bright lines of the flame, but to the augmentation of the intensity of
the adjacent spectrum. The experiment proved to demonstration, that
when the white light sent through the flame was sufficiently intense,
the quantity which the flame absorbed was far in excess of that which
it radiated.
Here then is a result of the utmost significance. Kirchhoff
immediately inferred from it that the salt flame, which could
intensify so remarkably the dark lines of Fraunhofer, ought also to be
able to _produce_ them. The spectrum of the Drummond light is known to
exhibit the two bright lines of sodium, which, however, gradually
disappear as the modicum of sodium, contained as an impurity in the
incandescent lime, is exhausted. Kirchhoff formed a spectrum of the
limelight, and after the two bright lines had vanished, he placed his
salt flame in front of the slit. The two dark lines immediately
started forth. Thus, in the continuous spectrum of the lime-light, he
evoked, artificially, the lines D of Fraunhofer.
Kirchhoff knew that this was an action not peculiar to the sodium
flame, and he immediately extended his generalisation to all coloured
flames which yield sharply defined bright bands in their spectra.
White light, with all its constituents complete, sent through such
flames, would, he inferred, have those precise constituents absorbed,
whose refrangibilities are the same as those of the bright bands; so
that after passing through such flames, the white light, if
sufficiently intense, would have its spectrum furrowed by bands of
darkness. On the occasion here referred to Kirchhoff also succeeded in
reversing a bright band of lithium.
The long-standing difficulty of Fraunhofer's lines fell to pieces in
the presence of facts and reflections like these, which also carried
with them an immeasurable extension of the chemist's power. Kirchhoff
saw that from the agreement of the lines in the spectra of terrestrial
substances with Fraunhofer's lines, the presence of these substances
in the sun and fixed stars might be immediately inferred. Thus the
dark lines D in the solar spectrum proved the existence of sodium in
the solar atmosphere; while the bright lines discovered by Brewster in
a nitre flame, which had been proved to coincide exactly with certain
dark lines between A and B in the solar spectrum, proved the existence
of potassium in the sun.
All subsequent research verified the accuracy of these first daring
conclusions. In his second paper, communicated to the Berlin Academy
before the close of 1859, Kirchhoff proved the existence of iron in
the sun. The bright lines of the spectrum of iron vapour are
exceedingly numerous, and 65 of them were subsequently proved by
Kirchhoff to be absolutely identical in position with 65 dark
Fraunhofer's lines. Ångström and Thalén pushed the coincidences to 450
for iron, while, according to the same excellent investigators, the
following numbers express the coincidences, in the case of the
respective metals to which they are attached:--
Calcium 75
Barium 11
Magnesium 4
Manganese 57
Titanium 118
Chromium 18
Nickel 33
Cobalt 19
Hydrogen 4
Aluminium 2
Zinc 2
Copper 7
The probability is overwhelming that all these substances exist in the
atmosphere of the sun.
Kirchhoff's discovery profoundly modified the conceptions previously
entertained regarding the constitution of the sun, leading him to
views which, though they may be modified in detail, will, I believe,
remain substantially valid to the end of time. The sun, according to
Kirchhoff, consists of a molten nucleus which is surrounded by a
flaming atmosphere of lower temperature. The nucleus may, in part, be
_clouds_, mixed with, or underlying true vapour. The light of the
nucleus would give us a continuous spectrum, like that of the Drummond
light; but having to pass through the photosphere, as Kirchhoff's beam
passed through the sodium flame, those rays of the nucleus which the
photosphere emit are absorbed, and shaded lines, corresponding to the
rays absorbed, occur in the spectrum. Abolish the solar nucleus, and
we should have a spectrum showing a bright line in the place of every
dark line of Fraunhofer, just as, in the case of Kirchhoff's second
experiment, we should have the bright sodium lines of the flame if the
lime-light were withdrawn. These lines of Fraunhofer are therefore not
absolutely dark, but dark by an amount corresponding to the difference
between the light intercepted and the light emitted by the
photosphere.
