THE SPECTROSCOPE.
WE now come to speak of an instrument which may fairly rank, after the
telescope and microscope, as one of the most wonderful discoveries of
modern optical science. By its means we have not only discovered four
new elementary bodies, which are found in certain minerals in
inconceivably small quantities, but we have also determined the chemical
composition of some of the remotest stars and nebulæ.
In 1701 Newton discovered that if an ordinary ray of white light was
admitted through a small hole into a dark chamber, and thence passed
through a triangular prism, it became decomposed into a coloured band,
known as the solar spectrum. As we have already explained that this
decomposition is caused by the different coloured rays that make up
white light being bent unequally by the action of the prism, we trust
the following explanations will be readily understood. In 1802 Dr.
Wollaston, an English philosopher, discovered that by using a narrow
slit, instead of a round hole, the resulting spectrum was no longer
continuous, but was divided at intervals by dark lines extending across
it in a direction parallel to the edges of the prism. These lines
attracted considerable attention at the time, but it was not until 1815,
that Fraunhofer, an optician of Munich, investigated them with accuracy.
He mapped and counted no less than six hundred of them, identifying
eight of the most conspicuous by the first eight letters of the
alphabet. Their positions are as follows:—
A. Beginning of red.
B. Middle of red.
C. Beginning of orange.
D. Middle of yellow.
E. Middle of green.
F. Beginning of blue.
G. Middle of indigo.
H. Middle of violet.
The designations of these lines have been retained to the present day,
and they have been named after the Munich philosopher, being known as
Fraunhofer’s lines. They are to be seen in all parts of the spectrum,
and increase in number and fineness according as the width of the slit
through which the light passes is diminished. It may be asked, how it
happens that they increase in proportion to the narrowness of the
aperture admitting the light? A little consideration will soon show the
reason of this.
When a beam of light is passed through a hole of, let us say, the eighth
of an inch in diameter and decomposed by a prism, the spectrum so
produced is imperfect, inasmuch as an infinite number of spectra are
thus superposed, and for this reason, that the rays of light entering on
the right side of the aperture will give a spectrum falling in a
different place to that formed by the rays entering on the left. In
order, therefore, to diminish the confusion caused by the superposition
of a number of spectra, the aperture ought to be reduced to a narrow
slit. When the thin slice of light passing through the slit is
decomposed by the prism, we find that not only is the purity of the
colours greatly increased, but the lines in question make their
appearance more or less in all parts of the coloured band.
These lines are very unequally distributed, some being crowded together
in masses, while others are extremely faint, and are separated by large
intervals. Their position is well marked and determined, no matter from
what source we obtain our beam of sunlight. Whether the spectrum be
produced from the sun itself, or from the reflected light proceeding
from the moon or planets, they are still found in the same place; only
that in the latter case they are not so numerous, on account of the
light being much fainter. For many years the cause of these lines
remained a complete mystery, and it was not until Bunsen and Kirchhoff
undertook their investigation that a satisfactory explanation of their
origin was arrived at. In order to explain this, we must consider
briefly the properties of the spectra of flames, and other luminous
bodies.
If, instead of the light of the sun, we examine prismatically the light
given off by an incandescent body, such as a white-hot piece of
platinum, we shall find that the lines seen in the solar spectrum are
absent, and that we have a continuous band of coloured light quite
uninterrupted by dark spaces or bands. The same absence of lines is seen
in the spectra of the electric light and the flame of an ordinary
candle, the light in each of these cases being produced by particles of
carbon in a state of vivid incandescence. But if we examine the flame of
incandescent gases, we shall find a spectrum of an entirely new kind.
Thus if we examine an ordinary gaslight through a slit with a prism, we
shall obtain a continuous spectrum, in consequence of the luminous
portion of the flame consisting of solid carbon in a state of
incandescence; but if we turn down the flame, so as to lessen the amount
of carbon to be burned, we shall find the whole of that body is
converted into feebly luminous gas, giving off a faint reddish blue
light. If we now again examine it in the same manner, we shall find that
the spectrum produced consists of black spaces, here and there crossed
by a few faint coloured lines or bands. The reason of this is obvious:
in the faint flame caused by the carbon and hydrogen in a state of
luminous vapour, which only have a few of the colours of the spectrum,
which, when passed through the prism, fall into their proper places. All
substances with which we are acquainted are capable of being converted
into luminous vapour by means of heat, and when thus burnt produce
flames of more or less faint luminosity, generally characteristically
coloured. A piece of soda inserted in the wick of a spirit-lamp gives a
yellow tinge to the flame; a morsel of saltpetre (nitrate of potash) or
nitrate of strontia will give a purple and crimson tint respectively.
