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Fraunhofer. They are generally known as Fraunhofer's lines. These dark
lines, which exist in great numbers, and of very varying strength, are ir-
regularly distributed over the whole spectrum. Some of them, in con-
sequence of their peculiar strength and their mutual position, may always
be easily recognized ; the more conspicuous are represented in fig. 58. The
same dark lines, though paler, and much more difficult to recognize, are

Fig. 58.

Red. Orange. Yellow. Green. Blue. Indigo. Tiolet.
A B C D "E"^ F G H



Sun



Na



Dark

lines.






Sr



Bright
lines.



observed in the spectrum of planets lighted by the sun ; for instance, in
the light emanating from Venus. On the other hand, the dark lines ob-
served in the spectra, which are produced by the light emanating from fixed
stars from Sirius, for instance differ in position from those previously
mentioned.

Sources of light which contain no volatile constituents incandescent
platinum wire, for example furnish continuous spectra, exhibiting no such
lines. But if volatile substances be present in the source of light, bright
lines are observed in the spectrum, which are frequently characteristic of
the volatile substances.

Professor Pliicker, of Bonn, has investigated the spectra which are pro-
duced by the electric light when developed in very rarefied gases. He
found the bright lines and the dark stripes between the lines varying con-
siderably with different gases. When the electric light was developed in a



88



LIGHT.



mixture of two gases, the spectrum thus obtained exhibited simultaneously
the peculiar spectra belonging to the two gases of which the mixture con-
sisted. When the experiment was made in gaseous compounds capable of
being decomposed by the electrical current, this decomposition was indicated
by the spectra of the separated constituents becoming perceptible.

Many years ago the spectra of colored flames were examined by Sir John
Herschel, Fox Talbot, and W. A. Miller. Within the last few years results
of the greatest importance have been obtained by Kirchhoff and Bunsen,

Fig. 59.




who have investigated the spectra furnished by the incandescence of vola-
tile substances: these researches have enriched chemistry with a new
method of analysis, the analysis by spectrum observations. In order to
recognize one of the metals of the alkalies or of the alkaline earths, it is
generally sufficient to introduce a minute quantity of a moderately volatile
compound of the metal on the loop of a platinum wire into the edge of the
very hot, but scarcely luminous flame, of a mixture of air and coal-gas, and
to examine the spectrum which is furnished by the flame containing the
vapor of the metal or its compound. Fig. 59 exhibits the apparatus which
is used in performing experiments of this description. The light of the
flame in which the metallic compound is evaporated passes through the fine
slit in the disc, s, into a tube, the opposite end of which is provided with a
convex lens. This lens collects the rays diverging from the slit, and throws
them parallel upon the prism, p. The light is decomposed by the prism,
and the spectrum thus obtained is observed by means of the telescope, which
may be turned round the axis of the stand carrying the prism. Foreign
light is excluded by an appropriate covering.

The limits of this elementary treatise do not permit us to describe the
ingenious arrangements which have been contrived for sending the light
from different sources through the same prism at different heights, whereby
their spectra, the solar spectrum, for instance, and that of a flame, may be
placed in a parallel position, the one above the other, and thus be compared.*
The spectra of flames in which different substances are volatilized frequently
exhibit such characteristically distinct phenomena, that they may be used
with the greatest advantage fcfr the discrimination of these substances. Thus
the spectrum of a flame containing sodium (Na) exhibits a bright line on

* See the article " Spectral Analysis," by Prof. Roscoe, in Watts's Dictionary of Chemistry,
vol. i.




LIGHT. 89

the yellow portion, the spectrum of potassium .(K) a characteristic bright
line at the extreme limit of the red, and another at the opposite violet limit
of the spectrum. Lithium (Li) shows a bright brilliant line in the red, and
a paler line in the yellow portion ; strontium (Sr) a bright line in the blue,
one in the orange, and six less distinct ones in the red portion of the spec-
trum. The diagram (fig. 58) exhibits the most remarkable of the dark
lines (Fraunhofers lines), and the position of the bright lines in the
spectra of flames containing the vapors of compounds of the several metals
enumerated.