Almost every great scientific discovery is approached
contemporaneously by many minds, the fact that one mind usually
confers upon it the distinctness of demonstration being an
illustration, not of genius isolated, but of genius in advance. Thus
Foucault, in 1849, came to the verge of Kirchhoff's discovery. By
converging an image of the sun upon a voltaic arc, and thus obtaining
the spectra of both sun and arc superposed, he found that the two
bright lines which, owing to the presence of a little sodium in the
carbons or in the air, are seen in the spectrum of the arc, coincide
with the dark lines D of the solar spectrum. The lines D he found to
he considerably strengthened by the passage of the solar light through
the voltaic arc.
Instead of the image of the sun, Foucault then projected upon the arc
the image of one of the solid incandescent carbon points, which of
itself would give a continuous spectrum; and he found that the lines D
were thus _generated_ in that spectrum. Foucault's conclusion from
this admirable experiment was 'that the arc is a medium which emits
the rays D on its own account, and at the same time absorbs them when
they come from another quarter.' Here he stopped. He did not extend
his observations beyond the voltaic arc; he did not offer any
explanation of the lines of Fraunhofer; he did not arrive at any
conception of solar chemistry, or of the constitution of the sun. His
beautiful experiment remained a germ without fruit, until the
discernment, ten years subsequently, of the whole class of phenomena
to which it belongs, enabled Kirchhoff to solve these great problems.
Soon after the publication of Kirchhoff's discovery, Professor Stokes,
who also, ten years prior to the discovery, had nearly anticipated it,
borrowed an illustration from sound, to explain the reciprocity of
radiation and absorption. A stretched string responds to aërial
vibrations which synchronize with its own. A great number of such
strings stretched in space would roughly represent a medium; and if
the note common to them all were sounded at a distance they would take
up or absorb its vibrations.
When a violin-bow is drawn across this tuning-fork, the room is
immediately filled with a musical sound, which may be regarded as the
_radiation_ or _emission_ of sound from the fork. A few days ago, on
sounding this fork, I noticed that when its vibrations were quenched,
the sound seemed to be continued, though more feebly. It appeared,
moreover, to come from under a distant table, where stood a number of
tuning-forks of different sizes and rates of vibration. One of these,
and one only, had been started by the sounding fork, and it was the
one whose rate of vibration was the same as that of the fork which
started it. This is an instance of the _absorption_ of the sound of
one fork by another. Placing two unisonant forks near each other,
sweeping the bow over one of them, and then quenching the agitated
fork, the other continues to sound; this other can re-excite the
former, and several transfers of sound between the two forks can be
thus effected. Placing a cent-piece on each prong of one of the forks,
we destroy its perfect synchronism with the other, and no such
communication of sound from the one to the other is then possible.
I have now to bring before you, on a suitable scale, the demonstration
that we can do with _light_ what has been here done with sound. For
several days in 1861 I endeavoured to accomplish this, with only
partial success. In iron dishes a mixture of dilute alcohol and salt
was placed, and warmed so as to promote vaporization. The vapour was
ignited, and through the yellow flame thus produced the beam from the
electric lamp was sent; but a faint darkening only of the yellow band
of a projected spectrum could be obtained. A trough was then made
which, when fed with the salt and alcohol, yielded a flame ten feet
thick; but the result of sending the light through this depth of flame
was still unsatisfactory. Remembering that the direct combustion of
sodium in a Bunsen's flame produces a yellow far more intense than
that of the salt flame, and inferring that the intensity of the colour
indicated the copiousness of the incandescent vapour, I sent through
the flame from metallic sodium the beam of the electric lamp. The
success was complete; and this experiment I wish now to repeat in your
presence.[25]
Firstly then you notice, when a fragment of sodium is placed in a
platinum spoon and introduced into a Bunsen's flame, an intensely
yellow light is produced. It corresponds in refrangibility with the
yellow band of the spectrum. Like our tuning-fork, it emits waves of a
special period. When the white light from the electric lamp is sent
through that flame, you will have ocular proof that the yellow flame
intercepts the yellow of the spectrum; in other words, that it absorbs
waves of the same period as its own, thus producing, to all intents
and purposes, a dark Fraunhofer's band in the place of the yellow.
