arrived at tend in any way to rob Nature of her beauty, or that our
sense of the loveliness of colour is in any danger of being blunted by
thus tracing out, as far as may be, the causes and sources of our
sensations. The poets have occasionally said harsh things of
science—indeed, one goes so far as to stigmatize the man of science as
one who would “untwist the rainbow” and “botanize upon his mother’s
grave;” and another thus laments dispelled illusions:
“When Science from Creation’s face
Enchantment’s veil withdraws,
What lovely visions yield their place
To cold material laws!”
Now, in the case we have been considering, the scientific view is surely
as beautiful as the ordinary one. We can, it is true, no longer regard
the objects as having in themselves the colours which common observation
attributes to them, but we look upon the material world as being, so to
speak, the neutral canvas upon which Light, the great painter, spreads
his varied tints, although, unlike the real canvas of an artist, which
is not only neutral, but receives indifferently whatever hues are laid
upon it, the objects around us exercise a selective effect—as if the
picture of Nature were produced by each part of the canvas refusing all
the tints save one, but itself supplying none. The tendency of the study
of science to increase our interest in the great spectacle of Nature,
and to enhance our appreciation of her charms, has been more justly
indicated by another poet—thus:
“Nor ever yet
The melting rainbow’s vernal tinctured hues
To me have shone so pleasing, as when first
The hand of Science pointed out the path
In which the sunbeams gleaming from the west
Fall on the watery cloud, whose darksome veil
Involves the orient.”
[Illustration:
FIG. 215.—_Portrait of Professor Kirchhoff._
]
THE SPECTROSCOPE.
Many of the modern discoveries and inventions already described in these
pages have been instances of practical applications of science to the
every-day wants of mankind; but the chief interest of the subject we now
enter upon flows mainly from other sources than direct applications of
its principles in useful arts, although these applications are already
neither few nor unimportant. But that which, in the highest degree,
claims our attention and excites our admiration in the revelations of
the spectroscope is the wonderful and wholly unexpected extent to which
this instrument has enlarged our knowledge of the universe, and the
apparently inadequate means by which this has been accomplished. A
little triangular piece of glass gives us power to rob the stars of
their secrets, and tells more about those distant orbs than the wildest
imagination could have deemed attainable to human knowledge. One of the
most acute philosophers of the present century, a profound thinker who
devoted his mind to the consideration of the mutual relations of the
sciences, declared emphatically, not very many years ago, that all we
could know of the heavenly bodies must ever be confined to an
acquaintance with their motions, and to such a limited acquaintance with
their features as the telescope reveals in the less distant ones. A
knowledge of their composition, he expressly asserted, could never be
attained, for we could have no means of chemically examining the matter
of which they are constituted. Such was the deliberate utterance of a
man by no means disposed to underrate the power of the human mind in the
pursuit of truth. And such might still have been the opinion of the
learned and of the unlearned, but for the remarkable train of
discoveries which has led us to the construction of instruments
revealing to us the nature of the substances entering into the
constitution of the heavenly bodies. We have now, for example, the same
certainty about the existence of iron in our sun, that we have about its
existence in the poker and tongs on the hearth. The last few years have
seen the dawn of a new science; and two branches of knowledge which
formerly seemed far as the poles asunder—namely, astronomy and
chemistry—have their interests united in this new science of celestial
chemistry. The progress which has been made in this department of
spectroscopic research is so rapid, and the field is so promising, that
the well-instructed juvenile of the future, instead of idly repeating
the simple lay of _our_ childhood:
“Twinkle, twinkle, little star,
How I _wonder_ what you are!”
will probably only have to direct his sidereal spectroscope to the
object of his admiration in order to obtain exact information as to what
the star is, chemically and physically.
[Illustration:
FIG. 216.
]
The results which have already been obtained in celestial chemistry, and
other branches of spectroscopic science, are so surprising, and
apparently so remote from the range of ordinary experience, that the
reader can only appreciate these wonderful discoveries by tracing the
steps by which they have been reached. A few fundamental phenomena of
light have already been spoken of in the foregoing article; and an
acquaintance with these will have prepared the reader’s mind for a
consideration of the new facts we are about to describe. In discussing,
in the foregoing pages, the subject of refraction, we have, in order
that the reader’s attention might not be distracted, omitted all mention
of a circumstance attending it, when a beam of ordinary light falls upon
a refracting surface, such as that represented in Fig. 203. The laws
there explained apply, in fact, to elementary rays, and not to ordinary
white light, which is a mixture of a vast multitude of elementary rays,
red, yellow, green, &c. When such a beam falls obliquely upon a piece of
glass, the ray is, at its entrance, broken up into its elements, for
these, being refracted in different degrees by the glass, each pursues a
different path in that medium, as represented by Fig. 216. Each
elementary ray obeys the laws which have been explained, and therefore
each emerges from the second surface of the plate parallel to the
incident ray, and, in consequence of this, the separation is not
perceptible under ordinary circumstances with plates of glass having
parallel surfaces. But, if the second surface be inclined so as to form
such an angle with the first that the rays are rendered still more
divergent in their exit, then the separation of the light into its
elementary coloured rays becomes quite obvious. Such is the arrangement
of the surfaces in a prism, and in the triangular pieces of glass which
are used in lustres.
