represented in Fig. 219. It has a single prism, made of glass, of great
power in dispersing the rays. The prism is supported on a little stage,
placed in the middle of a horizontal circular brass table about 6 in. in
diameter. On the left is seen a tube, about 15 in. long, at the outer
extremity of which is the slit, formed of pieces of metal very
accurately shaped. One of these pieces slides in a direction at right
angles to the slit, and, by means of a spring and a fine screw, can be
very nicely adjusted, so that an opening of any degree of fineness can
be readily obtained. In front of the slit is a small glass prism, with
its edges parallel to the slit, but only half its height. The bases of
this prism are formed of two sides of a square and its diagonal, and, as
shown in the figure, one side is parallel to the face of the slit, and
the other to the axis of the tube. Rays of light coming from a source on
the left of the slit (as seen in the figure) will, therefore, enter this
little prism, and be totally reflected (see page 399) by the diagonal
surface, down the axis of the tube through the lower half only of the
slit. This is the only office of this prism, which has nothing to do
with the dispersion of the rays: the use to which it is put will be seen
presently. It is fixed in such a manner that, when required, it can be
turned aside with the touch of a finger, and the _whole_ length of the
slit exposed. A peculiarity in these instruments of Mr. Browning’s is
the admirable arrangement for determining the position of any line in a
spectrum. For this purpose, the eye-piece of the telescope is provided
with a pair of cross-wires, and the telescope itself, which is about 18
in. in length, moves in a horizontal plane round the axis of the
circular brass table, from which an arm projects, carrying a ring into
which the telescope screws. This arm carries a _vernier_ along the limb
of the circular table, which is very accurately divided into thirds of
degrees, so that with the aid of the vernier the angular position of the
telescope can be read off to a minute, that is, to 1/60th of a degree.
The arm carrying the telescope is provided with a screw for clamping it
in any desired position while the readings are taken. On placing in
front of the slit the flame of a Bunsen’s burner, the spectrum produced
by any substance in this flame will, when the instrument is in proper
adjustment, be seen on looking through the telescope, and the
cross-wires being also in view, the point of their intersection may be
brought into coincidence with any line of the spectrum, and the
telescope being clamped in this position, the angular reading thus taken
determines the position of the line. Thus, for example, the angular
positions in which the principal Fraunhofer’s lines are seen having been
observed and recorded, the angular position of any line in another
spectrum will at once determine its position among the Fraunhofer lines;
or the spectrum may be mapped by laying down the angular readings of the
lines by means of a scale of equal parts. And, again, in the little
prism in front of the slit we have the means of bringing two spectra in
view at once, one being from a light directly in front, and the other
from a light at the side. The two spectra are seen one above the other,
and the coincidence or difference of their lines may be directly
observed. When the instrument is in use, the prism and the ends of the
tube are covered with a black cloth, loosely thrown over them, by which
all stray light is shut out. The author has had in use for several years
one of these instruments, and he cannot forbear expressing his perfect
satisfaction with its powers, which he finds amply sufficient for all
ordinary chemical purposes, while the accuracy of the workmanship is
really wonderful, considering the very moderate price of the instrument.
The substances the spectra of which are most conveniently examined are
the metals of the alkalies and alkaline earths. Small quantities of the
salts of these metals, placed in a loop of fine platinum wire, impart
characteristic colours to the flame of a Bunsen burner or to that of a
spirit-lamp. For the examination of the spectra the former is to be
preferred, as the lines come out much more vividly. Indeed, at
temperatures higher than that of the Bunsen’s burner, such as in the
flame of pure hydrogen, or in the voltaic arc, some substances give out
additional lines. In Plate XVII., Nos. 2 to 9, is shown the appearance
of the spectra produced by the Bunsen’s burner when salts of the metals
are held in the flame in the manner already mentioned, and the spectra
are examined with the instrument just described. One of the simplest of
these spectra is that produced by sodium compounds, such as common salt.
