1902 Encyclopedia > Spectroscopy
SPECTROSCOPY. The spectroscope is an instrument which separates luminous vibrations of different wave-lengths, as far as is necessary for the object in view. It consists of three parts,the collimator, the prism or grating, and the telescope. The collimator carries the slit through which the light is admitted and a lens which con-verts the diverging pencil of light into a parallel pencil. The pencils carrying light of different wave-lengths are turned through different angles by the prism or grating, which is therefore the essential portion of the spectro-scope. The telescope serves only to give the necessary magnifying power, and is dispensed with in small direct vision spectroscopes. For a description of the different kinds of prism used, see OPTICS; and for an explanation of the action of the grating, see WAVE THEORY. The most important adjustment in the spectroscope is that of the collimator. Especially in instruments of large resolving power it is essential for good definition that the light should enter the prism or fall on the grating as a parallel pencil. For a method allowing an easy and accurate adjustment for each kind of ray, see an article in Phil. Mag., vol. vii. p. 95 (1879).
Prisms are nearly always used in the position of minimum deviation, but, if the collimator is properly adjusted, this is by no means a necessary condition for good definition. Prisms as generally cut, with an isosceles base, give the greatest resolving power in the position of minimum deviation, but the loss in resolving power is not great for a small displacement. The dispersion and magnifying power of a prism can be considerably altered by a change of its position, and a knowledge of this fact is of great value to an experienced observer. The use of a prism in a position different from that of minimum deviation is, however, a luxury which only those acquainted with the laws of optics can indulge in with safety.
Lord Rayleigh has given the theory of the spectro-scope under OPTICS, and shown on what its resolving power depends. There is no connexion between resolving power and dispersion, any value of resolving power being consistent with any value of dispersion. To obtain large resolving power with small dispersion requires, however, the use of inconveniently large telescopes and prisms or gratings. It is easy, on the other hand, to obtain small resolving power together with large dispersion.
The following definitions would be found of general use if adopted. Resolving Power.The unit resolving power of a spectroscope in any part of the spectrum is that resolving power which allows the separation of two lines differing by the thousandth part of their own wave-length or wave-number,the wave-number being the number of waves in unit length. Purity.The unit purity of a spectrum is that purity which allows the separation of two lines differing by the thousandth part of their own wave-length or wave-number. We speak of the resolving power of a spectroscope and of the purity of a spectrum. The resolving power is a constant for each spectroscope, and independent of the width of the slit. The purity of a spectrum, on the other hand, depends on the width of the slit, unless that width is small compared to a certain quantity presently to be mentioned. The resolving power of a spectroscope is numerically equal to the greatest purity of spectrum obtainable by it.
Adopting these definitions, we get from Lord Rayleigh's equations for the resolving power _ of a grating
1000 R = mn,
where n is the total number of lines used on the grating and m the order of the spectrum. For a spectroscope with simple prisms we get
== IMAGE ==
where <2 and t-^ are the greatest and smallest lengths of paths in the dispersive medium. If we put for the re-fractive index of the medium /_ = A + -B/A2 we may write
== IMAGE ==
It will be seen that, while the resolving power of a spectroscope with grating depends only on the order of the spec-trum and is independent of the wave-length for each order, the resolving power of a spectroscope with prism will vary inversely as the third power of the wave-length A, so that the resolving power will be about eight times as great in the violet as in the red (see OPTICS). If compound prisms are used we must write
== IMAGE ==
where t2 is the greatest effective length of path in one medium, tx in the other medium, B2 and 2?_ being the dis-persive constants for the two media.
The purity P of a spectrum is given by the equation
== IMAGE ==
where d denotes the width of slit and ip is the angle sub-tended by the collimator lens at the slit. If the slit is sufficiently narrowed, d ip may be made small compared to A, and in that case the purity of the spectrum is independ-ent of the width of slit and equal to the resolving power. If, on the other hand, a wide slit is used, so that d \f> is large compared to A, the purity becomes inversely pro-portional to the width of slit. In actual work the slit is generally of such width that neither term in the denomi-nator of the expression for purity can be neglected.
