Chapter X: , Note 2 (2)

[22] _Potassium_ forms _alloys with sodium_ in all proportions. The
alloys containing 1 and 3 equivalents of potassium to one
equivalent of sodium are _liquids_, like mercury at the ordinary
temperature. Joannis, by determining the amount of heat developed
by these alloys in decomposing water, found the evolution for
Na_{2}K, NaK, NaK_{2} and NaK_{3} to be 44·5, 44·1, 43·8 and 44·4
thousand heat units respectively (for Na 42·6 and for K 45·4). The
formation of the alloy NaK_{2} is therefore accompanied by the
development of heat, whilst the other alloys may be regarded as
solutions of potassium or sodium in this alloy. In any case a fall
of the temperature of fusion is evident in this instance as in the
alloys of nitre (Note 14). The liquid alloy NaK_{2} is now used
for filling thermometers employed for temperatures above 360°,
when mercury boils.

The resemblance between _potassium_ and _sodium_ is so great that _their compounds_ can only be easily _distinguished_ in the form of certain of their salts. For instance, the acid potassium tartrate, C_{4}H_{5}KO_{6} (cream of tartar), is distinguished by its sparing solubility in water and in alcohol, and in a solution of tartaric acid, whilst the corresponding sodium salt is easily soluble. Therefore, if a solution of tartaric acid be added in considerable excess to the solutions of the majority of potassium salts, a precipitate of the sparingly-soluble acid salt is formed, which does not occur with salts of sodium. The chlorides KCl and NaCl in solutions easily give double salts K_{2}PtCl_{6} and Na_{2}PtCl_{6}, with platinic chloride, PtCl_{4}, and the solubility of these salts is very different, especially in a mixture of alcohol and ether. The sodium salt is easily soluble, whilst the potassium salt is insoluble or almost so, and therefore the reaction with platinic chloride is that most often used for the separation of potassium from sodium, as is more fully described in works on analytical chemistry.

It is possible to discover the least traces of these metals in admixture together, by means of their property of imparting different colours to _a flame_. The presence of a salt of sodium in a flame is recognised by a brilliant yellow coloration, and a pure potassium salt colours a colourless flame violet. However, in the presence of a sodium salt the pale violet coloration given by a potassium salt is quite undistinguishable, and it is at first sight impossible in this case to discover the potassium salt in the presence of that of sodium. But by decomposing the light given by a flame coloured by these metals or a mixture of them, by means of a prism, they are both easily distinguishable, because the yellow light emitted by the sodium salt depends on a group of light rays having a definite index of refraction which corresponds with the yellow portion of the solar spectrum, having the index of refraction of the Fraunhofer line (strictly speaking, group of lines) D, whilst the salts of potassium give a light from which these rays are entirely absent, but which contain rays of a red and violet colour. Therefore, if a potassium salt occur in a flame, on decomposing the light (after passing it through a narrow slit) by means of a prism, there will be seen red and violet bands of light situated at a considerable distance from each other; whilst if a sodium salt be present a yellow line will also appear. If both metals simultaneously occur in a flame and emit light, the spectrum lines corresponding to the potassium and the sodium will appear simultaneously.