The smallest particle of this substance imparts an intense yellow colour
to the flame, and the spectrum is found to take the form of a single
bright yellow line—No. 3. It has been estimated that the presence of the
(1/100000000)_th part of a grain_ of sodium can be detected by the
production of this line. Indeed, the very delicacy of this sodium
reaction renders it almost impossible to get rid of this line, for
sodium is found to be present in almost everything,—a fact the earlier
observers of spectra were not aware of, for they attributed this yellow
line to water, which was the only substance they knew to be so generally
diffused. If a platinum wire be heated in the flame of the Bunsen burner
until all the sodium indications have disappeared, it suffices to remove
the wire, and, without allowing it to come into contact with anything,
to leave it exposed to the air for a few minutes, to cause it again to
give the characteristic yellow colour when again plunged into the flame.
This is due to the fact that the element is contained in all the
floating particles which pervade the atmosphere. The spectroscope is not
required to show the presence of the sodium on the platinum which has
been exposed to the air, the colour imparted to the flame being plainly
visible to the eye, and it needs only the Bunsen burner and 2 in. of
platinum wire to prove the fact, and also to show that mere contact with
the fingers is enough to highly charge the wire with sodium compounds.
Any volatile compound of potassium gives the spectrum represented by No.
2, the principal lines being a red line and one in the extreme violet,
the latter being somewhat difficult to observe. There is also a third
rather ill-defined red line, and a portion of a faint continuous
spectrum. Salts of strontium impart a bright red colour to the flame,
and the spectrum they produce is shown by No. 6, in which are seen
several bright red lines and a fainter blue one. Calcium, which also
gives a reddish colour to flame, furnishes an entirely different set of
lines (No. 5), and barium salt another, containing numerous lines,
especially some very vivid green ones.
In all the cases we have named, and whenever bright-lined spectra are
furnished by substances placed in the flame of a lamp, or in burning
hydrogen gas, or in the intensely hot voltaic arc, there is evidence
that the substances are converted into vapour or gas. We have already
seen how hot solid carbon gives a continuous spectrum, while carbon in
the state of gaseous combination gives most of the bright lines seen in
the spectrum of coal-gas (No. 14). It is observed also that the more
readily volatized are the salts, the more vivid are the bright lines
they produce when heated in a flame. It must be understood that each
element gives it own characteristic lines, that these are always in
precisely the same position in the spectrum, that no substance produces
a line in exactly the same position as another, however near two lines
due to different substances may, in some cases, appear; and also, that
however the salts of the different metals are mixed together, each
produces its own lines, and each ingredient may be recognized. And this
is done in an instant by an experienced observer—a mere glance at the
superposed spectra of, perhaps, half a dozen metals, suffices to inform
him which are present. There is also a peculiarity in this optical mode
of recognizing the presence of bodies which gives the subject the
highest interest, namely, the circumstance that the spectrum is produced
and the bodies recognized, however far from the observer the luminous
gas may be placed, the only condition required being that the rays reach
the instrument.
Until Kirchhoff and Bunsen’s spectroscopic investigations, lithium was
supposed to be a rare metal, occurring only in a few minerals. It
happens that this substance yields a remarkable spectrum (No. 4), for it
gives an extremely vivid line of a splendid red colour, accompanied by
only one other, a feeble yellow line; and the reaction is of very great
delicacy, for 1/6000000 of a grain can easily be detected, and an eye
which has once seen the red line readily recognizes it again. A single
drop of a mineral water containing lithium has been found to distinctly
produce the red line, in cases where the quantity contained in a quart
of the water would have escaped ordinary chemical analysis. The
spectroscope has shown that lithium, so far from occurring in only four
or five minerals, is a substance very widely diffused in nature. In the
waters of the ocean, in mineral and river waters, in most plants, in
wines, tea, coffee, milk, blood, and muscle, this metal has been found.