In front of the slit (at L, fig. 56) through which the beam issues is
placed a Bunsen's burner (_b_) protected by a chimney (C). This beam,
after passing through a lens, traverses the prism (P) (in the real
experiment there was a pair of prisms), is there decomposed, and forms
a vivid continuous spectrum (S S) upon the screen. Introducing a
platinum spoon with its pellet of sodium into the Bunsen's flame, the
pellet first fuses, colours the flame intensely yellow, and at length
bursts into violent combustion. At the same moment the spectrum is
furrowed by an intensely dark band (D), two inches wide and two feet
long. Introducing and withdrawing the sodium flame in rapid
succession, the sudden appearance and disappearance of the band of
darkness is shown in a most striking manner. In contrast with the
adjacent brightness this band appears absolutely black, so vigorous is
the absorption. The blackness, however, is but relative, for upon the
dark space falls a portion of the light of the sodium flame.
[Illustration: Fig. 56.]
I have already referred to the experiment of Foucault; but other
workers also had been engaged on the borders of this subject before it
was taken up by Bunsen and Kirchhoff. With some modification I have on
a former occasion used the following words regarding the precursors of
the discovery of spectrum analysis, and solar chemistry:--'Mr. Talbot
had observed the bright lines in the spectra of coloured flames, and
both he and Sir John Herschel pointed out the possibility of making
prismatic analysis a chemical test of exceeding delicacy, though not
of entire certainty. More than a quarter of a century ago Dr. Miller
gave drawings and descriptions of the spectra of various coloured
flames. Wheatstone, with his accustomed acuteness, analyzed the light
of the electric spark, and proved that the metals between which the
spark passed determined the bright bands in its spectrum. In an
investigation described by Kirchhoff as "classical," Swan had shown
that 1/2,500,000 of a grain of sodium in a Bunsen's flame could be
detected by its spectrum. He also proved the constancy of the bright
lines in the spectra of hydrocarbon flames. Masson published a prize
essay on the bands of the induction spark; while Van der Willigen, and
more recently Plücker, have also given us beautiful drawings of
spectra obtained from the same source.
'But none of these distinguished men betrayed the least knowledge of
the connexion between the bright bands of the metals and the dark
lines of the solar spectrum; nor could spectrum analysis be said to be
placed upon anything like a safe foundation prior to the researches of
Bunsen and Kirchhoff. The man who, in a published paper, came nearest
to the philosophy of the subject was Ångström. In that paper,
translated by myself, and published in the "Philosophical Magazine"
for 1855, he indicates that the rays which a body absorbs are
precisely those which, when luminous, it can emit. In another place,
he speaks of one of his spectra giving the general impression of the
_reversal_ of the solar spectrum. But his memoir, philosophical as it
is, is distinctly marked by the uncertainty of his time. Foucault,
Thomson, and Balfour Stewart have all been near the discovery, while,
as already stated, it was almost hit by the acute but unpublished
conjecture of Stokes.'
Mentally, as well as physically, every year of the world's age is the
outgrowth and offspring of all preceding years. Science proves itself
to be a genuine product of Nature by growing according to this law. We
have no solution of continuity here. All great discoveries are duly
prepared for in two ways; first, by other discoveries which form their
prelude; and, secondly, by the sharpening of the inquiring intellect.
Thus Ptolemy grew out of Hipparchus, Copernicus out of both, Kepler
out of all three, and Newton out of all the four. Newton did not rise
suddenly from the sea-level of the intellect to his amazing elevation.
At the time that he appeared, the table-land of knowledge was already
high. He juts, it is true, above the table-land, as a massive peak;
still he is supported by the plateau, and a great part of his absolute
height is the height of humanity in his time. It is thus with the
discoveries of Kirchhoff. Much had been previously accomplished; this
he mastered, and then by the force of individual genius went beyond
it. He replaced uncertainty by certainty, vagueness by definiteness,
confusion by order; and I do not think that Newton has a surer claim
to the discoveries that have made his name immortal, than Kirchhoff
has to the credit of gathering up the fragmentary knowledge of his
time, of vastly extending it, and of infusing into it the life of
great principles.