For convenience in carrying on this kind of testing, _spectroscopes_ (fig. 72) are constructed,[23] consisting of a refracting prism and three tubes placed in the plane of the refracting angle of the prism. One of the tubes, C, has a vertical slit at the end, giving access to the light to be tested, which then passes into the tube (collimator), containing a lens which gives the rays a parallel direction. The rays of light having passed through the slit, and having become parallel, are refracted and dispersed in the prism, and the spectrum formed is observed through the eye-piece of the other telescope B. The third tube D contains a horizontal transparent scale (at the outer end) which is divided into equal divisions. The light from a source such as a gas burner or candle placed before this tube, passes through the scale, and is reflected on that face of the prism which stands before the telescope B, so that the image of the scale is seen through this telescope simultaneously with the spectrum given by the rays passing through the slit of the tube C. In this manner the image of the scale and the spectrum given by the source of light under investigation are seen simultaneously. If the sun's rays be directed through the slit of the tube C, then the observer looking through the eye-piece of B will see the solar spectrum, and (if the aperture of the slit be narrow and the apparatus correctly adjusted) the dark Fraunhofer lines in it.[24] Small-sized spectroscopes are usually so adjusted that (looking through B) the violet portion of the spectrum is seen to the right and the red portion to the left, and the Fraunhofer line D (in the bright yellow portion of the spectrum) is situated on the 50th division of the scale.[25] If the light emitted by an incandescent solid--for example, the Drummond light--be passed through the spectroscope, then all the colours of the solar spectrum are seen, but not the Fraunhofer lines. To observe the result given by a flame coloured by various salts a Bunsen gas burner (or the pale flame of hydrogen gas issuing from a platinum orifice) giving so pale a flame that its spectrum will be practically invisible is placed before the slit. If any compound of sodium be placed in the flame of the gas burner (for which purpose a platinum wire on whose end sodium chloride is fused is fixed to the stand), then the flame is coloured yellow, and on looking through the spectroscope the observer will see a bright _yellow_ line falling upon the 50th division of the scale, which is seen together with the spectrum in the telescope. No yellow lines of other refractive index, nor any rays of any other colour, will be seen, and, therefore, the spectrum corresponding with sodium compounds consists of yellow rays of that index of refraction which belong to the Fraunhofer (black) line D of the solar spectrum. If a potassium salt be introduced into the flame instead of a sodium salt, then two bands will be seen which are much feebler than the bright sodium band--namely, one red line near the Fraunhofer line A and another violet line. Besides which, a pale, almost continuous, spectrum will be observed in the central portions of the scale. If a mixture of sodium and potassium salts be now introduced into the flame, three lines will be seen simultaneously--namely, the red and pale violet lines of potassium and the yellow line of sodium. In this manner it is possible, by the aid of the spectroscope, to determine the relation between the spectra of metals and known portions of the solar spectrum. The continuity of the latter is interrupted by dark lines (that is, by an absence of light of a definite index of refraction), termed the Fraunhofer lines of the solar spectrum. It has been shown by careful observations (by Fraunhofer, Brewster, Foucault, Ångstrom, Kirchhoff, Cornu, Lockyer, Dewar, and others) that there exists an exact _agreement between the spectra_ of certain _metals_ and certain of the _Fraunhofer_ lines. Thus the bright yellow sodium line exactly corresponds with the dark Fraunhofer line D of the solar spectrum. A similar agreement is observed in the case of many other metals. This is not an approximate or chance correlation. In fact, if a spectroscope having a large number of refracting prisms and a high magnifying power be used, it is seen that the dark line D of the solar spectrum consists of an entire system of closely adjacent but definitely situated fine and wide (sharp, distinct) dark lines,[26] and an exactly similar group of bright lines is obtained when the yellow sodium line is examined through the same apparatus, so that each bright sodium line exactly corresponds with a dark line in the solar spectrum.[26 bis] This conformity of the bright lines formed by sodium with the dark lines of the solar spectrum cannot be accidental. This conclusion is further confirmed by the fact that the bright lines of other metals correspond with dark lines of the solar spectrum. Thus, for example, a series of sparks passing between the iron electrodes of a Ruhmkorff coil gives 450 very distinct lines characterising this metal. All these 450 bright lines, constituting the whole spectrum corresponding with iron, are repeated, as Kirchhoff showed, in the solar spectrum as dark Fraunhofer lines which occur in exactly the same situations as the bright lines in the iron spectrum, just as the sodium lines correspond with the band D in the solar spectrum. Many observers have in this manner studied the solar spectrum and the spectra of different metals simultaneously, and discovered in the former lines which correspond not only with sodium and iron, but also with many other metals.[27] The spectra of such elements as hydrogen, oxygen, nitrogen, and other gases may be observed in the so-called Geissler's tubes--that is, in glass tubes containing rarefied gases, through which the discharge of a Ruhmkorff's coil is passed. Thus hydrogen gives a spectrum composed of three lines--a red line corresponding with the Fraunhofer line C, a green line corresponding with the line F, and a violet line corresponding with one of the lines between G and H. Of these rays the red is the brightest, and therefore the general colour of luminous hydrogen (with an electric discharge through a Geissler tube) is reddish.

[23] For accurate measurements and comparative researches more
complicated spectroscopes are required which give a greater
dispersion, and are furnished for this purpose with several
prisms--for example, in Browning's spectroscope the light passes
through six prisms, and then, having undergone an internal total
reflection, passes through the upper portion of the same six
prisms, and again by an internal total reflection passes into the
ocular tube. With such a powerful dispersion the relative position
of the spectral lines may be determined with accuracy. For the
absolute and exact determination of the wave lengths it is
particularly important that the spectroscope should be furnished
with diffraction gratings. The construction of spectroscopes
destined for special purposes (for example, for investigating the
light of stars, or for determining the absorption spectra in
microscopic preparations, &c.) is exceedingly varied. Details of
the subject must be looked for in works on physics and on spectrum
analysis. Among the latter the best known for their completeness
and merit are those of Roscoe, Kayser, Vogel, and Lecoq de
Boisbaudran.

[24] The arrangement of all the parts of the apparatus so as to give
the clearest possible vision and accuracy of observation must
evidently precede every kind of spectroscopic determination.
Details concerning the practical use of the spectroscope must be
looked for in special works on the subject. In this treatise the
reader is supposed to have a certain knowledge of the physical
data respecting the refraction of light, and its dispersion and
diffraction, and the theory of light, which allows of the
determination of the length of the waves of light in absolute
measure on the basis of observations with diffraction gratings,
the distance between whose divisions may be easily measured in
fractions of a millimetre; by such means it is possible to
determine the wave-length of any given ray of light.

[25] In order to give an idea of the size of the scale, we may observe
that the ordinary spectrum extends from the zero of the scale
(where the red portion is situated) to the 170th division (where
the end of the visible violet portion of the spectrum is
situated), and that the Fraunhofer line A (the extreme prominent
line in the red) corresponds with the 17th division of the scale;
the Fraunhofer line F (at the beginning of the blue, near the
green colour) is situated on the 90th division, and the line G,
which is clearly seen in the beginning of the violet portion of
the spectrum, corresponds with the 127th division of the scale.

[26] The two most distinct lines of D, or of sodium, have wave-lengths
of 589·5 and 588·9 millionths of a millimeter, besides which
fainter and fainter lines are seen whose wave-lengths in
millionths of a millimeter are 588·7 and 588·1, 616·0 and 615·4,
515·5 and 515·2, 498·3 and 498·2, &c., according to Liveing and
Dewar.

[26 bis] In the ordinary spectroscopes which are usually employed in
chemical research, one yellow band, which does not split up into
thinner lines, is seen instead of the system of sodium lines,
owing to the small dispersive power of the prism and the width of
the slit of the object tube.