Dr. Roscoe states that the ash of a cigar, when moistened with
hydrochloric acid, and held in a platinum wire in the flame of the
Bunsen’s burner, at once shows the principal lines of sodium, potassium,
calcium, and lithium. Salts of lithium and of strontium both impart a
rich crimson tint to flames, and it is hardly possible to detect any
difference in these colours with the naked eye; but, as the reader may
see on comparing spectra No. 4 and No. 6, the prism makes a wide
distinction.
Matter for a very interesting chapter in the history of prismatic
analysis has been furnished by the discovery of four new elements by
means of the spectroscope. In 1860 Bunsen observed that the residue,
after evaporation, of a certain mineral water, yielded spectra with
bright lines which he had not seen before. He concluded that they were
due to some unknown elements, and, in order to separate these, he
evaporated many tons of the water, and was rewarded by the discovery of
two alkaline metals, _cæsium_ and _rubidium_. The delicacy of the
spectrum reaction may be inferred from the fact of a ton weight of the
water containing only three grains of the salts of each of these
substances. Rubidium gives a splendid spectrum, containing red, yellow,
and green lines, and also two characteristic violet lines; while cæsium
has orange, yellow, and green lines, and two very beautiful blue lines,
by which it is easily recognized.
About the same time, Mr. W. Crookes discovered, in a mineral from the
Hartz, another elementary body, the existence of which was first
indicated to him by the characteristic spectrum it produces, namely, a
single splendid green line (No. 8 spectrum). In 1864 two German chemists
discovered, also in the Hartz, a fourth new element, which was detected
by two well-defined lines in the more refrangible end of the
spectrum—(see spectrum No. 9, in the plate). This metal was named
Indium, in reference to the colour of its lines, and the names of the
other three—cæsium, rubidium, and thallium, are also derived from the
colours of their characteristic lines.
[Illustration:
FIG. 220.—_Miniature Spectroscope._
]
Although the reader may, from such representations of the spectra as
those given in Plate XVII., form some idea of their appearance, he would
find his knowledge of the subject much clearer if he had the opportunity
of examining for himself the actual phenomena. We have already
recommended the performance of certain easy experiments involving no
outlay, but, in the matter of spectroscopes, carefully finished optical
and mechanical work is absolutely necessary in the appliances. It
fortunately happens that one eminent optician, at least, has made it his
study to produce good spectroscopic apparatus at the lowest possible
cost, and if the reader be interested in this subject, and desirous of
trying experiments himself, he can, for a very moderate sum, be equipped
with all the appliances for examining the phenomena we have described.
He has only to procure, in the first place, a small direct-vision
spectroscope, such as that represented of its actual size in Fig. 220,
which is sold by Mr. Browning for twenty-two shillings; secondly, a
Bunsen’s burner, a few feet of india-rubber tubing, two inches of
platinum wire, and a few grains of the salts of lithium, strontium,
thallium, &c. The whole expense will probably be covered by adding four
shillings to the cost of the spectroscope, and the reader will then be
in a position to see for himself the principal Fraunhofer lines, the
spectra of the metals already referred to, and the absorption bands of
the gases which have been mentioned, as well as the absorption bands in
liquids which will be spoken of in the sequel.