For the fundamental experimental fact of our subject, we must go back
two centuries, when we shall find Sir Isaac Newton making his celebrated
analysis of light by means of the glass prism. We shall describe
Newton’s experiment, for, although it was performed so long ago, and is
generally well known, it will render our view of the present subject
more complete; and it will also serve to impress on the reader an
additional instance of the world’s indebtedness to that great mind, when
we thus trace the grand results of modern discovery from their source.
“It is well,” is the remark of a clear thinker and eloquent writer, “to
turn aside from the fretful din of the present, and to dwell with
gratitude and respect upon the services of ‘those mighty men of old, who
have gone down to the grave with their weapons of war,’ but who, while
they lived, won splendid victories over ignorance.”
[Illustration:
FIG. 217.—_Newton’s Experiment._
]
The experiment of Sir Isaac Newton will be readily understood from Fig.
217, where C is the prism, and A C represents the path of a beam of
sunlight allowed to enter into a dark apartment through a small _round_
hole in a shutter, all other light being excluded from the apartment. In
this position of the prism, the rays into which the sunbeam is broken at
its entrance into the glass were bent upwards, and at their emergence
from the glass they were again bent upwards, still more separated, so
that when a white screen was placed in their path, instead of a white
circular image of the sun appearing, as would have been the case had the
light been merely refracted and not split up, Newton saw on the screen
the variously-coloured band, D D, which he termed the _spectrum_. The
letters in the figure indicate the relative positions of the various
colours, red, orange, yellow, green, blue, &c., by their initial
letters. The spectrum, or prolonged coloured image of the sun, is red at
the end, R, where the rays are least refracted, and violet at the other
extremity, where the refraction is greatest, while, in the intermediate
spaces, yellow, green, and blue pass by insensible gradations into each
other. Newton varied his experiment in many ways, as, for example, by
trying the effect of refraction through a second prism on the
differently coloured rays. He found that the second prism did not divide
the yellow rays, for instance, into any other colour, but merely bent
them out of the straight course, to form on the second screen a somewhat
broader band of yellow, and similarly with regard to the others. From
these, and a number of other experiments described in his “Opticks,” (A.
D. 1675), Newton concludes, “that if the sun’s light consisted of but
one sort of rays, there would be but one colour in the whole world, nor
would it be possible to produce any new colour by reflections and
refractions, and, by consequence, the variety of colours depends upon
the composition of light.” ... “And if, at any time, I speak of light
and rays, or coloured, or endued with colours, I would be understood to
speak not philosophically and properly, but grossly, and accordingly to
such conceptions as vulgar people in seeing all these experiments would
be apt to frame. For the rays, to speak properly, are not coloured. In
them there is nothing else than a certain power and disposition to stir
up a sensation of this or that colour. For, as sound in a bell, a
musical string, or other sounding body, is nothing but a trembling
motion, and in the air nothing but that motion propagated from the
object, and in the sensorium ‘tis a sense of that motion under the form
of a sound; so colours in the object are nothing but a disposition to
reflect this or that sort of rays more copiously than the rest: in the
rays they are nothing but their dispositions to propagate this or that
motion into the sensorium, and in the sensorium they are sensations of
these motions under the form of colours.”
These memorable investigations of Newton’s have been the admiration of
succeeding philosophers, and even poets have caught inspiration from
this theme:
“Nor could the darting beam of speed immense
Escape his swift pursuit and measuring eye.
E’en Light itself, which everything displays.
Shone undiscovered, till his brighter mind
Untwisted all the shining robe of day;
And, from the whitening undistinguished blaze,
Collecting every ray into his kind,
To the charmed eye educed the gorgeous train
Of parent colours. First the flaming red
Sprung vivid forth; the tawny orange next;
And next delicious yellow—by whose side
Fell the kind beams of all-refreshing green;
Then the pure blue, that swells autumnal skies,
Ethereal played; and then, of sadder hue
Emerged the deepened indigo, as when
The heavy-skirted evening droops with frost,
While the last gleamings of refracted light
Died in the fainting violet away.
These, when the clouds distil the rosy show,
Shine out distinct adown the watery bow;
While o’er our heads the dewy vision bends
Delightful—melting on the fields beneath.