[27] The most accurate investigations made in this respect are carried
on with spectra obtained by diffraction, because in this case the
position of the dark and bright lines does not depend on the index
of refraction of the material of the prism, nor on the dispersive
power of the apparatus. The best--that is, the most general and
accurate--method of expressing the results of such determinations
consists in determining the lengths of the waves corresponding to
the rays of a definite index of refraction. (Sometimes instead of
this the fraction of 1 divided by the square of the wave-length is
given.) We will express this _wave-length_ in _millionth parts of
a millimetre_ (the ten-millionth parts are already doubtful, and
fall within the limits of error). In order to illustrate the
relation between the wave-lengths and the positions of the lines
of the spectrum, we will cite the wave-lengths corresponding with
the chief Fraunhofer lines and colours of the spectrum.

Fraunhofer line A B C D E b
Wave-length 761·0 687·5 656·6 589·5-588·9 527·3 518·7
+-----------------+ +---------+ +----+ +---+
Colour red orange yellow green

Fraunhofer line F G H
Wave-length 486·5 431·0 397·2
+----------+ +----+
Colour blue violet

In the following table are given the _wave-lengths_ of the light
rays (the longest and most distinct, _see_ later) for certain
elements, those in black type being the most clearly defined and
distinct lines, which are easily obtained either in the flame of a
Bunsen's burner, or in Geissler's tubes, or in general, by an
electric discharge. These lines refer to the elements (the lines
of compounds are different, as will be afterwards explained, but
many compounds are decomposed by the flame or by an electric
discharge), and moreover to the elements in an incandescent and
rarefied gaseous state, for the spectra sometimes vary
considerably with a variation of temperature and pressure.

It may be mentioned that the _red_ colour corresponds with lines
having a wave-length of from 780 (with a greater wave-length the
lines are hardly visible, and are ultra red) to 650, the _orange_
from 650 to 590, the _yellow_ from 590 to 520, the _green_ from
520 to 490, the _blue_ from 490 to 420, and the _violet_ from 420
to 380 millionth parts of a millimetre. Beyond 380 the lines are
scarcely visible, and belong to the ultra-violet. For fluorine
Moissan found as many as 13 bright lines from 744 to 623.

In the table (p. 565) which is arranged in conformity with the
image of the spectrum as it is seen (the red lines on the
left-hand and the violet on the right-hand side), the figures in
black type correspond with lines which are so bright and
distinctly visible that they may easily be made use of, both in
determining the relation between the divisions of the scale and
the wave-lengths, and in determining the admixture of a given
element with another. Brackets join those lines between which
several other lines are clearly visible if the dispersive power of
the spectroscope permits distinguishing the neighbouring lines. In
the ordinary laboratory spectroscopes with one prism, even with
all possible precision of arrangement and with a brilliancy of
light permitting the observations being made with a very narrow
aperture, the lines whose wave-lengths only differ by 2-3
millionths of a millimetre, are blurred together; and with a wide
aperture a series of lines differing by even as much as 20
millionths of a millimetre appear as one wide line. With a faint
light (that is, with a small quantity of light entering into the
spectroscope) only the most _brilliant_ lines are clearly visible.
The _length_ of the lines does not always correspond with their
brilliancy. According to Lockyer this length is determined by
placing the carbon electrodes (between which the incandescent
vapours of the metals are formed), not horizontally to the slit
(as they are generally placed, to give more light), but vertically
to it. Then certain lines appear long and others short. As a rule
(Lockyer, Dewar, Cornu), the longest lines are those with which it
is easiest to obtain _reversed_ spectra (_see_ later).
Consequently, these lines are the most characteristic. Only the
longest and most brilliant are given in our table, which is
composed on the basis of a collection of the data at our disposal
for _bright_ spectra of the _incandescent and rarefied vapours of
the elements_. As the spectra change with great variations of
temperature and vapour density (the faint lines become brilliant
whilst the bright lines sometimes disappear), which is
particularly clear from Ciamician's researches on the halogens,
until the method of observation and the theory of the subject are
enlarged, particular theoretical importance should not be given to
the wave-lengths showing the maximum brilliancy, which only
possess a practical significance in the common methods of
spectroscopic observations. In general the spectra of metals are
simpler than those of the halogens, and the latter are variable;
at an increased pressure all spectral lines become broader.

+-------+-------+--------+--------+-------+-------+------+------+
| N_{2} | O_{2} | Cl_{2} | Br_{2} | I_{2} | Pb | Sn | Tl |
+-------+-------+--------+--------+-------+-------+------+------+
| -- | -- | -- | -- | -- | -- | -- | -- |
| -- | -- | -- | -- | -- | -- | -- | -- |
| 662 | -- | -- | -- | -- | -- | -- | -- |
| 632} | -- | -- | 636 | -- | -- | 645 | -- |
| 620} | 615 | -- | -- | 621 | -- | -- | -- |
| 585} | -- | -- | -- | 613 | 605·7 | 580} | -- |
| 574} | -- | 546} | -- | 579} | 560·7 | 556} | -- |
| 544} | 543 | 539} | 544} | 560} | 554·7 | -- | 549 |
| 535} | 533 | 528} | 523} | 545} | 537 | -- | 535 |
| 527} | 516 | 519} | 517 | 506} | -- | -- | -- |
| 516} | 495} | 494} | 479 | -- | -- | -- | 489 |
| -- | 494} | 480} | 470} | -- | -- | -- | -- |
| 457 | 470} | -- | 462} | -- | -- | 452 | -- |
| 442 | 465} | -- | 454 | 445 | -- | -- | -- |
| 436} | 447} | 436} | 437 | -- | -- | -- | -- |
| 426} | 432} | 431} | -- | -- | -- | -- | -- |
| -- | -- | -- | -- | 421 | -- | -- | -- |
| -- | -- | -- | -- | -- | -- | -- | -- |
| 409 | -- | -- | -- | -- | 406 | -- | -- |
| -- | -- | -- | -- | -- | -- | -- | -- |
+-------+-------+--------+--------+-------+-------+------+------+

