF THE VAPOUR
DISCOVERY OF RUBIDIUM, CÆSIUM, AND THALLIUM
RELATION OF EMISSION TO ABSORPTION
THE LINES OF FRAUNHOFER
THEIR EXPLANATION BY KIRCHHOFF
SOLAR CHEMISTRY INVOLVED IN THIS EXPLANATION
FOUCAULT'S EXPERIMENT
PRINCIPLES OF ABSORPTION
ANALOGY OF SOUND AND LIGHT
EXPERIMENTAL DEMONSTRATION OF THIS ANALOGY
RECENT APPLICATIONS OF THE SPECTROSCOPE
SUMMARY AND CONCLUSION.
We have employed as our source of light in these lectures the ends of
two rods of coke rendered incandescent by electricity. Coke is
particularly suitable for this purpose, because it can bear intense
heat without fusion or vaporization. It is also black, which helps the
light; for, other circumstances being equal, as shown experimentally
by Professor Balfour Stewart, the blacker the body the brighter will
be its light when incandescent. Still, refractory as carbon is, if we
closely examined our voltaic arc, or stream of light between the
carbon-points, we should find there incandescent carbon-vapour. And if
we could detach the light of this vapour from the more dazzling light
of the solid points, we should find its spectrum not only less
brilliant, but of a totally different character from the spectra that
we have already seen. Instead of being an unbroken succession of
colours from red to violet, the carbon-vapour would yield a few bands
of colour with spaces of darkness between them.
What is true of the carbon is true in a still more striking degree of
the metals, the most refractory of which can be fused, boiled, and
reduced to vapour by the electric current. From the incandescent
vapour the light, as a general rule, flashes in groups of rays of
definite degrees of refrangibility, spaces existing between group and
group, which are unfilled by rays of any kind. But the contemplation
of the facts will render this subject more intelligible than words can
make it. Within the camera is now placed a cylinder of carbon hollowed
out at the top; in the hollow is placed a fragment of the metal
thallium. Down upon this we bring the upper carbon-point, and then
separate the one from the other. A stream of incandescent
thallium-vapour passes between them, the magnified image of which is
now seen upon the screen. It is of a beautiful green colour. What is
the meaning of that green? We answer the question by subjecting the
light to prismatic analysis. Sent through the prism, its spectrum is
seen to consist of a single refracted band. Light of one degree of
refrangibility--that corresponding to this particular green--is
emitted by the thallium-vapour.
We will now remove the thallium and put a bit of silver in its place.
The are of silver is not to be distinguished from that of thallium; it
is not only green, but the same shade of green. Are they then alike?
Prismatic analysis enables us to answer the question. However
impossible it is to distinguish the one _colour_ from the other, it is
equally impossible to confound the _spectrum_ of incandescent
silver-vapour with that of thallium. In the case of silver, we have
two green bands instead of one.
If we add to the silver in our camera a bit of thallium, we shall
obtain the light of both metals. After waiting a little, we see that
the green of the thallium lies midway between the two greens of the
silver. Hence this similarity of colour.
But why have we to 'wait a little' before we see this effect? The
thallium band at first almost masks the silver bands by its superior
brightness. Indeed, the silver bands have wonderfully degenerated
since the bit of thallium was put in, and for a reason worth knowing.
It is the _resistance_ offered to the passage of the electric current
from carbon to carbon, that calls forth the power of the current to
produce heat. If the resistance were materially lessened, the heat
would be materially lessened; and if all resistance were abolished,
there would be no heat at all. Now, thallium is a much more fusible
and vaporizable metal than silver; and its vapour facilitates the
passage of the electricity to such a degree, as to render the current
almost incompetent to vaporize the more refractory silver. But the
thallium is gradually consumed; its vapour diminishes, the resistance
rises, until finally you see the two silver bands as brilliant as they
were at first.[24]
We have in these bands a perfectly unalterable characteristic of the
two metals. You never get other bands than these two green ones from
the silver, never other than the single green band from the thallium,
never other than the three green bands from the mixture of both
metals. Every known metal has its own particular bands, and in no
known case are the bands of two different metals alike in
refrangibility. It follows, therefore, that these spectra may be made
a sure test for the presence or absence of any particular metal. If we
pass from the metals to their alloys, we find no confusion. Copper
gives green bands; zinc gives blue and red bands; brass--an alloy of
copper and zinc--gives the bands of both metals, perfectly unaltered
in position or character.