These hues are caused by the metals sodium, potassium, and strontium
contained in these salts being converted into luminous vapour. On
analyzing these coloured flames with a prism, as before, we should find
in the case of the soda a single broad yellow line, situated just in the
middle of the yellow portion of the spectrum, the rest of the space
where the spectrum should be being perfectly dark. The reason of this is
pretty simple. Sodium burns with a pure yellow flame, consequently when
passed through a prism it cannot split into any other colours, but takes
its place in the position belonging to yellow of that particular hue.
Were it a little more orange or green in tint, it would take its place
nearer to the red or violet end of the spectrum. The light from
saltpetre, which contains potassium may next be examined. It will be
found to tinge the flame with the spirit-lamp of a beautiful purple. We
can almost guess what will happen when this flame is submitted to the
action of the prism. We shall find that the purple light emitted will
split into red and violet, which will immediately arrange themselves in
their proper positions according to their hues. If in like manner we
substitute nitrate of strontia for saltpetre, we shall get a splendid
crimson flame which is decomposed by the prism into red, orange, or
blue.
On submitting the compounds of the other elements to the same tests, we
shall find that each of them, when converted into luminous gas, is
capable of producing coloured lines of various kinds when the light of
their flames is passed through a prism. If, therefore, we had a number
of salts of whose composition we were ignorant, all we need do is to
burn them in a spirit-lamp, and by the number and position in the lines
of their spectra we should be able to tell immediately of what they were
composed.
The spectra of nearly all the elements capable of being connected with
luminous gas have been determined with great accuracy. Perhaps the
number and position of the lines of a few spectra will be interesting to
the student.
_Sodium._—This is the metallic base of soda salts, and gives a double
bright yellow line in the middle of the yellow.
_Potassium._—The base of the various salts of potash. It gives one line
in the extreme red, one in the middle of the red, one in the violet, and
a peculiar glow in the centre of the spectrum.
_Strontium._—The base of the strontia salts, of which the nitrate is
used as the principal ingredient in the red fire of the theatres. It
gives a group of lines in the red and orange, and a beautiful blue one
in the middle of the blue.
_Barium._—The base of the baryta salts, one of which is used in making
green fire. It gives several strong lines in the green, and a few in the
red, orange, and yellow.
After the position of the spectral lines of most of the elements had
been discovered, Messrs. Bunsen and Kirchhoff were one day examining the
saline deposit of a spring which issues from the earth near Durkheim, in
the Palatinate, and were surprised to find that a blue line belonging to
no known metal made its appearance in addition to the potassium, sodium,
and other lines produced by the saline ingredients of the water. These
philosophers immediately concluded that the unknown line was caused by
an unknown metal, and they at once set to work to obtain a larger
quantity of the saline residue from the spring. They evaporated down no
less than forty tons of water, and succeeded in isolating the new
substance, which turned out to be a metal resembling potassium. While
examining the residue more carefully, a new, dark _red_ line, beyond
that belonging to potassium, was discovered, pointing to the existence
of a second new element, which was also afterwards obtained in the pure
state. These two new metals, which closely resemble potassium in their
properties, were named in accordance with the lines given by them when
converted into luminous gas. The first was called cæsium, from _cœsius_,
Lat. light blue; and the other, rubidium, from _rubidus_, Lat. dark red.
Since the publication of MM. Bunsen and Kirchhoff’s experiments, these
two elements have been found in comparatively large quantities in
various minerals, and these properties have been closely studied.