The delicacy of these spectral reactions is very considerable, but unequal
in the case of different metals. The presence of YTre.imT.Tnnj' S ra i n f sodium
in the flame is still easily recognizable by the bright yellow line in the
spectrum. Lithium, when introduced in the form of a volatile compound,
imparts to the flame a red color; but this coloration is no longer perceptible
when a volatile sodium compound is simultaneously present, the yellow
coloration of the flame predominating under such circumstances. On the
other hand, when a mixture of one part of lithium and 1000 parts of
sodium is volatilized in a flame, the spectrum of the flame exhibits, together
with the bright yellow sodium line, likewise the red line characteristic of
lithium. The observation of bright lines not belonging to any of the pre-
viously known bodies has led to the discovery of new elements. Thus,
Bunsen and Kirchhoff, when examining the spectrum of a flame in which a
mixture of alkaline salt was evaporated, observed some bright lines, which
could not be attributed to any of the known elements, and were thus led to
the discovery of the two new metals, caesium and rubidium. By the same
method a new element, thallium, has been more recently discovered by Mr.
Crookes.

For the examination of the bright lines in the spectra of metals, the
electric spark, passing between two points of the metal under examination,
may be conveniently employed as a source of light. Small quantities of
the metal are invariably volatilized ; and the spectrum developed by the
electric light exhibits the bright lines characteristic of the metal employed.
These lines were observed by Wheatstone as early as 1835. This method of
investigation is more especially applicable to the examination of the spectra
of the heavy metals.

By a series of theoretical considerations, Professor Kirchhoff has arrived
at the conclusion that the spectrum of an incandescent gas is reversed i. e.,
that the bright lines become dark lines, if there be behind the incandescent
gas a very luminous source of light, which by itself furnishes a continuous
spectrum. Kirchhoff and Bunsen have fully confirmed this conclusion by
experiment. Thus a volatile lithium salt produces, as just pointed out, a
very distinct bright line in the red portion of the spectrum ; but if bright
sunlight, or the light emitted by a solid body heated to the most powerful
incandescence, be allowed to fall through the flame upon the prism, the
spectrum exhibits, in the place of this bright line, a black line similar in
every respect to Fraunhofers lines in the solar spectrum. In like manner
the bright strontium line is reversed into a dark line. Kirchhoff and Bunsen
have expressed the opinion that all the Fraunhofer lines in the solar spec-
trum are bright lines thus reversed. In their conception, the sun is sur-
rounded by aluminous atmosphere, containing a certain number of volatilized
substances, which would give rise in the spectrum to certain bright lines,
if the light of the solar atmosphere alone could reach the prism ; but the
intense light of the powerful incandescent body of the sun which passes
through the solar atmosphere, causes these bright lines to be reversed, and
to appear as dark lines on the ordinary solar spectrum. Kirchhoff and
Bunsen have thus been enabled to attempt the investigation of the chemical
constituents of the solar atmosphere, by ascertaining the elements which,

8*



90 LIGHT.

when in the state of incandescent vapor, develop bright spectral lines, co-
inciding with Fraunhofer's lines in the solar spectrum. Fraunhofer's line
D (fig. 58) coincides most accurately with the bright spectral line of sodium,
and may be artificially produced by reversing the latter; sodium would thus
appear to be a constituent of the solar atmosphere. Kirchhoff has proved,
moreover, that sixty bright lines perceptible in the spectrum of iron cor-
respond, both as to position and distinction, most exactly with the same
number of dark lines in the solar spectrum; and, accordingly, he believes
iron, in the state of vapor, to be present in the solar atmosphere. In a
similar manner this physicist has endeavored to establish the presence of
several other elements in the solar atmosphere.