There is a necessary limit to the resolving power of all optical instruments, depending on the fact that light con-sists of a series of groups of waves incapable of interfering with each other. If it is true, as is generally believed, but without sufficient reason, that a retardation of 50,000 wave-lengths is sufficient to destroy the capability of interfer-encethat is to say, that the groups consist on the average of approximately 50,000 wavesthe maximum purity obtainable in any spectroscope is 50. The closest line resolved with a grating, as far as the present writer is aware, requires a resolving power of about 100. Professor Piazzi Smyth has with prisms realized a purity of 50. It would seem, therefore, that the theoretical limit of purity has very nearly been reached, for, though the estimate of 50,000 waves to the group is in all probability too small, there are other considerations which render it highly improbable that the total number of waves to the group should, for sunlight at any rate, be more than two or three times larger. The limit of possible purity will very likely depend on the temperature of the luminous body.
Almost the greatest practical difficulty which the spectro-scopist has to contend with generally is the want of suffi-cient light. The following remarks apply to line spectra principally, but they hold also almost entirely for the spectra of fluted bands, which break up into lines under high resolving power. The maximum illumination for any line is obtained when the angular width of the slit is equal to the angle subtended by one wave-length at a distance equal to the collimator aperture. In that case d\p = \ and the purity is half the resolving power. Hence when light is a consideration we shall not, as a rule, realize more than half the resolving power of the spectroscope. If the visual impression depended only on the intensity of illumination, a further widening of the slit should not increase the visi-bility of a line. As a matter of fact spectroscopists gener-ally work with slits wider than that which theoretically gives full illumination. The explanation of the fact is physiological, visibility depending on the apparent width of the object. If different spectroscopes have their slits of such width that the apparent width of a line as seen by the eye is the same, and if the magnifying power is such that the pupil is just filled with light, the purity of the spectrum is directly proportional to the resolving power. We come to the conclusion, therefore, that for both narrow and wide slits the efficiency of a spectroscope depends ex-clusively on its resolving power. It has been pointed out by Lord Eayleigh that, owing to the want of definition in the optical images on the retina when the full aperture of the pupil is used, the pencil must be contracted to a third or a quarter of its natural width, if full resolving power is to be obtained. This is accompanied with a serious loss of light, which can be partly obviated by contracting the horizontal aperture only (the refracting edge being supposed vertical). There are two ways of doing this. One con-sists in the use of magnifying half prisms. But the loss of light by reflexion in simple half prisms more than counterbalances the advantage ; compound half prisms like those used by Christie may, however, be employed. We may also use prisms of three or four times the height of the effective horizontal aperture, with correspondingly large telescopes, and then by the eye-piece contract the beam until its vertical section fills the pupil. The latter plan, though theoretically best, involves more expensive appa-ratus and prisms of very homogeneous material.
The question of illumination is important also when photography is used for spectroscopic analysis. For a given intensity of the source of light the intensity of the image on the sensitive film will be directly proportional to the solid angle of the cone of light forming the last image, and will be independent of the arrangement of inter-mediate lenses. Hence lenses with as short a focus com-pared to aperture as is consistent with good definition should be used in the camera.
The methods of recording and reducing spectroscopic observations are described in all books and treatises on the subject and may therefore be passed over here.
A lens is often used to concentrate the light of the source on the slit. There is some loss of light due to reflexion from the surface of the lens, but its position, aperture, and focal length do not affect the luminosity of the spectrum seen as long as the whole collimator is filled with light.
Bodies are rendered luminous for spectroscopic investi-gation either by being placed in the Bunsen flame or by the help of the electric current. A little difficulty may arise where the body is given in solution and does not show its characteristic lines in the flame. Lecoq de Bois-baudran takes the spark from the surface of the solution. The present writer has found the tube sketched in the figure on the next page a great improvement on those commonly used, if a sufficient quantity of the solution is at hand ; otherwise the method is too wasteful. The current is brought into the solution by a platinum wire, sealed into a small glass tube; the platinum wire reaches about to the level of the open end of the tube. A capillary of thick-walled glass tubing is placed over the platinum wire; the liquid rises in the capillary and sparks can be taken as from a solid. The lines due to the glass are easily eliminated. If a small quantity of material only is avail-able, the plan adopted by Bunsen and ex-tensively used by Hartley seems the most successful. Pointed pieces of charcoal (Bun-sen) or pieces of graphite pointed to a knife edge (Hartley) are impregnated with the liquid, and the spark is taken from them, stances, when introduced into a vacuum tube, especially near the negative pole, and under great exhaustion, show a characteristic phosphorescence. Becquerel was the first to examine the spectra shown under these circumstances, and Crookes has lately used the same method with great success.