But we are not confined to the metals themselves; the _salts_ of these
metals yield the bands of the metals. Chemical union is ruptured by a
sufficiently high heat; the vapour of the metal is set free, and it
yields its characteristic bands. The chlorides of the metals are
particularly suitable for experiments of this character. Common salt,
for example, is a compound of chlorine and sodium; in the electric
lamp it yields the spectrum of the metal sodium. The chlorides of
copper, lithium, and strontium yield, in like manner, the bands of
these metals.
When, therefore, Bunsen and Kirchhoff, the illustrious founders of
_spectrum analysis_, after having established by an exhaustive
examination the spectra of all known substances, discovered a spectrum
containing bands different from any known bands, they immediately
inferred the existence of a new metal. They were operating at the time
upon a residue, obtained by evaporating one of the mineral waters of
Germany. In that water they knew the unknown metal was concealed, but
vast quantities of it had to be evaporated before a residue could be
obtained sufficiently large to enable ordinary chemistry to grapple
with the metal. They, however, hunted it down, and it now stands
among chemical substances as the metal _Rubidium_. They subsequently
discovered a second metal, which they called _Cæsium_. Thus, having
first placed spectrum analysis on a sure foundation, they demonstrated
its capacity as an agent of discovery. Soon afterwards Mr. Crookes,
pursuing the same method, discovered the bright green band of
_Thallium_, and obtained the salts of the metal which yielded it. The
metal itself was first isolated in ingots by M. Lamy, a French
chemist.
All this relates to chemical discovery upon earth, where the materials
are in our own hands. But it was soon shown how spectrum analysis
might be applied to the investigation of the sun and stars; and this
result was reached through the solution of a problem which had been
long an enigma to natural philosophers. The scope and conquest of this
problem we must now endeavour to comprehend. A spectrum is _pure_ in
which the colours do not overlap each other. We purify the spectrum by
making our beam narrow, and by augmenting the number of our prisms.
When a pure spectrum of the sun has been obtained in this way, it is
found to be furrowed by innumerable dark lines. Four of them were
first seen by Dr. Wollaston, but they were afterwards multiplied and
measured by Fraunhofer with such masterly skill, that they are now
universally known as Fraunhofer's lines. To give an explanation of
these lines was, as I have said, a problem which long challenged the
attention of philosophers, and to Professor Kirchhoff belongs the
honour of having first conquered this problem.
(The positions of the principal lines, lettered according to
Fraunhofer, are shown in the annexed sketch (fig. 55) of the solar
spectrum. A is supposed to stand near the extreme red, and J near the
extreme violet.)
[Illustration: Fig. 55.]
The brief memoir of two pages, in which this immortal discovery is
recorded, was communicated to the Berlin Academy on October 27, 1859.
Fraunhofer had remarked in the spectrum of a candle flame two bright
lines, which coincide accurately, as to position, with the double dark
line D of the solar spectrum. These bright lines are produced with
particular intensity by the yellow flame derived from a mixture of
salt and alcohol. They are in fact the lines of sodium vapour.
Kirchhoff produced a spectrum by permitting the sunlight to enter his
telescope by a slit and prism, and in front of the slit he placed the
yellow sodium flame. As long as the spectrum remained feeble, there
always appeared two bright lines, derived from the flame, in the place
of the two dark lines D of the spectrum. In this case, such absorption
as the flame exerted upon the sunlight was more than atoned for by the
radiation from the flame. When, however, the solar spectrum was
rendered sufficiently intense, the bright bands vanished, and the two
dark Fraunhofer lines appeared with much greater sharpness and
distinctness than when the flame was not employed.