+-----+-----+-----+-------+-----+------+------+-----+-------+-----+
| In | Ga | Al | Ba | Sr | Ca | Mg | Zn | Cd | Hg |
|-----+-----+-----+-------+-----|------+------|-----+-------+-----+
| -- | -- | -- | -- | -- | -- | -- | -- | -- | -- |
| -- | -- | -- | -- | -- | -- | -- | -- | -- | -- |
| -- | -- | -- | -- | -- | 646 | -- | -- | -- | -- |
| -- | -- | 624 | 649·7 | 641 | 644 | -- | 636 | 643·8 | -- |
| 619 | -- | 623 | 614 | 606 | -- | -- | -- | -- | 615 |
| -- | -- | -- | -- | -- | 612 | -- | -- | -- | 579 |
| -- | -- | 572 | 553·5 | -- | -- | -- | -- | -- | 577 |
| -- | -- | 570 | 549 | 548 | 559 | -- | -- | 537·7 | 546 |
| 525 | -- | -- | -- | 524 | -- | 518 | -- | 533·6 | -- |
| -- | -- | -- | -- | -- | -- | 516 | 492 | 508·5 | -- |
| -- | -- | -- | 493·3 | -- | -- | -- | 481 | -- | -- |
| -- | -- | -- | -- | -- | -- | 471 | 472 | 479·9 | -- |
| 451 | -- | 466 | 455 | 460 | -- | -- | 468 | 467·7 | -- |
| -- | -- | -- | -- | -- | 445} | 448 | -- | -- | -- |
| -- | -- | -- | -- | -- | 442} | -- | -- | -- | 436 |
| -- | -- | -- | -- | 430 | -- | -- | -- | -- | -- |
| -- | 417 | -- | -- | 421 | 423 | -- | -- | -- | -- |
| -- | -- | -- | 413 | -- | -- | -- | -- | -- | -- |
| 410 | 403 | 396 | -- | 408 | 397 | 384} | -- | -- | 404 |
| -- | -- | 394 | -- | -- | 393 | 383} | - -| -- | -- |
+-----+-----+-----+-------+-----+------+------+-----+-------+-----+

+------+------+-----+-------+-----+-------+------+-------+-------+
| Mn | Fe | Cu | Ag | Cs | Rb | K | Na | Li |
|------+------+-----+-------+-----+-------+------+-------+-------+
| -- | -- | -- | -- | -- | 780 | 770 | -- | -- |
| -- | -- | -- | -- | -- | -- | 766 | -- | -- |
| -- | -- | -- | -- | -- | -- | -- | -- | 670·6 |
| -- | 640 | -- | -- | -- | -- | -- | -- | -- |
| 602} | -- | -- | -- | 622 | 629·6 | -- | -- | 610 |
| 601} | 561} | 578 | -- | 600 | -- | 583} | 589·5 | -- |
| 551} | 544} | 570 | 546·4 | -- | -- | 578} | 588·9 | -- |
| 534} | 537 | 522 | -- | -- | -- | -- | -- | -- |
| -- | 532} | 515 | 520·8 | -- | -- | 535 | -- | -- |
| -- | 521} | 511 | -- | -- | -- | 532} | -- | -- |
| 482} | 496} | -- | -- | -- | -- | -- | -- | 497 |
| 471} | 489} | -- | -- | -- | -- | -- | -- | -- |
| -- | -- | -- | -- | 459 | -- | -- | -- | 460·3 |
| -- | 441} | -- | -- | 456 | -- | -- | -- | -- |
| -- | 430} | -- | -- | -- | -- | -- | -- | -- |
| -- | 427 | -- | -- | -- | -- | -- | -- | -- |
| 424} | -- | -- | 421 | -- | 420 | -- | -- | -- |
| 403} | 407} | -- | -- | -- | -- | -- | -- | -- |
| -- | 404} | -- | -- | -- | -- | 404 | -- | -- |
| -- | -- | -- | -- | -- | -- | -- | -- | -- |
+------+------+-----+-------+-----+-------+------+-------+-------+

+-------+
| H_{2} |
+-------+
| -- |
| -- |
| -- |
| 656·2 |
| -- |
| -- |
| -- |
| -- |
| -- |
| -- |
| 486·1 |
| -- |
| -- |
| -- |
| 434 |
| -- |
| -- |
| -- |
| -- |
| -- |
+-------+