[Illustration:
FIG. 221.—_The Gassiot Spectroscope._
]
The splitting up of a beam of light into its elements—which it is the
office of the prism to produce—is accomplished by a single prism to a
certain degree only. It separates the red from the green, for example;
but the colours pass into each by insensible gradations through orange,
yellow, and greenish yellow. If we allow the rays to fall upon a second
prism after emerging from the first, the separation is carried further;
the red, for instance, is spread out into different kinds of red, and so
on with the rest. And the greater the number of prisms, the greater is
the extension which is given to the spectrum. Now, just as by increasing
the power of the telescope, new stars become visible, whose light was
before too faint, and nebulæ, or stars which before seemed single, are
resolved into clusters of individual stars—so, by increasing the power
of the spectroscope by employing two, four, or more prisms, lines which
appear single by the less powerful instruments are, in some instances,
resolved into groups of lines, and new lines come into view, which
before were too faint to show themselves. For example, if we view the
Fraunhofer lines through a spectroscope like that in Fig. 220, but
having two prisms instead of one, we shall see that the D line is not
really a single line, but is formed of two lines close together. If we
use greater dispersive power by employing a greater number of prisms, we
shall observe with solar light that when these two D lines are
sufficiently separated, several other lines make their appearance
between them. In this way the number of dark lines in sunlight, which
have been carefully mapped by Kirchhoff and others, amount to upwards of
2,000; and no doubt there are many more lines waiting a still more
powerful instrument. Fig. 221 is copied from a large spectroscope made
by Mr. Browning for Mr. Gassiot. It has nine or more highly dispersive
glass prisms; the telescope and the tube bearing the slit have focal
lengths of 18 in., the lenses having a diameter of 1½ in.; the telescope
is provided with a slow motion for taking the angular position; and
there is a third tube provided with a micrometer, by which the position
of the lines can be measured to 1/10000th of an inch.
The instruments we have mentioned, except the miniature spectroscope,
show only a portion of the spectra at once, a movement of the telescope
being requisite to bring each part into view. It has been already stated
that the only position of the prism which will make the lines clear and
well defined is that in which the _deviation_ is the least. In using
trains of prisms it is therefore necessary to adjust each prism for the
part of the spectrum which may be under observation. This is a tedious
process, and it has been obviated by a useful invention of Mr.
Browning’s, by which the adjustment is rendered automatic—that is, the
movements of the telescope are communicated to the prisms in such a
manner that they place themselves into the proper position for producing
clear images of the slit, whatever may be the refrangibility of the rays
under examination: Fig. 222 shows the arrangement as it appears when
viewed from above. The train of six prisms can be so arranged that the
ray after passing through six of them shall be totally reflected by a
surface of the last prism, and pursue again its path through the six
prisms in the reverse direction, becoming more and more dispersed by
each prism until it emerges parallel to the axis of the telescope. The
power of the instrument is, therefore, equivalent to that of one with
twelve prisms; but it can be used at pleasure with any dispersive power,
from two to twelve prisms.
[Illustration:
FIG. 222.—_Browning’s Automatic Adjustment of Prisms._
]
By making use of one of the Bunsen burners, the lines which are
characteristic of some ten or twelve metals are readily seen when one of
their more volatile salts is converted into vapour. For this purpose
their chlorides are usually employed, but the reactions are common to
all their salts. It is necessary that the metal should exist in the
flame in the state of highly heated vapour or gas, in order that its
characteristic rays should be given off. We usually introduce compounds
of these metals into the flame; but there is reason to believe that
these are decomposed in the flame, and the disassociated metal takes the
form of glowing gas, a small quantity of which suffices for the
production of the bright lines. No doubt the other constituent of the
compound, the chlorine for example, is also set free in the gaseous
form; but since the spectrum of the metal only is visible, we may infer
that at the temperature of the flame, the non-metallic elements are not
sufficiently luminous to produce a spectrum. When we repeat the
experiments with salts of the less volatile metals, we obtain no
spectra—the temperature of the flame not being sufficiently high to
convert these into vapour. Other methods have, therefore, to be resorted
to, and advantage is taken of the fact discovered by Faraday, that an