A good deal of discussion has taken place on the spectra of the metalloids, owing to the fact that they seem to be able to give different spectra under different circumstances. Spectra have occasionally been assigned to the elements which on further investiga-tion were found to belong to some compound present. According to the general opinion of spectroscopists at present, different spectra of the same elements are always due to different allotropic condi-tions. If a complex molecule breaks up into simpler molecules the breaking up is always accompanied by a change of spectrum.
Nitrogen.(a) The line spectrum appears whenever a strong spark (jar discharge) is taken in nitrogen gas. It is always present when metallic spectra are examined by the ordinary method of allowing the jar discharge to pass between metallic poles. Hartley (Phil. Trans., 1884, part i.) has measured the ultra-violet lines of the air spectrum, but has not separated the oxygen from the nitrogen lines. (5) The band spectrum of the positive discharge, which is generally called the band spectrum of nitrogen, always appears when the discharge is sufficiently reduced in intensity. The spectrum consists of two sets of bands of different appearance, one in the less refrangible part and one in the more refrangible part of the spectrum, the two sets of bands overlapping in the green. Hence some observers believe the spectrum to be made up of two distinct spectra. Plueker and Hittorf (Phil. Trans., 1865) give a coloured drawing of this spectrum, which is one of the most beautiful that can be observed. The most complete drawing of it is given by Piazzi Smyth (Trans. Roy. Soc. Edin., vol. xxxii. part iii.), and there is also a good drawing by Hassclberg (Mem. Acad. Imp. de St.Petersb., vol. xxxii.). (c) The glow which surrounds the negative electrode in an exhausted tube shows in many cases a spectrum which, as a rule, is not seen in any other part of the tube. The memoir of Hasselberg contains a drawing of it. The spectrum seen when a weak spark is taken in a current of ammonia is neither that of nitrogen nor that of hydrogen, but must be due to a compound of these gases. When the pressure of the gas is reduced, a single band is seen having a wave-length from 5686 to 5627 ATth metres (Nature, vi. p. 359). When a spark is taken from a liquid solution of ammonia a more complicated spectrum appears (Lecoq de Bois-baudran), and, if ammonia and hydrogen are burnt together either in air or oxygen, a complicated spectrum is obtained the chemical origin of which has not been satisfactorily explained. Drawings of it are given by Dibbits (Pogg. Ann., cxxii. p. 518) and by Hofmann (Pogg. Ann., cxlvii. p. 95). The absorption spectrum of the red fumes of nitrogen tetroxide has often been mapped ; the most perfect drawing is given by Dr B. Hasselberg (Mem. Acad. Imp. de St. Pet., xxvi.). According to Moser (Pogg. Ann., elx. p. 177), three bands close to the solar line C disappear when the vapour is heated. Recently Deslandes has obtained in vacuum tubes some ultra-violet bands which seem to be due to a compound of nitrogen and oxygen (O.R., chap. i. p. 1256, 1885).
Oxygen.(a) The elementary line spectrum of oxygen is that which appears at the highest temperature to which we can subject oxygen, that is, whenever the jar and air break are introduced into the electric circuit. It consists of a great number of lines, especially in the more refrangible part of the spectrum, (b) The compound line spectrum of oxygen appears at lower temperatures than the first. It consists, according to Piazzi Smyth, of six triplets and a number of single lines. This spectrum corresponds to the band spectrum of nitrogen, (c) The continuous spectrum of oxygen appears at the lowest temperature at which oxygen is luminous. The wide part of a Plueker tube, for instance, filled with pure oxygen generally shines with a faint yellow light, which gives a continuous spectrum. Even at atmospheric pressure this spectrum can be ob-tained by putting the contact breaker of the induction coil out of adjustment, so that the spark is weakened, (d) The spectrum of the negative glow was first accurately described by Wiillner, and is always seen in the glow surrounding the negative electrode in oxygen. It consists of five bands, three in the red and two in the green. For further information respecting these spectra, see Schuster (Phil. Trans., clxx. p. 37, 1879) and Piazzi Smyth (Trans. Roy. Soc. Edin., vol. xxxii. part iii.). According to Egoroff, the A and B lines of the solar spectrum are due to absorption by oxygen in our atmosphere, and some recent observations of Janssen seem to support this view.