The correlation of the Fraunhofer lines with the spectra of metals depends on the phenomenon of the so-called _reversal of the spectrum_. This phenomenon consists in this, that instead of the bright spectrum corresponding with a metal, under certain circumstances a similar dark spectrum in the form of Fraunhofer lines may be obtained, as will be explained directly. In order to clearly understand the phenomenon of reversed spectra, it must be known that when light passes through certain transparent substances these substances retain rays of a certain refrangibility. The colour of solutions is a proof of this. Light which has passed through a yellow solution of a uranium salt contains no violet rays, and after having passed through a red solution of a permanganate, does not contain many rays in the yellow, blue, and green portions of the spectrum. Solutions of copper salts absorb nearly all red rays. Sometimes colourless solutions also absorb rays of certain definite refractive indexes, and give _absorption spectra_. Thus solutions of salts of didymium absorb rays of a certain refrangibility, and therefore an impression of black lines is received,[28] as shown in fig. 73. Many vapours (iodine) and gases (nitric peroxide) give similar spectra. Light which has passed through a deep layer of aqueous vapour, oxygen, or nitrogen also gives an absorption spectrum. For this reason the peculiar (winter) dark lines discovered by Brewster are observed in sunlight, especially in the evening and morning, when the sun's rays pass through the atmosphere (containing these substances) by a longer path than at mid-day. It is evident that the Fraunhofer lines may be ascribed to the absorption of certain rays of light in its passage from the luminous mass of the sun to the earth. The remarkable progress made in all spectroscopic research dates from the investigations made by _Kirchhoff_ (1859) on the relation between absorption spectra and the spectra of luminous incandescent gases. It had already been observed long before (by Fraunhofer, Foucault, Ångstrom) that the bright spectrum of the sodium flame gives two bright lines which are in exactly the same position as two black lines known as D in the solar spectrum, which evidently belong to an absorption spectrum. When Kirchhoff caused diffused sunlight to fall upon the slit of a spectroscope, and placed a sodium flame before it, a perfect superposition was observed--the bright sodium lines completely covered the black lines D of the solar spectrum. When further the continuous spectrum of a Drummond light showed the black line D on placing a sodium flame between it and the slit of the spectroscope--that is, when the Fraunhofer line of the solar spectrum was artificially produced--then there was no doubt that its appearance in the solar spectrum was due to the light passing somewhere through incandescent vapours of sodium. Hence a new theory of _reversed spectra_[29] arose--that is, of the relation between the waves of light emitted and absorbed by a substance under given conditions of temperature; this is expressed by Kirchhoff's law, discovered by a careful analysis of the phenomena. This law may be formulated in an elementary way as follows: At a given temperature the relation between the intensity of the light emitted (of a definite wave-length) and the absorptive capacity with respect to the same colour (of the same wave-length) is a constant quantity.[30] As a black dull surface emits and also absorbs a considerable quantity of heat rays whilst a polished metallic surface both absorbs and emits but few, so a flame coloured by sodium emits a considerable quantity of yellow rays of a definite refrangibility, and has the property of absorbing a considerable quantity of the rays of the same refractive index. In general, the medium which emits definite rays also absorbs them.

[28] The method of observing absorption spectra consists in taking a
continuous spectrum of white light (one which does not show either
dark lines or particularly bright luminous bands--for instance,
the light of a candle, lamp, or other source). The collimator
(that is, the tube with the slit) is directed towards this light,
and then all the colours of the spectrum are visible in the ocular
tube. A transparent absorptive medium--for instance, a solution or
tube containing a gas--is then placed between the source of light
and the apparatus (or anywhere inside the apparatus itself in the
path of the rays). In this case either the entire spectrum is
uniformly fainter, or absorption bands appear on the bright field
of the continuous spectrum in definite positions along it. These
bands have different lengths and positions, and distinctness and
intensity of absorption, according to the properties of the
absorptive medium. Like the luminous spectra given by incandescent
gases and vapours, the absorption spectra of a number of
substances have already been studied, and some with great
precision--as, for example, the spectrum of the brown vapours of
nitrogen dioxide by Hasselberg (at Pulkowa), the spectra of
colouring matters (Eder and others), especially of those applied
to orthochromatic photography, the spectra of blood, chlorophyll
(the green constituent of leaves), and other similar substances,
all the more carefully as by the aid of their spectra the presence
of these substances may be discovered in small quantities (even in
microscopical quantities, by the aid of special appliances on the
microscope), and the changes they undergo investigated.

The absorption spectra, obtained at the ordinary temperature and
proper to substances in all physical states, offer a most
extensive but as yet little studied field, both for the general
theory of spectroscopy, and for gaining an insight into the
structure of substances. The investigation of colouring matters
has already shown that in certain cases a definite change of
composition and structure entails not only a definite change of
the colours but also a displacement of the absorption bands by a
definite number of wave-lengths.

[29] A number of methods have been invented to demonstrate the
reversibility of spectra; among these methods we will cite two
which are very easily carried out. In Bunsen's method sodium
chloride is put into an apparatus for evolving hydrogen (the spray
of the salt is then carried off by the hydrogen and colours the
flame with the yellow sodium colour), and the hydrogen is ignited
in two burners--in one large one with a wide flame giving a bright
yellow sodium light, and in another with a small fine orifice
whose flame is pale: this flame will throw a dark patch on the
large bright flame. In Ladoffsky's method the front tube (p. 561)
is unscrewed from a spectroscope directed towards the light of a
lamp (a continuous spectrum), and the flame of a spirit lamp
coloured by a small quantity of NaCl is placed between the tube
and the prism; a black band corresponding to sodium will then be
seen on looking through the ocular tube. This experiment is always
successful if only there be the requisite relation between the
strength of light of the two lamps.