electric spark is nothing but highly heated matter. The spectroscope
gives us reason to believe that this matter, which is formed of the
substances between which the spark passes, is in the gaseous state; for
it is found, on examining sparks passing between two pieces of each
metal, that characteristic bright lines are produced. If one of the
metals already named is submitted to this examination, the same lines
are found which are seen in the spectra produced by the salts of the
metal volatized in the flame, but in some cases additional bright lines
appear in the spark spectrum. With the heavier metals the spark, or the
electric arc, is, however, the only means of igniting their vapours. The
usual mode of doing this is to make the discharges of a large induction
coil pass between the two fine wires of the metals, placed about a
quarter of an inch apart. A Leyden jar is commonly employed to condense
the discharge, and thus produce a still higher temperature. Mr. Browning
has contrived the neat little apparatus shown in Fig. 223, in which the
jar is superseded by a more compact and convenient condenser inside of
the box, so that it is only necessary to attach one terminal of the coil
to the binding-screw, seen outside of the end of the box, and place the
other wire from the coil in the binding-screw of one or the other of the
pieces of apparatus supported by the upright rod. Of these it is the one
on the right which at present engages our attention. Within a small
glass cylinder are two sliding rods, terminated by screw-clips, which
hold finely-pointed pieces of the metal under examination. The slit of
the spectroscope is placed close to the glass cylinder, and when a very
rapid succession of sparks is passing, the bright lines are seen
continuously. The spectra of metals examined in this way are found to
yield a very large number of lines. Thus the spectrum of calcium has 75
lines, and that of iron no fewer than 450 lines. Our limits will not
permit of an account of many interesting particulars relating to these
spectra, which include those of all the 50 metallic elements. It should,
perhaps, be stated that a modified mode of producing spectra by sparks
is sometimes found useful. This consists in causing sparks to pass
between a solution of some salt of the metal and a piece of platinum
wire. The apparatus for this purpose is that shown on the left side of
the upright in Fig. 223.
[Illustration:
FIG. 223.—_Apparatus for Spark Spectra._
]
It remains to describe the method of producing spectra of the gaseous
non-metallic elements, such as oxygen, nitrogen, hydrogen, &c. For this
purpose electricity is again made use of. It has been found that while
an electric discharge cannot take place across a perfect vacuum, and air
or gas, at ordinary densities, offers much resistance to the passage of
electricity, on the other hand, a highly rarefied gas permits the
discharge to take place through it with great facility. This is seen in
Geissler’s tubes, where a succession of discharges from a Ruhmkorff’s
coil causes the tubes to appear filled with light—due to the heating to
incandescence of a very minute quantity of the gas. The eye readily
recognizes difference of colour in the light given off by the different
gases, and when this light is examined by the spectroscope, bright
lines, characteristic of each gas, are observed. Nos. 12 and 13, in
Plate XVII., are the spectra of hydrogen and of nitrogen respectively,
which appear when the gases are examined in the manner just described.
In this manner the spectra of chlorine, bromine, iodine, oxygen,
sulphur, phosphorus, &c., may be studied. Silicon and some other solid
non-metallic elements present great difficulties to the spectroscopist,
for these elements cannot be volatized at any temperature we can
command, and the spectra of their elements can only be inferred from
those of their compounds. But unfortunately the spectra are found to
vary with the nature of the compound, and thus it happens that in the
case of carbon, for example, no definite spectrum can be assigned to the
element. The flame of coal-gas, burning in the air, as in the Bunsen
burner, gives the spectrum No. 14; but if this is compared with the
spectrum of the flame of burning _cyanogen_ (a compound of carbon and
nitrogen), the two are found to differ greatly. The cyanogen spectrum
has the two pale broad bands of violet-blue, the four blue lines, the
two green lines, and the brightest of the greenish yellow which are seen
in the coal-gas spectrum. But it has in addition a characteristic series
of violet lines, a series of bright blue, two or three crimson and red
lines, and bands in the orange, and several green lines, none of which
occur in the coal-gas spectrum. These additional lines are not due to
nitrogen, for, with perhaps the exception of some red lines, they do not
coincide in position with any of the nitrogen lines. The spectrum of
hydrogen, No. 12, should be noticed, as its three lines are very
distinct, and it will be observed that they exactly coincide in their
position with the three Fraunhofer lines, C, F, and G, in No. 1.