Carbon.(a) The line spectrum appears when a very strong spark is sent through carbonic oxide or carbonic acid. The ultra-violet lines observed by Hartley when sparks are taken from graphite electrodes also belong probably to this spectrum, (b) Considerable discussion has taken place as to the origin of the spectrum seen at the base of a candle or a gas flame. At first observations seemed to point to the fact that it was due to a hydrocarbon. It has been ascertained, however, that sparks taken in cyanogen gas, even when dried with all care, show the spectrum, and a flame of cyanogen and oxygen gives the same bands brilliantly. These facts have convinced the majority of observers that the spectrum is a true carbon spectrum. The best drawing is given by Piazzi Smyth, who ascribes the spectrum, however, to a hydrocarbon. The flame of cyanogen, which had already been examined by Faraday and Draper before the days of spectrum analysis, shows a series of bands in the red, reaching into the green. There is no doubt that they are due to a compound of nitrogen and oxygen. Another series of bands in the blue, violet, and ultra-violet have been also proved by Liveing and Dewar to be due to a compound of nitrogen and carbon. If the discharge is passed at low pressure through carbonic acid or carbonic oxide a spectrum is seen which seems to belong to carbonic oxide. A very beautiful and remarkable drawing of this spectrum, especially of its most brilliant band, has been published by Piazzi Smyth.
Very little need be said of the remaining metalloids, as we do not possess a sufficiently careful examination of their spectra. Chlorine, bromine, and iodine show bands by absorption. If a spark is passed through the gases line spectra appear. Sulphur volatilized in a vacuum tube may show either a line or a band spectrum under the influence of the electric discharge. The absorption through the vapour of sulphur is continuous at first on volatilization, but as the vapour is heated to 1000° the continuous spectrum gives way to a band spectrum. A spark through the vapour of phosphorus gives a line spectrum. We may obtain the spectra of fluorine, silicon, and boron by comparing the spectra given by sparks taken in atmospheres of fluoride of boron and fluoride of silicon.
Spectra of Metals and their Compounds.
Hydrogen.If sparks are taken through hydrogen, four well-known lines appear in the visible region of the spectrum. The remarkable series of ultra-violet lines photographed by Dr Huggins in the spectra of some stars which in their visible part show hydro-gen chiefly has suggested the question whether the whole series is not due to that gas. This has now been proved to be the case by Cornu, who has recently examined the hydrogen spectrum with great care. In vacuum tubes filled with hydrogen a complicated spectrum often appears which is so persistent that nearly all 'ob-servers have ascribed it to hydrogen (though Salet had given reasons against that conclusion). According to Cornu, the purer the gas the feebler does this spectrum become, so that the above-mentioned line spectrum seems to be the only true hydrogen spectrum. A flame of hydrogen in air or oxj'gen shows a number of lines in the ultra-violet belonging apparently to an oxide of hydrogen (Live-ing and Dewar, Huggins). Aqueous vapour gives an absorption spectrum principally in the yellow.