Spectrum analysis has yielded us two more new metals since first these
philosophers applied the prism to the determination of the chemical
composition of various bodies. Mr. W. Crookes, F.R.S., an English
chemist of eminence, while examining the flame of a deposit obtained
during the manufacture of sulphuric acid from a certain sulphur mineral
found in the Hartz mountains, perceived a brilliant green line with
which he was previously unacquainted, which quickly flashed into view,
and then disappeared. After numerous experiments on various other
minerals (for the deposit he had first experimented upon only yielded
him a few grains of the new body), Mr. Crookes succeeded in discovering
a comparatively large quantity of it in a sulphur mineral found in
Belgium. The new element was found to be a heavy metal, closely
resembling lead in its properties. It was named by the discoverer,
thallium, from the Greek word _thallos_, a green twig, from the
brilliancy of the single green line that indicates its presence. In like
manner, Messrs. Reich and Richter have discovered a fourth new metal,
which has been named _indium_, from its principal lines being found in
the centre of the _indigo_ of the spectrum.
The delicacy of spectrum analysis may be imagined from the fact that a
quantity of sodium amounting to less than the _two-millionth_ of a grain
can be detected by its means. Indeed, it has taught us that sodium in
one form or other exists almost everywhere. This mode of analysis is
only serviceable to indicate the composition of any salt or other
substance, the quantities of the different elements found by its use
having no influence on the appearances brought out by the prism. Thus, a
substance which has only been contaminated with sodium from being
handled by warm fingers, will show the yellow bands as strongly as if it
contained a large proportion of that metal.
For ordinary experiments in spectrum analysis the apparatus used is very
simple. It consists of a tube with a fine slit at one end, and a convex
lens at the other, for concentrating the light from the coloured flame
upon the centre of the prism. After the light passes through the prism,
it is examined by a small telescope of low magnifying power. The lamp
used may be either a spirit-lamp or a colourless gas flame into which
the substance to be examined is introduced upon a platinum wire.
We now come to another very important discovery, made by means of our
prism and narrow slit—the determination of the composition of the
photosphere or mass of luminous vapour surrounding the body of the sun.
A simple experiment will show how this brilliant discovery was arrived
at. The light of a candle or other flame containing incandescent _solid_
matter is passed through the spectroscope, and is found to decompose
into a continuous spectrum, uninterrupted by dark lines. Between the
light and the slit a spirit-lamp is placed, but no difference in the
appearance of the spectrum is perceived. Introduce, however, the
smallest portion of a soda salt into the non-luminous flame of the
second-lamp, and a broad black line is immediately seen, crossing the
middle of the yellow portion of the band of colour. Remove the sodium
flame and the band disappears; but do the same with the lamp producing
the spectrum, and the spectrum of course disappears, and the dark band
caused by the sodium flame is changed to the yellow line produced by
that metal. The same experiments may be tried with potassium, strontium,
and other metals; and we shall always find that when a coloured flame is
introduced between an incandescent solid and its continuous spectrum, it
produces a series of black lines corresponding to the substances by
which it is coloured. Thallium, in like manner, would give a black band
in the middle of the green, and indium a similar one in the indigo.
(Fig. 6, Frontispiece).
The exact position of the black band in the middle of the yellow is
shown in the coloured figure of the spectrum so beautifully printed in
the frontispiece of this book, and it has been found to correspond
exactly with the dark line D of the solar spectrum. The inference from
this fact is obvious. The incandescent portion of the sun gives off
light corresponding in its properties to that emitted by the solid
matter contained in the candle flame, but the photosphere containing the
vapour of sodium cuts off that portion corresponding to the sodium line.
Accurate measurements prove that numberless other lines occurring in the
solar spectrum are due to the vapours of other well known metals
existing on the earth. Amongst these may be mentioned potassium, calcium
(the base of lime), iron, nickel, chromium, and several others. This
discovery with regard to the sun has resulted in the spectral
examination of a large number of the fixed stars and nebulæ. For
centuries the fixed stars refused to answer all questions put to them by
mortals. The telescope showed them merely as bright points. Their nature
and origin remained a beautiful mystery, until Dr. Miller, Mr. Huggins,
Father Secchi, and a few other philosophers interrogated them in a
manner that could not fail to draw forth an answer. They brought their
light within range of their prisms, and forthwith they declared
themselves to be suns like our own. It is true that before this they
were looked on by most astronomers as bodies analogous to our own sun,
but it was only reasoning from analogy, after all; but we are now able
to assert with all the certainty that is compatible with human
fallibility that many of these heavenly bodies are possessed of an
incandescent centre, surrounded by a photosphere or envelope of gaseous
matter in a luminous condition. It would be impossible to give a list of
all the stars that have been examined up to the present time; the
composition of the photospheres of a few must therefore suffice. It is
singular that the elements hitherto discovered in the stars are those
which are more or less abundant on the earth. Amongst them we may name
hydrogen, nitrogen, sodium, magnesium, barium, iron, antimony, bismuth,
tellurium, and mercury. The bright star in the constellation of Orion
known as Betelgeux is one of the most singular in composition, the lines
of its spectrum indicating the absence of hydrogen. If, as Messrs.