Myriads of mingling dyes from these result,
And myriads still remain.—Infinite source
Of beauty! ever blushing—ever new!
Did ever poet image aught so fair,
Dreaming in whispering groves, by the hoarse brook,
Or prophet, to whose rapture Heaven descends?”
The spectra which Newton obtained by admitting the solar beams through a
circular aperture, were, however, not simple spectra. The circular beam
may be considered as built up of flat and very thin bands of light,
parallel to the edges of the prism, and a simple ray would be formed by
one of these flat bands; as the round opening would allow an indefinite
number of such rays to enter, each would produce its own spectrum on the
screen, and the actual image would be formed of a number of spectra
overlapping each other. When the aperture by which the light is admitted
consists merely of a narrow slit, or line, parallel to the edges of the
prism, we obtain what is termed a _pure spectrum_. When the prism is
properly placed, an eye, viewing the fine slit through it, sees a
spectrum formed, as it were, of a succession of virtual images of the
slit in all the elementary coloured rays.
The person who first examined the solar spectrum in this manner was the
English chemist Wollaston, who, in 1802, found that the spectrum thus
observed was not continuous, but that it was crossed at intervals by
dark lines. Wollaston saw them by placing his eye directly behind the
prism. Twelve years later, namely, in 1814, the German optician
Fraunhofer devised a much better mode of viewing the spectrum; for,
instead of looking through the prism with the naked eye, he used a
telescope, placing the prism and the telescope at a distance of 24 ft.
from the slit, the virtual image of which was thus considerably
magnified. The prism was so placed that the incident and refracted rays
formed nearly equal angles with its faces, in which circumstance the ray
is least deflected from its direction, and the position is therefore
spoken of as being that of _minimum deviation_. It can be shown that
this position is the only one in which the refracted rays can produce
clear and sharp virtual images of the slit, and therefore it is
necessary in all instruments to have the prism so adjusted. Fraunhofer
then saw that the dark lines were very numerous, and he found that they
always kept the same relative positions with regard to the coloured
spaces they crossed; that these positions did not change when the
material of which the prism was made was changed; and that a variation
in the refracting angle of the prism did not affect them. He then made a
very careful map, laying down upon it the position of 354 of the lines
out of about 600 which he counted, and indicated their relative
intensities, for some are finer and less dark than others. The most
conspicuous lines he distinguished by letters of the alphabet, and these
are still so indicated; and the dark lines in the solar spectrum are
called “Fraunhofer’s Lines.” These lines, as will appear in the sequel,
are of great importance in our subject. A few of the more obvious ones
are shown in No. 1, Plate XVII. Fraunhofer found that these lines were
always produced by sunlight, whether direct, or diffused, or reflected
from the moon and planets; but that the light from the fixed stars
formed spectra having different lines from those in the sun—although he
recognized in some of the spectra a few of the same lines he found in
the solar spectrum. The fact of these differences in the spectra of the
sun and fixed stars proved that the cause of the dark lines, whatever it
might be, must exist in the light of these self-luminous bodies, and not
in our atmosphere. It was, however, some years afterwards ascertained
that the passage of the sun’s light through the atmosphere does give
rise to some dark bands in the spectrum; for it was found that certain
lines make their appearance only when the sun is near the horizon, and
its rays consequently pass through a much greater thickness of air.
Sir D. Brewster first noticed in 1832 that certain coloured gases have
the power of absorbing some of the sun’s rays, so that the spectrum,
when the rays are made to pass through such a gas before falling on the
prism, is crossed by a series of dark lines—altogether different from
Fraunhofer’s lines, though these are also present. The gas in which this
property was first noticed is that called “nitric peroxide”—a
brownish-red gas, of which even a thin stratum produces a well-marked
series of dark lines. The same property was soon discovered in the
vapours of bromine, iodine, and a certain compound of chlorine and
oxygen. Each substance furnishes a system of lines peculiar to itself:
thus the vapour of bromine, although it has almost exactly the same
colour as nitric peroxide, gives a totally different set of lines.
These, therefore, do not depend on the mere colour of the gas or vapour,
and this is conclusively proved by the fact of many coloured vapours
producing no dark lines whatever: the vapour of tungsten chloride, for
example, although in colour so exactly like bromine vapour that the two
cannot be distinguished by the eye, yields no lines whatever.