With one additional point we will wind up our illustrations of the
principles of solar chemistry. Owing to the scattering of light by
matter floating mechanically in the earth's atmosphere, the sun is
seen not sharply defined, but surrounded by a luminous glare. Now, a
loud noise will drown a whisper, an intense light will overpower a
feeble one, and so this circumsolar glare prevents us from seeing many
striking appearances round the border of the sun. The glare is
abolished in total eclipses, when the moon comes between the earth and
the sun, and there are then seen a series of rose-coloured
protuberances, stretching sometimes tens of thousands of miles beyond
the dark edge of the moon. They are described by Vassenius in the
'Philosophical Transactions' for 1733; and were probably observed even
earlier than this. In 1842 they attracted great attention, and were
then compared to Alpine snow-peaks reddened by the evening sun. That
these prominences are flaming gas, and principally hydrogen gas, was
first proved by M. Janssen during an eclipse observed in India, on the
18th of August, 1868.
But the prominences may be rendered visible in sunshine; and for a
reason easily understood. You have seen in these lectures a single
prism employed to produce a spectrum, and you have seen a pair of
prisms employed. In the latter case, the dispersed white light, being
diffused over about twice the area, had all its colours
proportionately diluted. You have also seen one prism and a pair of
prisms employed to produce the bands of incandescent vapours; but here
the light of each band, being absolutely monochromatic, was incapable
of further dispersion by the second prism, and could not therefore be
weakened by such dispersion.
Apply these considerations to the circumsolar region. The glare of
white light round the sun can be dispersed and weakened to any extent,
by augmenting the number of prisms; while a monochromatic light,
mixed with this glare, and masked by it, would retain its intensity
unenfeebled by dispersion. Upon this consideration has been founded a
method of observation, applied independently by M. Janssen in India
and by Mr. Lockyer in England, by which the monochromatic bands of the
prominences are caused to obtain the mastery, and to appear in broad
daylight. By searching carefully and skilfully round the sun's rim,
Mr. Lockyer has proved these prominences to be mere local juttings
from a fiery envelope which entirely clasps the sun, and which he has
called the _Chromosphere_.
It would lead us far beyond the object of these lectures to dwell upon
the numerous interesting and important results obtained by Secchi,
Respighi, Young, and other distinguished men who have worked at the
chemistry of the sun and its appendages. Nor can I do more at present
than make a passing reference to the excellent labours of Dr. Huggins
in connexion with the fixed stars, nebulae, and comets. They, more
than any others, illustrate the literal truth of the statement, that
the establishment of spectrum analysis, and the explanation of
Fraunhofer's lines, carried with them an immeasurable extension of the
chemist's range. The truly powerful experiments of Professor Dewar are
daily adding to our knowledge, while the refined researches of Capt.
Abney and others are opening new fields of inquiry. But my object here
is to make principles plain, rather than to follow out the details of
their illustration.
SUMMARY AND CONCLUSION.
My desire in these lectures has been to show you, with as little
breach of continuity as possible, something of the past growth and
present aspect of a department of science, in which have laboured some
of the greatest intellects the world has ever seen. I have sought to
confer upon each experiment a distinct intellectual value, for
experiments ought to be the representatives and expositors of
thought--a language addressed to the eye as spoken words are to the
ear. In association with its context, nothing is more impressive or
instructive than a fit experiment; but, apart from its context, it
rather suits the conjurer's purpose of surprise, than the purpose of
education which ought to be the ruling motive of the scientific man.