Huggins and Miller suggest, the worlds revolving round this star are
also deficient in this element, they would be without water, like our
moon.
Upon a very clear night it may be noticed that the stars are not all of
the same colour, but that many of them appear to be of a ruddy or
yellowish tint. The cause of this is plainly seen when they are
submitted to spectral analysis. Thus, Sirius, which is a brilliant white
star, shows but three dark lines, while one of the stars in the
constellation of Hercules shows several groups of bands in the red,
blue, and green portions of its spectrum, fully accounting for its
orange tint.
The double star _β_ Cygni is a very beautiful example of the
distribution of colour between two members of a stellar group. One star
shows a strong spectrum with the blue and violet portions almost totally
blotted out, while its companion is similarly circumstanced with respect
to the yellow and orange portions of its spectrum. The colour of one is
consequently orange, while the other is of a delicate blue. If these
stars are the principal members of a system, the alternation of blue and
orange days must be indeed a singular phenomenon to those who inhabit
their planets.
In some of the stars lines have been discovered which do not possess any
equivalent amongst those produced by terrestrial matter; they
consequently contain elements of which we know nothing; at the same
time, however, it has been found that terrestrial elements exist in some
of the remote nebulæ, which are so distant that their light takes many
thousands of years to reach our earth.
Spectrum analysis has decided the grand question of the physical
composition of the nebulæ. Those bodies were supposed, with some reason,
to be aggregations of stars, like our Milky Way, which only required
telescopes of sufficient power to resolve them. That they partly consist
of gaseous matter in a luminous condition is evidenced by their showing
a series of bright lines in the spectroscope, exactly like those
produced by terrestrial gases. Their light is therefore not emitted by a
solid or liquid incandescent body, but by a glowing gas. The lines
mentioned by Messrs. Huggins and Miller showed that the nebula in the
sword-handle of Orion consists of hydrogen and nitrogen in a state of
luminous incandescence. Not the slightest trace of a continuous spectrum
can be detected in the light emanating from this body; consequently,
according to present hypotheses, it contains no solid matter at all. A
number of other nebulæ have given similar results.
There are numerous star clusters which, unlike the true nebulæ, give
continuous spectra when their light is submitted to the action of the
prism. Of these may be specially mentioned the great clusters in
Andromeda and Hercules, which give continuous spectra, interrupted by
dark bands on the red and orange. The light thrown by these experiments
upon the nebular hypotheses of Sir William Herschel, who considered that
true nebulæ consisted of the primordial gaseous matter out of which suns
and stars have been elaborated, is very great, and will be appreciated
even by those whose knowledge of astronomy is small.
Spectral analysis has also been the means of our witnessing a celestial
conflagration, and understanding the cause of this marvellous event. It
is well known to most people that from time to time stars have suddenly
burst upon us, and have almost as suddenly disappeared. The theories
advanced to account for these singular celestial visitors, have been
more numerous than satisfactory. In May 1866, a star of the second
magnitude suddenly burst forth in the Northern Crown, and was almost
immediately noticed by Mr. Huggins who brought every power of prism and
telescope to bear upon this extraordinary celestial phenomenon. He found
the spectrum of the star to consist of two distinct spectra, one being
formed by four bright lines, the other analogous to the spectra of the
sun and stars. Consequently two kinds of light were given off by this
star; one forming a series of bright lines indicative of luminous gas,
the other consisting of a continuous spectrum, crossed by dark lines,
showing the existence of a solid body in a state of incandescence,
surrounded by a photosphere of luminous vapours. Two of the bright lines
undoubtedly showed the presence of hydrogen in a state of illumination,
the great brightness of the lines indicating that the burning gas was
hotter than the photosphere. These facts taken in conjunction with the
suddenness of the outburst in the star, and its immediate decline in
brightness from the second down to the eighth magnitude in twelve days,
suggest the startling speculation that the star had become suddenly
wrapped in the flames of burning hydrogen, consequent possibly on some
violent convulsion in the interior of the star having set free enormous
quantities of this gas. As the free hydrogen became exhausted, the
spectrum showing the bright lines gradually waned until the star
decreased in brilliancy. It must not be forgotten that the event seen by
Mr. Huggins occurred many years ago, and that the light emitted by this
marvellous celestial convulsion has been travelling to us ever since.