[30] The absorptive capacity is the relation between the intensity
of the light (of a given wave-length) falling upon and retained by
a substance. Bunsen and Roscoe showed by direct experiment that
this ratio is a constant quantity for every substance. If _A_
stand for this ratio for a given substance at a given
temperature--for instance, for a flame coloured by sodium--and _E_
be the intensity of the light of the same wave-length emitted at
the same temperature by the same substance, then Kirchhoff's law,
the explanation and deduction of which must be looked for in
text-books of physics, states that the fraction _A/E_ is a
constant quantity depending on the nature of a substance (as _A_
depends on it) and determined by the temperature and wave-length.

Thus the bright spectral rays characteristic of a given metal may be reversed--that is, converted into dark lines--by passing light which gives a continuous spectrum through a space containing the heated vapours of the given metal. A similar phenomenon to that thus artificially produced is observed in sunlight, which shows dark lines characteristic of known metals--that is, the Fraunhofer lines form an absorption spectrum or depend on a reversed spectrum; it being presupposed that the sun itself, like all known sources of artificial light, gives a continuous spectrum without Fraunhofer lines.[31] We must imagine that the sun, owing to the high temperature which is proper to it, emits a brilliant light which gives a continuous spectrum, and that this light, before reaching our eyes, passes through a space full of the vapours of different metals and their compounds. As the earth's atmosphere[32] contains very little, or no, metallic vapours, and as they cannot be supposed to exist in the celestial space,[32 bis] the only place in which the existence of such vapours can be admitted is in the _atmosphere surrounding the sun itself_. As the cause of the sun's luminosity must be looked for in its high temperature, the existence of an atmosphere containing metallic vapours is readily understood, because at that high temperature such metals as sodium, and even iron, are separated from their compounds and converted into vapour. The sun must be imagined as surrounded by an atmosphere of incandescent vaporous and gaseous matter,[33] including those elements whose reversed spectra correspond with the Fraunhofer lines--namely, sodium, iron, hydrogen, lithium, calcium, magnesium, &c. Thus in spectrum analysis we find a means of determining the composition of the inaccessible heavenly luminaries, and much has been done in this respect since Kirchhoff's theory was formulated. By observations on the spectra of many heavenly bodies, changes have been discovered going on in them,[34] and many of the elements known to us have been found with certainty in them.[35] From this it must be concluded that the same elements which exist on the earth occur throughout the whole universe, and that at that degree of heat which is proper to the sun those simple substances which we accept as the elements in chemistry are still undecomposed and remain unchanged. A high temperature forms one of those conditions under which compounds most easily decompose; and if sodium or a similar element were a compound, in all probability it would be decomposed into component parts at the high temperature of the sun. This may indeed be concluded from the fact that in ordinary spectroscopic experiments the spectra obtained often belong to the metals and not to the compounds taken; this depends on the decomposition of these compounds in the heat of the flame. If common salt be introduced into the flame of a gas-burner, a portion of it is decomposed, first forming, in all probability, with water, hydrochloric acid and sodium hydroxide, and the latter then becoming partially decomposed by the hydrocarbons, giving metallic sodium, whose incandescent vapour emits light of a definite refrangibility. This conclusion is arrived at from the following experiment:--If hydrochloric acid gas be introduced into a flame coloured by sodium it is observed that the sodium spectrum disappears, owing to the fact that metallic sodium cannot remain in the flame in the presence of an excess of hydrochloric acid. The same thing takes place on the addition of sal-ammoniac, which in the heat of the flame gives hydrochloric acid. If a porcelain tube containing sodium chloride (or sodium hydroxide or carbonate), and closed at both ends by glass plates, be so powerfully heated that the salt volatilises, then the sodium spectrum is not observable; but if the salt be replaced by sodium, then either the bright line or the absorption spectra is obtained, according to whether the light emitted by the incandescent vapour be observed, or light passing through the tube. Thus the above spectrum is not given by sodium chloride or other sodium compound, but is proper to the metal sodium itself. This is also the case with other analogous metals. The chlorides and other halogen _compounds_ of barium, calcium, copper, &c., give independent spectra which differ from those of the metals. If barium chloride be introduced into a flame, it gives a mixed spectrum belonging to metallic barium and barium chloride. If besides barium chloride, hydrochloric acid or sal-ammoniac be introduced into the flame, then the spectrum of the metal disappears, and that of the chloride remains, which differs distinctly from the spectrum of barium fluoride, barium bromide, or barium iodide. A certain common resemblance and certain common lines are observed in the spectra of two different compounds of one and the same element obtained in the above-described manner, and also in the spectrum of the metal, but they all have their peculiarities. The independent spectra of the compounds of copper are easily observed (fig. 75). Thus certain compounds which exist in a state of vapour, and are luminous at a high temperature, give their independent spectra. In the majority of cases the spectra of compounds are composed of indistinct luminous lines and complete bright bands, whilst metallic elements generally give a few clearly-defined spectral lines.[36] There is no reason for supposing that the spectrum of a compound is equal to the sum of the spectra of its elements--that is, _every compound_ which is not decomposed by heat _has its own proper spectrum_. This is best proved by absorption spectra, which are essentially only reversed spectra observed at low temperatures. If every salt of sodium, lithium, and potassium gives one and the same spectrum, this must be ascribed to the presence in the flame of the free metals liberated by the decomposition of their salts. Therefore _the phenomena of the spectrum are determined by molecules, and not by atoms_--that is, the molecules of the metal sodium, and not its atoms, produce those particular vibrations which determine the spectrum of a sodium salt. Where there is no free metallic sodium there is no sodium spectrum.