Alkali Metals.The metals of the alkali group are distinguished by the fact that their salts give the true metal spectra when ren-dered luminous in the Bunsen burner ; that is to say, their salts are decomposed and the radiation of their metallic base is sufficiently powerful to be visible at the temperature of the flame. Their spectra are not so easily seen if sparks are taken from the liquid solution, but Lecoq de Boisbaudran has obtained fine spectra of sodium and potassium by taking the spark from a semi-fluid bead of the sulphates. The most complete description of the spectra of sodium and potassium seen when the metals are heated up in the voltaic arc is given by Liveing and Dewar (Proc. Roy. Soc., xxix. p. 378, 1879), who have also mapped their ultra-violet lines (Phil. Trims., 1883, pt. i.). Abney has found a pair of infra-red lines belonging to sodium, with wave-lengths 8187 and 8199 (Proc. Roy. Soc, xxxii. p. 443, 1881). Becquerel finds lines in the infra-red at 11,420. The vapour of sodium and potassium heated up in a tube is coloured and shows a spectrum of fluted band; but in the case of sodium the yellow line is always present at the same time. It is probable that the band spectrum belongs to the vapour, con-taining two atoms in each molecule, and that at higher tempera-tures the molecules are split up, the single atoms showing the line spectra. Both potassium and sodium show an additional absorption line (5510 for Na and 5730 for Ka) at the temperature at which the fluted bands appear. According to a suggestion of Liveing and Dewar, these lines may depend on the presence of hydrogen, which it is very difficult to exclude. These experimenters have also de-scribed interesting but complicated absorption phenomena depend-ing on the simultaneous presence of two or more metals. Thus sodium and magnesium show a band in the green (\ = 5300), which does not appear when sodium alone or magnesium alone is volati-lized. Potassium and magnesium show similarly two lines in the red (Proc. Boy. Soc, xxvii. p. 350, 1878). If a spark is taken from potassium in an atmosphere of carbonic oxide a band appears (5700) depending probably on a combination between the potassium and the carbonic oxide. Lockyer has observed certain curious phenomena (Proc. Boy. Soc, vol. xxii. p. 378) taking place at the temperature at which the band spectrum of sodium changes into the line spectrum ; these phenomena deserve a fuller investigation. Lithium furnishes a good example of a change in the relative in-tensity of lines at different temperatures. At the temperature of the flame the red line is the most powerful, an orange line being also seen. When a spark is taken from a liquid solution the orange line is far the strongest, and a blue line is seen, which in its turn rapidly gains in intensity as the temperature is raised. When the spark is taken from solutions of different strengths the more con-centrated solution shows a change in relative intensity of lines in the direction in which an increase of temperature would act. Com-bination of the metals with transparent acids does not when in solution show any appreciable absorption in the visible part of the spectrum ; but Soret has mapped their ultra-violet absorption.
Metals of Alkaline Earths.Calcium, strontium, and barium are distinguished by the fact that their volatile compounds give fine spectra in the Bunsen flame. The more stable salts, as the phos-phates and silicates, give the reaction only feebly or not at all. When a salt like the chloride of barium is introduced into the flame the spectrum is seen to change gradually; the spectrum seen at first is different according as the chloride, bromide, or iodide is used, while the spectrum which finally establishes itself is the same for the different salts of the same metal. Mitscherlich, who was the first to investigate carefully these phenomena (Pogg. Ann., exxi. p. 459, 1864), ascribes the spectra seen at first to the compound placed in the flame, while gradually the oxide spectrum gets the upper hand. This explanation has always been accepted, and receives support from the fact that the bromide spectrum is strengthened by introducing bromine vapour into the flame, and the other compound spectra can be similarly strengthened by introducing suitable vapours. There is an observation, however, made by Pro-fessors Liveing and Dewar which in one case is not compatible with Mitscherlich's explanation. "A mixture of barium carbonate, aluminium filings, and lamp-black heated in a porcelain tube gave two absorption lines in the green, corresponding in position to bright lines seen when sparks are taken from a solution of barium chloride, at wave-lengths 5242 and 5136, marked a and /3 by Lecoq de Boisbaudran." These two lines, or rather bands, are the brightest in the spectrum commonly ascribed to barium chloride. In addi-tion to the compound spectra the brightest of the metallic lines seen at a low temperature appear in the flame. The metallic line is in the violet with calcium, in the blue with strontium, and in the green with barium. Sparks taken from a solution of the metallic salts show the compound spectra well, and in addition more of the true metallic lines than the flame. The best drawings of the compound spectra are those given in Lecoq de Boisbaudran's Atlas ; but measurements with higher resolving powers are much wanted. When the salts are introduced into the voltaic arc numer-ous metallic lines appear which have been mapped by Thalen. Liveing and Dewar have investigated those lines which can be reversed and have also mapped the ultra-violet spectra. Captain Abney has mapped a pair of infra-red lines belonging to calcium between 8500 and 8600, and, according to Becquerel, with the help of a phosphorescent screen bands or lines appear of still lower refrangibility (8830 to 8880). Lockyer (Phil. Trans., clxiii. p. 253, 1873, and clxiv. p. 805, 1874) has measured and mapped as regards their length the lines of these as well as of many of the other metals.