Absorption Spectra. The relative quantities of the several colored rays
absorbed by a colored medium of given thickness may be observed by view-
ing a line of light through a prism and the colored medium ; the spectrum
will then be seen to be diminished in brightness in some parts, and perhaps
cut oif altogether in others. This mode of observation is often of great use
in chemical analysis, as many colored substances when thus examined afford
very characteristic spectra, the peculiarities of which may often be dis-
tinguished, even though the solution of the substance under examination
contains a sufficient amount of colored impurities to change its color very
considerably. The following method of making the observation is given by
Professor Stokes.*

A small prism is to be chosen of dense flint glass, ground to an angle of
60, and just large enough to cover the eye comfortably. The top and
bottom should be flat, for convenience of holding the prism between the
thumb and fore-finger, and laying it down on a table, so as not to scratch
or soil the faces. A fine line of light is obtained by making a vertical slit
in a board six inches square, or a little longer in a horizontal direction, and
adapting to the aperture two pieces of thin metal. One of the metal pieces
is movable, to allow of altering the breadth of the slit. About the fiftieth
of an inch is a suitable breadth for ordinary purposes. The board and
metal pieces should be well blackened.

On holding the board at arm's length against the sky or a luminous flame,
the slit being, we will suppose, in a vertical direction, and viewing the line
of light thus formed through the prism held close to the eye, with its edge
vertical, a pure spectrum is obtained at a proper azimuth of the prism.
Turning the prism round its axis alters the focus, and the proper focus is
got by trial. The whole of the spectrum is not, indeed, in perfect focus at
once, so that in scrutinizing one part after another it is requisite to turn
the prism a little. When daylight is used, the spectrum is known to be
pure by its showing the principal fixed lines ; in other cases the focus is got
by the condition of seeing distinctly the other objects, whatever they may
be, which are presented in the spectrum. To observe the absorption-spec-
trum of a liquid, an elastic band is put round the board near the top, and a
test-tube containing the liquid is slipped under the band, which holds it in
its place behind the slit. The spectrum is then observed just as before, the
test-tube being turned from the eye.

To observe the whole progress of the absorption, different degrees of
strength must be used in succession, beginning with a strength which
does not render any part of the spectrum absolutely black, unless it be one
or more very narrow bands, as otherwise the most distinctive features of
the absorption might be missed. If the solution be contained in a wedge-
shaped vessel instead of a test-tube, the progress of the absorption may be
watched in a continuous manner by sliding the vessel before the eye. Some
observers prefer using a wedge-shaped vessel in combination with the slit,

* Chem. Soc. Journ. xvii. 306.



LIGHT.



91



the slit being perpendicular to the edge of the wedge. In this case each
element of the slit forms an elementary spectrum corresponding to a thick-
ness of the solution which increases in a continuous manner from ihe edg
of the wedge, where it vanishes. This is the mode of observation adopted
by Gladstone.*

Fig. 00 represents the effect produced in this way by a solution of
chromic chloride, and fig. 61 that produced by a solution of potassium
permanganate.



Fig. 60.



Fig. SI.





C d F 3E D



OB



The right-hand side of these figures corresponds with the red end of the
spectrum ; the letters refer to Fraunhofer's lines. The lower part of each
figure shows the pure spectrum seen through the thinnest part of the wedge ;
and the progress of the absorption, as the thickness of the liquid increases,
is seen by the gradual obliteration of the spectrum towards the upper part
of the figures.

Fluorescence. An examination into a peculiar mode of analysis of light,
discovered by Sir John Herschel, in a solution of quinine sulphate, has
within the last few years led to the discovery of a most remarkable fact.
Mr. Stokes has observed that light of certain refrangibility and color is
capable of experiencing a peculiar influence in being dispersed by certain
media, and of undergoing thereby an alteration of its refrangibility and color.
This curious change, called fluorescence, can be produced by a great number
of bodies, both liquid and solid, transparent and opaque. Frequently the
change affects only the extreme limits; at other times larger portions, and
in a few cases even the whole, or, at all events, the major part of the spec-
trum. A dilute solution of quinine sulphate, for instance, changes the
violet and the dark-blue light to sky-blue ; by a decoction of madder in a
solution of alum all rays of higher refrangibility than yellow are converted
into yellow; by an alcoholic solution of the coloring matter of leaves all the
rays of the spectrum become red. In all cases in which this peculiar phe-
nomenon presented itself in a greater or less degree, Mr. Stokes observed
that it consisted in a diminution of the refrangibility. Thus, rays of so
high a degree of refrangibility, that they extend far beyond the extreme
limits of the spectrum visible under ordinary circumstances, may be ren-
dered luminous, and converted into blue and even red light.