[31] Heated metals begin to emit light (only visible in the dark) at
about 420° (varying with the metal). On further heating, solids
first emit red, then yellow, and lastly white light. Compressed or
heavy gases (_see_ Chapter III., Note 44), when strongly heated,
also emit white light. Heated liquids (for example, molten steel
or platinum) also give a white compound light. This is readily
understood. In a dense mass of matter the collisions of the
molecules and atoms are so frequent that waves of only a few
definite lengths cannot appear; the reverse is possible in
rarefied gases or vapours.

[32] Brewster, as is mentioned above, first distinguished the
atmospheric, cosmical Fraunhofer lines from the solar lines.
Janssen showed that the spectrum of the atmosphere contains lines
which depend on the absorption produced by aqueous vapour.
Egoreff, Olszewski, Janssen, and Liveing and Dewar showed by a
series of experiments that the oxygen of the atmosphere gives rise
to certain lines of the solar spectrum, especially the line A.
Liveing and Dewar took a layer of 165 c.m. of oxygen compressed
under a pressure of 85 atmospheres, and determined its absorption
spectrum, and found that, besides the Fraunhofer lines A and B, it
contained the following groups: 630-622, 581-568, 535, 480-475.
The same lines were found for liquid oxygen.

[32 bis] If the material of the whole heavenly space formed the
absorbent medium, the spectra of the stars would be the same as
the solar spectrum; but Huyghens, Lockyer, and others showed not
only that this is the case for only a few stars, but that the
majority of stars give spectra of a different character with dark
and bright lines and bands.

[33] Eruptions, like our volcanic eruptions, but on an incomparably
larger scale, are of frequent occurrence on the sun. They are seen
as protuberances visible during a total eclipse of the sun, in the
form of vaporous masses on the edge of the solar disc and emitting
a faint light. These protuberances of the sun are now observed at
all times by means of the spectroscope (Lockyer's method), because
they contain luminous vapours (giving bright lines) of hydrogen
and other elements.

[34] The great interest and vastness of astro-physical observations
concerning the sun, comets, stars, nebulæ, &c., render this new
province of natural science very important, and necessitate
referring the reader to special works on the subject.

The most important astro-physical data since the time of Kellner
are those referring to the _displacement_ of the lines of the
spectrum. Just as a musical note changes its pitch with the
approach or withdrawal of the resonant object or the ear, so the
pitch of the luminous note or wave-length of the light varies if
the luminous (or absorbent) vapour and the earth from which we
observe it approach or recede from each other; this expresses
itself in a visible displacement of the spectral lines. The solar
eruptions even give broken lines in the spectrum, because the
rapidly moving eruptive masses of vapour and gases either travel
in the direction of the eye or fall back towards the sun. As the
earth travels with the solar system among the stars, so it is
possible to determine the direction and velocity with which the
sun travels in space by the displacement of the spectral lines and
light of the stars. The changes proceeding on the sun in its mass,
which must be pronounced as vaporous, and in its atmosphere, are
now studied by means of the spectroscope. For this purpose, many
special astro-physical observatories now exist where these
investigations are carried on.

We may remark that if the observer or luminous object moves with a
velocity ±_v_, the ray, whose wave-length is [Greek: l], has an
apparent wave-length [Greek: l](_n_±_v_)/_n_, where _n_ is the
velocity of light. Thus Tolon, Huyghens, and others proved that
the star Aldebaran approaches the solar system with a velocity of
30 kilometres per second, while Arcturus is receding with a
velocity of 45 kilometres. The majority of stars give a distinct
hydrogen spectrum, besides which nebulæ also give the spectrum of
nitrogen. Lockyer classes the stars from their spectra, according
to their period of formation, showing that some stars are in a
period of increasing temperature (of formation or aggregation),
whilst others are in a period of cooling. Altogether, in the
astro-physical investigation of the spectra of heavenly bodies we
find one of the most interesting subjects of recent science.

[35] Spectrum analysis has proved the indubitable existence in the sun
and stars of a number of elements known in chemistry. Huyghens,
Secchi, Lockyer, and others have furnished a large amount of
material upon this subject. A compilation of existing information
on it has been given by Prof. S. A. Kleiber, in the Journal of the
Russian Physico-chemical Society for 1885 (vol. xviii. p. 146).
Besides which, a peculiar element called helium has been
discovered, which is characterised by a line (whose wave-length is
587·5, situated near D), which is seen very brightly in the
projections (protuberances) and spots of the sun, but which does
not belong to any known element, and is not reproducible as a
reversed, dark line. This may be a right conclusion--that is to
say, it is possible that an element may be discovered to which the
spectrum of helium corresponds--but it may be that the helium line
belongs to one of the known elements, because spectra vary in the
brilliancy and position of their lines with changes of temperature
and pressure. Thus, for instance, Lockyer could only see the line
423, at the very end of the calcium spectrum, at comparatively low
temperatures, whilst the lines 397 and 393 appear at a higher
temperature, and at a still higher temperature the line 423
becomes quite invisible.