Metals of Magnesium Group.Beryllium presents comparatively simple spectroscopic phenomena, as far as it has hitherto been investigated. Two green lines were mapped by Thalen and five in the ultra-violet by Hartley (Jour. Chem. Soc, June 1883). The spectrum of magnesium is well known from its green triplet; but the vibrations of the metal seem very sensitive to a change of conditions. Full details are given by Liveing and Dewar in Proc Boy. Soc, xxxii. p. 189. These authors have found that some of the bands seen occasionally, when magnesium wire is burned in air, are due to a compound of magnesium and hydrogen. The spec-trum appears when sparks are taken from magnesium poles in an atmosphere containing hydrogen. For a description of the pecu-liarities of the flame, arc, and spark spectrum, the reader is referred to the original paper. The ultra-violet spectrum, which contains several repetitions of the green triplet, has also been mapped and measured by Hartley and Adeney (Phil. Trans., clxxv., 1874, pt. i.). The spectra of zinc and cadmium are obtained either by sparks from liquid solution or by the spark, with Leyden jar, from the metal poles. The ultra-violet spectra show for both elements a remarkable series of triplets, the lines of the cadmium triplet being about three times as far apart as those of the zinc triplets. The least refrangible of the series is in the blue with wave-lengths 5085-1, 4799-1, 4677'0 for cadmium, and 4809-7, 4721-4, 4679'5 for zinc.
Lead Group.The spectrum of lead is best obtained by taking the spark from the metallic poles. Care must be taken, however, to renew the surface frequently, otherwise the oxide spectrum will gradually make its appearance. The oxide itself shows its spectrum, according to Lecoq de Boisbaudran, in the Bunsen burner. The salts of thallium show the principal metal line at the temperature of the flame. The spark spectrum is more complicated. The ultra-violet spectra of both lead and thallium have been mapped.
Copper Group.The spectra of the metals belonging to this group are easily obtained in the ordinary way. When copper chloride is introduced into the Bunsen flame a fine spectrum of bands is seen. It is the same spectrum which is found when com-mon salt is thrown upon white hot coals. This reaction for copper chloride is very sensitive, but it has never been satisfactorily decided whether the presence of copper is really necessary for its production or whether the spectrum belongs to a peculiar condition of chlorine vapour. Silver when first volatilized gives a green vapour, which at a low temperature shows continuous absorption, but at a higher temperature a spectrum of fluted bands (Lockyer). Mercury shows its lines with great brilliancy if introduced and heated in a vacuum tube. Some of the lines widen easily, and at higher pressures a con-tinuous spectrum completely covers the background. The copper salts in aqueous solution absorb principally the red end of the spectrum, the green salts also the violet end. The glass, coloured green with oxide of copper, transmits through sufficient thickness exclusively the yellow and green rays between D and E (H. W. Vogel).
Cerium Group. Yttrium gives a good spark spectrum from the solution of the chloride ; the salts show no absorption bands. Crookes has found, however, that a certain substance yields brilliant phosphorescent bands under the influence of the negative pole in a vacuum tube. These bands he has, after a lengthy investigation, put down to yttrium compounds, and explained the changes they undergo in different compounds and the sensitiveness of the reaction. Lecoq de Boisbaudran, who obtains the same spectrum by taking a spark (without Leyden jar) from solutions, making the solution the positive pole, has expressed an opinion that the bands are not due to yttrium but to two substances provisionally called by him Za and Z/3. He has also under certain conditions seen a higher temperature spectrum, which he ascribes to Z7, leaving it undecided whether Z7 is a new substance or identical with Za (Phil. Trans., 1883, p. 891, and C.R., ci. p. 552, cii. p. 153).Banlhanumis easily recognized by a strong spark spectrum.Cerium, like yttrium and lanthanum, has no peculiar absorption spectrum when in combin-ation and solution ; although the salts are strongly coloured yellow, its line spectrum has characteristic lines in the blue.Didymium is characterized speetroscopically by the fine absorption spectra of its salts. Different salts show slightly different