Comets and meteors have been submitted to the test of spectral analysis.
The former erratic visitors have been but few and small since stellar
spectrum analysis has been perfected. In January 1866, Mr. Huggins
brought his apparatus to bear upon a small comet, which gave a somewhat
unexpected result. When the object was viewed in the spectroscope, two
spectra were distinguishable—a very faint continuous spectrum of the
tail, showing that it reflected solar light, and a bright space towards
the centre of the spectrum, indicating that the nucleous was
self-luminous and gaseous.
Mr. Alexander Herschel—the nephew and the grandson of Sir John and Sir
William Herschel—has recently succeeded in obtaining indications of the
composition of the meteors that people the heavens in the months of
August and November. The principal result of his observations appears to
be, that sodium in a state of luminous vapour is present in the trains
left behind these singular bodies.
Lightning has also been similarly examined, and lines showing that
hydrogen and nitrogen were rendered luminous during the electrical
discharge, were seen with great distinctness. In fact, the applications
of the prism to scientific discovery are almost endless, and in
describing them it is difficult to tell where to draw the line.
Before quitting this subject, it will be as well to say a few words on
the fluorescent rays of the spectrum, to which allusion has already been
made towards the end of Chapter IV., Part II. It was there said that the
chemical power of the spectrum extends to some distance beyond the
extreme violet, a fact that may be readily proved by exposing a piece of
photographic paper to the action of the dark portion of the spectrum.
Professor Stokes found that there were means of rendering these rays
visible to the eye by altering their rate of vibration. This he found
was possible by passing them through the solutions of certain
substances, such as sulphate of quinine, horse-chestnut bark, &c. We
have already said, that light vibrating at the rate of from 458 to 727
billion times a second, was capable of exciting luminous sensations upon
the optic nerve. The latter is the rate of vibration of the extreme
violet ray, and it has been found that the eyes of many persons are not
sufficiently sensitive to be influenced by it; it is, therefore, just
probable that there are animals whose eyes are so much more sensitive
than ours, that they can see rays that exist far beyond those seen by
us. Now, as difference of colour is produced by difference in the rate
of vibration, it follows that those whose eyes are sensitive enough to
perceive the extreme violet rays, see tints of violet that are
inappreciable by others.
The power of sulphate of quinine in reducing the luminous vibrations is
easily seen by passing a tube filled with the solution successively
through each of the colours of the spectrum formed by a quartz prism;
the ordinary colours will pass through the liquid as if it were simply
water, but on arriving near the violet extremity a gleam of pale blue
light will shoot across the tube, and continue to increase. As it is
moved onwards the light will gradually die away, until a point is
reached nearly equal in length to the whole of the visible spectrum,
when it will disappear altogether. It is somewhat singular that no
substance has yet been found that will increase the refrangibility of
the dark rays beyond the red end of the spectrum. There are many
artificial flames which produce this dark light (if we may use such a
paradoxical expression) in greater quantity than the sun, whose light is
no doubt greatly deteriorated in this respect during its passage through
the atmosphere. The substance of which the prism is made also greatly
influences the length of the invisible portion of the spectrum. By using
a quartz prism and lenses of the same material Professor Stokes, found
that the spectrum of the electric light could be traced for a distance
equal to six times that of the visible portion.
The action of certain substances in rendering the invisible rays of
light perceptible may be easily shown by any one possessing a
horse-chestnut tree. A weak decoction of the inner portion of the bark
having been made and filtered through blotting-paper, or at any rate
allowed to settle, the room is made quite dark and a piece of common
brimstone is ignited. The pale blue light given off is comparatively
feeble, but it is very rich in the ultra-violet rays; consequently, when
the infusion of horse-chestnut bark is poured into a tall jar of water,
beautiful waves of phosphorescent light are seen flashing backwards and
forwards as the two liquids mingle. The tincture of stramonium is also
possessed of this property, and characters traced on paper with it,
although nearly invisible by ordinary daylight, appear distinctly when
examined by the light of burning sulphur.
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