[Illustration:
FIG. 218.—_Bunsen’s Burner on a stand._
]
In Fig. 218 is represented a lamp for burning coal-gas, which is
constantly used by chemists as a source of heat. It is known as
“Bunsen’s burner,” from its inventor the celebrated German chemist. It
consists of a metal tube, 3 in. or 4 in. long, and ⅓ in. in diameter, at
the bottom of which the gas is admitted by a small jet communicating
with the elastic tube which brings the gas to the apparatus. A little
below the level of the jet there are two lateral openings which admit
air to the tube. The gas, therefore, becomes mixed with air within the
tube, and this inflammable mixture streams from the top of the tube and
readily ignites on the approach of a flame, the mixture burning with a
pale bluish flame of a very high temperature. This little apparatus is
not only the most useful pieces of chemical apparatus ever devised, but
it furnishes highly instructive illustrations of several points in
chemical and physical science; and to some of these we invite the
reader’s attention, as they have an immediate bearing on our present
subject. Coal-gas is a mixture of various compounds of the two
elementary bodies, hydrogen and carbon; and when the gas burns, these
substances are respectively uniting with the oxygen of the air,
producing water and carbonic acid gas. Now, when coal-gas is burnt in
the ordinary manner as a source of light, the supply of oxygen is too
small to admit of the complete combustion of all its constituents; and
as the oxygen more eagerly seizes upon the hydrogen than upon the
carbon, a large proportion of the latter thus set free from its hydrogen
compound is deposited in the flame in the solid form, and is there
intensely heated. The presence of solid carbon in an ordinary gas flame
is easily proved by holding in it a cold fragment of porcelain, or a
piece of metal, which will become covered with soot. In the flame of the
Bunsen burner there is no soot, because the increased supply of oxygen,
afforded by previously mixing the gas with air, enables the whole of the
constituents of the gas to be completely burnt; and this is of the
greatest advantage to the chemist, who always desires to have the
vessels he heats free from soot, in order that he may observe what is
taking place within them. The flame of Bunsen’s lamp becomes that of an
ordinary sooty gas flame, when the two orifices which admit the air at
the bottom of the tube are closed up, and then, of course, the
temperature cannot be so high as when the whole constituents of the gas
are completely burnt, but the flame becomes highly luminous; whereas
when the orifices are open it gives so little light, that in a dark room
one cannot see a finger held 20 in. from the lamp. Plainly the cause of
this difference is connected with the presence or absence of the heated
particles of solid carbon. The non-luminous flame contains no solid
particles; the bright part of the other flame is full of them. To these
heated particles of solid carbon we are, then, indebted for the light
which burning coal-gas supplies. And, since we are able by such
artificial illumination to distinguish colours, the white-hot carbon
must give off rays of all degrees of refrangibility, and we should
expect to find in the spectrum produced by such a flame, the red,
yellow, green, and other coloured rays. And such is indeed the spectrum
which these incandescent carbon particles produce: it resembles the
solar spectrum, but _there is an entire absence of dark lines_, so that
the appearance is that represented in No. 1, Plate XVII., if we suppose
the Fraunhofer lines removed. If the pale blue flame of the Bunsen’s
burner be similarly examined, the spectrum, No. 14, Plate XVII., shows
that only a few rays of certain refrangibilities are emitted, forming
bright lines here and there, but of little intensity, while the whole of
the other rays are absent. This shows that while the highly heated solid
gives off all rays from red to violet without interruption, the still
more highly heated gases give off only a few selected rays.
It has long been known that some substances impart certain colours to
flames, and such substances have been long employed to produce coloured
effects in fireworks, &c. But coloured flames do not appear to have been
examined by the prism until 1822, when Sir John Herschel described the
spectra of strontium, copper, and of some other substances, remarking
that “The colours thus communicated by the different bases to flame
afford in many cases a ready and neat way of detecting extremely minute
quantities of them.” A few years later, Fox Talbot described the method
of obtaining a monochromatic flame, by using in a spirit-lamp diluted
alcohol in which a little salt has been dissolved. The paper in which he
describes this and other observations concludes thus: “If this opinion
should be correct and applicable to the other definite rays, a glance at
the prismatic spectrum of flame may show it to contain substances which
it would otherwise require a laborious chemical analysis to detect.”
Here we have the first hint of that spectrum analysis which has provided
the chemist with a method of surpassing delicacy for the detection of
metallic elements. The spectra of coloured flames were also subsequently
examined and described by Professor W. A. Miller, but the most complete
investigation into the subject was made by Professors Kirchhoff and
Bunsen, who also contrived a convenient instrument, or _spectroscope_,
for the examination and comparison of different spectra. The instrument
has received many improvements and modifications, but the essential
parts are one or more prisms; a slit, through which the light to be
examined is allowed to enter; a tube, having at the other end a lens to
render parallel the rays from the slit; a telescope, through which the
spectrum is viewed; and usually some apparatus by which the positions of
the different lines may be identified.
[Illustration:
PLATE XVII. SPECTRA.
]
[Illustration:
FIG. 219.—_Spectroscope with one Prism._
]