[36] Spectroscopic observations are still further complicated by the
fact that one and the same substance gives different spectra at
different temperatures. This is especially the case with gases
whose spectra are obtained by an electric discharge in tubes.
Plücker, Wüllner, Schuster, and others showed that at low
temperatures and pressures the spectra of iodine, sulphur,
nitrogen, oxygen, &c. are quite different from the spectra of the
same elements at high temperatures and pressures. This may either
depend on the fact that the elements change their molecular
structure with a change of temperature, just as ozone is converted
into oxygen (for instance, from N_{2} molecules are obtained
containing only one atom of nitrogen), or else it may be because
at low temperature certain rays have a greater relative intensity
than those which appear at higher temperatures. If we suppose that
the molecules of a gas are in continual motion, with a velocity
dependent on the temperature, then it must be admitted that they
often strike against each other and rebound, and thus communicate
peculiar motions to each other and the supposed ether, which
express themselves in luminiferous phenomena. A rise of the
temperature or an increase in the density of a gas must have an
influence on the collision of its molecules and luminiferous
motions thus produced, and this may be the cause of the difference
of the spectra under these circumstances. It has been shown by
direct experiment that gases compressed by pressure, when the
collision of the molecules must be frequent and varied, exhibit a
more complex spectrum on the passage of an electric spark than
rarefied gases, and that even a continuous spectrum appears. In
order to show the variability of the spectrum according to the
circumstances under which it proceeds, it may be mentioned that
potassium sulphate fused on a platinum wire gives, on the passage
of a series of sparks, a distinct system of lines, 583-578, whilst
when a series of sparks is passed through a solution of this salt
this system of lines is faint, and when Roscoe and Schuster
observed the absorption spectrum of the vapour of metallic
potassium (which is green) they remarked a number of lines of the
same intensity as the above system in the red, orange, and yellow
portions.

The spectra of solutions are best observed by means of Lecoq de
Boisbaudran's arrangement, shown in fig. 76. A bent capillary
tube, D F, inside which a platinum wire, A _a_ (from 0·3 to 0·5
mm. in diameter) is fused, is immersed in a narrow cylinder, C (in
which it is firmly held by a cork). The projecting end, _a_, of
the wire is covered by a fine capillary tube, _d_, which extends
1-2 mm. beyond the wire. Another straight capillary tube, E, with
a platinum wire, B _b_, about 1 mm. in diameter (a finer wire soon
becomes hot), is held (by a cork or in a stand) above the end of
the tube, D. If the wire A be now connected with the positive, and
the wire B with the negative terminal of a Ruhmkorff's coil (if
the wires be connected in the opposite order, the spectrum of air
is obtained), a series of sparks rapidly following each other
appear between _a_ and _b_, and their light may be examined by
placing the apparatus in front of the slit of a spectroscope. The
variations to which a spectrum is liable may easily be observed by
increasing the distance between the wires, altering the direction
of the current or strength of the solution, &c.

_Spectrum analysis_ has not only endowed science with a knowledge of the composition of distant heavenly bodies (of the sun, stars, nebulæ, comets, &c.), but has also given a new _method_ for studying the matter of the earth's surface. With its help Bunsen discovered two new elements belonging to the group of the alkali metals, and thallium, indium, and gallium were afterwards discovered by the same means. The spectroscope is employed in the study of rare metals (which in solution often give distinct absorption spectra), of dyes, and of many organic substances, &c.[37] With respect to the metals which are analogous to sodium, they all give similar very volatile salts and such very characteristic spectra that the least traces of them[38] are discovered with great ease by means of the spectroscope. For instance, _lithium_ gives a very brilliant red coloration to a flame and a very bright red spectral line (wave-length, 670 millionths mm.), which indicates the presence of this metal in admixture with compounds of other alkali metals.

[37] The importance of the spectroscope for the purpose of chemical
research was already shown by Gladstone in 1856, but it did not
become an accessory to the laboratory until after the discoveries
of Kirchhoff and Bunsen. It may be hoped that in time
spectroscopic researches will meet certain wants of the
theoretical (philosophical) side of chemistry, but as yet all that
has been done in this respect can only be regarded as attempts
which have not yet led to any trustworthy conclusions. Thus many
investigators, by collating the wave-lengths of all the light
vibrations excited by a given element, endeavour to find the law
governing their mutual relations; others (especially Hartley and
Ciamician), by comparing the spectra of analogous elements (for
instance, chlorine, bromine, and iodine), have succeeded in
noticing definite features of resemblance in them, whilst others
(Grünwald) search for relations between the spectra of compounds
and their component elements, &c.; but--owing to the multiplicity
of the spectral lines proper to many elements, and (especially in
the ultra-red and ultra-violet ends of the spectrum) the existence
of lines which are undistinguishable owing to their faintness, and
also owing to the comparative novelty of spectroscopic
research--this subject cannot be considered as in any way
perfected. Nevertheless, in certain instances there is evidently
some relationship between the wave-lengths of all the spectral
lines formed by a given element. Thus, in the hydrogen spectrum
the wave-length = 364·542 _m_^2/(_m_^{2}-4), if _m_ varies as a
series of whole numbers from 3 to 15 (Walmer, Hagebach, and
others). For example, when _m_ = 3, the wave-length of one of the
brightest lines of the hydrogen spectrum is obtained (656·2), when
_m_ = 7, one of the visible violet lines (396·8), and when _m_ is
greater than 9, the ultra-violet lines of the hydrogen spectrum.

[38] In order to show the degree of sensitiveness of spectroscopic
reactions the following observation of Dr. Bence Jones may be
cited: If a solution of 3 grains of a lithium salt be injected
under the skin of a guinea-pig, after the lapse of four minutes,
lithium can be discovered in the bile and liquids of the eye, and,
after ten minutes, in all parts of the animal.