DOUBLE REFRACTION AND POLARIZATION. A ray of common light made
to pass through certain crystals of a particular order is found to undergo a
.very remarkable change. It becomes split or divided into two rays, one of



* Chem. Soc. Journ. x. 79.



92



LIGHT.



Fig. 62.



which follows the general law of refraction, while the other takes a new
and extraordinary course, dependent on the position of the crystal. This
effect, which is called double refraction, is beautifully illustrated in the case
of Iceland spar, or crystallized calcium carbonate. On placing a rhomb of
this substance on a piece of white paper on which a mark or line has been
made, the object will be seen double.

Again, if a ray of light be suffered to fall on a plate of glass at an angle
of 56 45', the portion of the ray which suffers reflection will be found to
have acquired properties which it did not before possess ; for on throwing
it, at the same angle, upon a second glass plate, it will be observed that
there are two particular positions of the latter, namely, those in which the
planes of incidents are at right angles to one another, when the ray of light
is no longer reflected, but entirely refracted. Light which has suffered this
change is said to be polarized.

The light which passes through the first or polarizing plate is, also, to a
certain extent, in this peculiar condition, and by employing a series of
similar plates held parallel to the first, this effect may
be greatly increased ; a bundle of fifteen or twenty
such plates may be used with great convenience for the
experiment. It is to be remarked, also, that the light
polarized by transmission in this manner is in an oppo-
site state to that polarized by reflection; that is, when
examined by a second or analyzing plate, held at the
angle before mentioned, it will be seen to be reflected
when the other is transmitted, and to be dispersed when
the first is reflected.

It is not every substance which is capable of polar-
izing light in this manner; glass, water, and certain
other bodies bring about the change in question, each
having a particular polarizing angle at which the effect
is greatest. The metals also can, by reflection, polarize
the light, but they do so very imperfectly. The two rays into which a
pencil of common light divides itself in passing through a doubly refracting
crystal are found on examination to be polarized in a very complete manner,
and also transversely, the one being capable of reflection when the other
vanishes or is transmitted. The two rays are said to be polarized in op-
posite directions. With a rhomb of transparent Iceland spar of tolerably
large dimensions, the two oppositely polarized rays may be widely separated
and examined apart.

Certain doubly refracting crystals absorb the one of these rays, but not
the other. Through a plate of such a crystal one ray passes and becomes
entirely polarized ; the other, which is likewise polarized, but in another
plane, is removed by absorption. The best known of these media is tour-
maline. When two plates of this mineral, cut parallel to the axis of the
crystal, are held with their axes parallel, as in fig. 63, light traverses them
both freely; but when one of them is turned round in the manner shown in
fig. 64, so as to make the axes cross at right angles, the light is almost




Fig. 03.



Fig. 64.





LIGHT. 93

wholly stopped, if the tourmalines are good. A plate of the mineral thus
becomes an excellent test for discriminating between polarized light and
that which has not undergone the change.

Some of the most splendid phenomena of the science of light are ex-
hibited when thin plates of doubly refracting substances are interposed
between the polarizing arrangement and the analyzer.

Instead of the tourmaline plate, which is always colored, frequent use is
made of two Nichol's prisms, or conjoined prisms of calcium carbonate,
which, in consequence of a peculiar cutting and combination, possess the
property of allowing only one of the oppositely polarized rays to pass. A
more advantageous method of cutting and combining prisms has been given
by M. Foucault. His prisms are as serviceable as and less expensive than
those of Nichol. If two Nichol's or Foucault's prisms be placed one behind
the other in precisely similar positions, the light polarized by the one goes
through the other unaltered. But when one prism is slightly turned round
in its setting, a cloudiness is produced ; and by continuing to turn the prism,
this increases until perfect darkness ensues. This happens, as with the tour-
maline plates, when the two prisms cross one another. The phenomenon
is the same with colorless as with colored light.