spectra, but they can be recognized at first sight as didymium spectra. The crystals of didymium salts show remarkable differences in the absorption spectra according to the direction in which the ray traverses the crystal. Light reflected from the powdered salts shows the character-istic spectrum. According to Auer von Welsbach (Monatsschr. f. Chemie, vi. p. 477), didymium has lived up to its name didvpoi, " twins," for by fractional crystallization he has found it to be an intimate mixture of two substances, each of them giving half the ab-sorption spectrum and half the emission spectrum of didymium. Terbium has a characteristic line spectrum when the spark is taken from a solution of the salts.The salts of erbium give a characteristic absorption spectrum, but till recently the drawings of it contained also absorption bands due to thulium and holmium. The spectrum of erbium, as previously mapped by Thalen, belongs almost exclusively to ytterbium ; but he has recently mapped the lines belonging to what is now known as erbium (C.R., xci. p. 326). Erbium salts heated in the Bunsen burner show a spectrum of bright bands without apparent volatilization. Ytterbium, discovered by Marignac (atomic weight 17'3, Kilson), gives an ab-sorption band in the ultra-violet. Its luminous spectrum is rich in lines (Thalen, C.R., xci. p. 326).Samarium, also discovered by Marignac and called by him originally Y/3, gives absorption bands in the visible part and in the ultra-violet (Soret, G.B., xc. p. 212). It frequently occurs with didymium, and most of the maps of the didymium spectrum contain the samarium bands. When pre-cipitated with another metal it shows a brilliant phosphorescent spectrum (Crookes), which, however, is slightly different accord-ing to the metal. The peculiar yttrium spectrum is very weak even when it is mixed in considerable quantities with samarium. But when the quantity of yttrium is increased to about 60 per cent, a very rapid change takes place, and afterwards it is the samarium spectrum which is very weak. A band in the orange peculiar to the mixture, weak in pure samarium and absent in yttrium, is strongest in a mixture containing about 80 per cent, of samarium and 20 per cent, of yttrium.Holmium, identified as a separate element by Soret (C.R., xci. p. 378), has absorption bands in the visible part of the spectrum (6405, 5363, 4855 on Lecoq's map of chloride of erbium), and also a strongly marked ultra-violet absorption spectrum.Thulium, likewise first recognized by Soret, is band 6840 on Lecoq's drawing of chloride of erbium, and also possesses a band at 4645. Thalen has measured the bright line spectrum (C.R., xci. p. 376, 1880).Scaiulium is characterized by a bright line spectrum (Thalen, O.R., xci. p. 48,1880).Gadolinium (Marignac's Ya) has a weak absorption spectrum in the ultra-violet and a characteristic phosphorescent spectrum (Proe. Roy. Soc, February 1886); but the latest researches of Crookes have rendered it probable that it is a mixture of several new elements (Proa. Roy. Hoc, 10th June 1886).The mosandrium of Lawrence Smith seems a mixture of gadolinium and terbium. philippium of De la Fontaine was a mixture of yttrium and terbium; and the latest dccipium of the same chemist is probably holmium.
Aluminium Group.The spectra of the metals belonging to this group can be obtained in the ordinary way by means of the electric spark. The chloride of indium shows the two strongest metallic lines, one in the indigo and one in the violet, when intro-duced into the Bunsen flame. According to Claydon and Heycock, a number of other lines appear when the spark is taken from the metal electrodes. When a weak spark is taken from aluminium electrodes in air a band spectrum is often seen belonging apparently to the oxide, for it disappears when the spark is taken in hydrogen. Gallium, another metal belonging to this group, was first discovered by means of its spectroscopic reaction. The chloride shows two violet lines feebly in the Bunsen flame, but strongly if a spark is taken from the liquid solution. The ultra-violet lines of indium and of aluminium have been photographed by Hartley and Adeney, as well as by Liveing and Dewar. Some of the lines had been pre-viously mapped by Cornu, whose researches extend furthest into the ultra-violet. According to Stokes, aluminium shows lines more refrangible than those of any other metal, and the wave-lengths of their lines as measured by Cornu are for one double line 1934, 1929, and for another 1860, 1852.