And now a brief summary of our work will not be out of place. Our
present mastery over the laws and phenomena of light has its origin in
the desire of man to _know_. We have seen the ancients busy with this
problem, but, like a child who uses his arms aimlessly, for want of
the necessary muscular training, so these early men speculated vaguely
and confusedly regarding natural phenomena, not having had the
discipline needed to give clearness to their insight, and firmness to
their grasp of principles. They assured themselves of the rectilineal
propagation of light, and that the angle of incidence was equal to the
angle of reflection. For more than a thousand years--I might say,
indeed, for more than fifteen hundred years--the scientific intellect
appears as if smitten with paralysis, the fact being that, during this
time, the mental force, which might have run in the direction of
science, was diverted into other directions.
The course of investigation, as regards light, was resumed in 1100 by
an Arabian philosopher named Alhazen. Then it was taken up in
succession by Roger Bacon, Vitellio, and Kepler. These men, though
failing to detect the principles which ruled the facts, kept the fire
of investigation constantly burning. Then came the fundamental
discovery of Snell, that cornerstone of optics, as I have already
called it, and immediately afterwards we have the application, by
Descartes, of Snell's discovery to the explanation of the rainbow.
Following this we have the overthrow, by Roemer, of the notion of
Descartes, that light was transmitted instantaneously through space.
Then came Newton's crowning experiments on the analysis and synthesis
of white light, by which it was proved to be compounded of various
kinds of light of different degrees of refrangibility.
Up to his demonstration of the composition of white light, Newton had
been everywhere triumphant--triumphant in the heavens, triumphant on
the earth, and his subsequent experimental work is, for the most part,
of immortal value. But infallibility is not an attribute of man, and,
soon after his discovery of the nature of white light, Newton proved
himself human. He supposed that refraction and chromatic dispersion
went hand in hand, and that you could not abolish the one without at
the same time abolishing the other. Here Dollond corrected him.
But Newton committed a graver error than this. Science, as I sought to
make clear to you in our second lecture, is only in part a thing of
the senses. The roots of phenomena are embedded in a region beyond the
reach of the senses, and less than the root of the matter will never
satisfy the scientific mind. We find, accordingly, in this career of
optics the greatest minds constantly yearning to break the bounds of
the senses, and to trace phenomena to their subsensible foundation.
Thus impelled, they entered the region of theory, and here Newton,
though drawn from time to time towards truth, was drawn still more
strongly towards error; and he made error his substantial choice. His
experiments are imperishable, but his theory has passed away. For a
century it stood like a dam across the course of discovery; but, as
with all barriers that rest upon authority, and not upon truth, the
pressure from behind increased, and eventually swept the barrier away.
In 1808 Malus, looking through Iceland spar at the sun, reflected from
the window of the Luxembourg Palace in Paris, discovered the
polarization of light by reflection. As stated at the time, this
discovery ushered in the darkest hour in the fortunes of the wave
theory. But the darkness did not continue. In 1811 Arago discovered
the splendid chromatic phenomena which we have had illustrated by the
deportment of plates of gypsum in polarized light; he also discovered
the rotation of the plane of polarization by quartz-crystals. In 1813
Seebeck discovered the polarization of light by tourmaline. That same
year Brewster discovered those magnificent bands of colour that
surround the axes of biaxal crystals. In 1814 Wollaston discovered the
rings of Iceland spar. All these effects, which, without a theoretic
clue, would leave the human mind in a jungle of phenomena without
harmony or relation, were organically connected by the theory of
undulation.
The wave theory was applied and verified in all directions, Airy being
especially conspicuous for the severity and conclusiveness of his
proofs. A most remarkable verification fell to the lot of the late Sir
William Hamilton, of Dublin, who, taking up the theory where Fresnel
had left it, arrived at the conclusion that at four special points of
the 'wave-surface' in double-refracting crystals, the ray was divided,
not into two parts but into an infinite number of parts; forming at
these points a continuous conical envelope instead of two images. No
human eye had ever seen this envelope when Sir William Hamilton
inferred its existence. He asked Dr. Lloyd to test experimentally the
truth of his theoretic conclusion. Lloyd, taking a crystal of
arragonite, and following with the most scrupulous exactness the
indications of theory, cutting the crystal where theory said it ought
to be cut, observing it where theory said it ought to be observed,
discovered the luminous envelope which had previously been a mere idea
in the mind of the mathematician.