CIRCULAR POLARIZATION. Supposing that polarized light, colored, for ex-
ample, by going through a plate of red glass, has passed through the first
Nichol's prism, and been altogether obstructed in consequence of the posi-
tion of the second prism, then, if between the two prisms a plate of rock-
crystal formed by a section at right angles to the principal axis of the crystal,
be interposed, the light polarized by the first prism will, by passing through
the plate of quartz, be enabled partially to pass through the second Nichol's
prism. Its passage through the second prism can then again be interrupted
by turning the second prism round to a certain extent. The rotation re-
quired varieg with the thickness of the plate of rock-crystal, and also with
the color of the light employed. It increases from red in the following
order yellow, green, blue, violet.

This property of rock-crystal was discovered by Arago. The kind of
polarization has been called circular polarization. The direction of the
rotation is with many plates towards the right hand ; in other plates it is
towards the left. The one class is said to possess right-handed polarization,
the other class left-handed polarization. For a long time quartz was the
only solid body known to exhibit circular polarization. Others have since
been found which possess this property in a far higher degree. Thus, a
plate of cinnabar acts fifteen times more powerfully than a plate of quartz
of equal thickness.

Biot observed that many solutions of organic substances exhibit the
property of circular polarization, though to a far less extent than rock-
crystal. Thus, solutions of cane-sugar, glucose, and tartaric acid, possess
right-handed polarization; whilst albumen, uncrystallizable sugar, and oil
of turpentine, are left-handed. In all these solutions the amount of circular
polarization increases with the concentration of the liquid and the thickness
of the column through which the light passes. Hence circular polarization
is an important auxiliary in chemical analysis. In order to determine the
amount of polarization which any liquid exhibits, it is put into a glass tube
not less than from ten to twelve inches long, which is closed with glass
plates. This is then placed between the two Nichol's prisms, which have
previously been so arranged with regard to each other that no light could
pass through. An apparatus of this description, the saccharimeter, is used
for determining the concentration of solutions of cane-sugar.

The form of this instrument may be seen in fig. (35. The two Nichol's
prisms are enclosed in the corresponding fastenings a and 6. Between the
two there is a space to receive the tube, which is filled with the solution of



94



LIGHT.



sugar. If the prisms are crossed in the way above mentioned before the
tube is put in its place, that is, if they are placed so that no light passes
them, then by the action of the solution of sugar the light is enabled to
pass, and the Nichol's prism, a, must be turned through a certain angle
before the light is again perfectly stopped. The magnitude of this angle is
observed on the circular disk s s, which is divided into degrees, and upon
which, by the turning of the prism, an index z is moved along the division.
When the tube is exactly ten inches long, and is closed at both ends by flat
glass plates, and when it is filled with solution containing 10 per cent, by
weight of cane-sugar, and free from any other substance possessing an ac-
tion on light, the angle of rotation is 13-35. Since the magnitude of this
angle stands in direct relation to the length of the column of liquid and
also to the quantity of sugar in solution, it is clear that the quantity of
sugar in any given solution, when the length of the column of liquid is I
inches, and the angle of rotation is a degrees, can be determined by the

a X I
equation ' = 33^-.

This process is not sufficient when the solution contains cane-sugar and
uncrystallizable sugar : for the latter rotates the ray to the left ; in that

Fig. 65.

c



dud*




case only the difference of the two actions is obtained. But if the whole
quantity of sugar be changed into uncrystallizable sugar, and the experi-
ment be repeated, then from the results of the two observations the quan-
tity of both kinds of sugar can easily be calculated.

It is difficult to find exactly that position of the Nichol's prisms in which






LIGHT. 95

the greatest darkness prevails. To make the measurements more exact and
easy, Soleil has made some additions to the apparatus. At <?, before the
prism 6, a plate of rock-crystal cut at right angles to the axis is placed.
It is divided in the centre of the field of vision, half consisting of quartz
rotating to the right hand, and half of the variety which rotates to the left ;
it is 0-148 inch (3'75 millimetre) thick, this thickness being found by ex-
periment to produce the greatest difference in the color of the two halves,
when one prism is slightly rotated. The solution of sugar has precisely the
same action on the rotation, since it increases the action of the half which
has a right-handed rotation, and lessens the action of the half which rotates
to the left. Hence the two halves will assume a different color when the
smallest quantity of sugar is present in the liquick By slightly turning the



Online LibraryGeorge FownesA manual of elementary chemistry: theoretical and practical → online text (page 10 of 114)