Metals of the Iron Group.The spectroscopic phenomena of this group are somewhat complicated. The line spectra can be obtained either by taking sparks from the metal or from the solution of a salt, and also by placing the metal in the voltaic arc. The lines are very numerous and very liable to alter in relative intensity under different circumstances. The great difference shown, for instance, between the arc and spark spectra of iron in the ultra-violet region is shown in the map by Liveing and Dewar in Phil. Trans., 1885, pt. i. The visible part has also been investigated by the same authors and by Lockyer, and much information has thus been added to the knowledge previously obtained by Kirchhoff, Angstrom, and Thalen. That part of the iron spectrum lying between a wave-length of 4071 and 2947 has been mapped by Cornu; Liveing and Dewar's observations refer chiefly to the more re-frangible region. Considering the very important part which the iron spectrum plays in solar observations, a full investigation of its changes by a variation of temperature would at the present time be of great value. If observations with the method adopted by Lecoq de Boisbaudran were repeated with higher resolving powers they would add much to our knowledge. Some of the manganese salts, such as the chloride or carbonate, seem to be the only salts belonging to this group which show a characteristic spectrum when heated in the Bunsen burner or the oxyhydrogen flame. The spectrum observed in these cases is, according to Watts, the characteristic spectrum of the Bessemer flame, which disappears at the right moment for stopping the blast; it is probably due to an oxide of manganese. When a spark spectrum is taken from a solution of the chloride the same spectrum is seen, but the relative intensity of the lines depends on the length and the strength of the spark. The green - coloured manganates show a continuous absorption at the two ends of the spectrum, transmitting in con-centrated solutions almost exclusively the green part of the spec-trum. The absorption bands of permanganate of potassium are well known and seem to be due to the permanganic acid, as they appear also with other permanganates. The green salts of nickel show a continuous absorption at the two ends of the spectrum. The cobalt salts show well-defined absorption bands. Their careful investigation by Dr W. J. Russell deserves special notice (Proc. Roy. Soc., xxxii. p. 258, 1881).
Metals of Chromium Group.The metallic spectra of this group have been measured principally by Thalen in the usual way. Lockyer and Roberts have obtained a channelled spectrum of chromium by absorption. As regards the spectra of compounds of chromium, the absorption of the vapour of chloro - chromic anhydride has been measured by Emerson and Reynolds (Phil. Mag., xlii. p. 41, 1871), and consists of a series of regularly dis-tributed bands. The chromium salts all possess a decided colour and show interesting absorption phenomena. The chromates ab-sorb the violet and blue completely, also the extreme red, and transmit only the orange, yellow, and in dilute solutions part ol the green. The most complete investigation of the salts in which chromium plays the part of a base is due to Erhard in a dissertation published at Freiburg. Potassium chrom-alum, ammonia chrom-alum, sulphate of chromium, when in solution, give an identical absorption for the same amount of chromium. The extreme red is freely transmitted by the violet solution, but the absorption grows rapidly towards the yellow. An indistinct absorption band (X = 6790 to X=6740) is seen when the layer is thick or the solution concentrated. The strongest absorption takes place for a wave-length of 5800. The green is transmitted again more freely, the minimum absorption taking place for a wave-length 4880; the absorption then grows rapidly towards the violet. When the solutions are heated the colour changes to green, the absorp-tion is increased throughout the spectrum, except in the green, where it remains nearly unchanged, and the minimum of absorption shifts to a wave-length of 5090. The solution, which remains green on cooling, has, when compared with its original state, an increased absorption in the red and blue and a slightly diminished absorption in the green. When light is sent through plates cut out of crystals of potassium chrom-alum or ammonia chrom-alum, three absorption bands (6860, 6700, 6620) are seen in the red. The green and blue show the same absorption as the solution. The chloride in solution gives the same absorption as the chrom-alums,transmitting, how-ever, slightly more light for the same quantity of chromium. The hot solution also shows the same changes, but with this difference that colour and absorption phenomena are almost entirely recovered on cooling. The nitrate (solution of chromic hydroxide in nitric acid) agrees with chrom-alum, but transmits more light. Red crystals of potassic chromic oxalate only transmit the red with an absorption band slightly less refrangible than B (\=6867). The blue salt has the absorption band at a wave-length of 7040 and transmits part of the light in the green and blue. The solutions of the salts show the same absorption as the crystals, with the position of the absorption band apparently unchanged. The warm solutions absorb more than the cold ones. The oxalate of chromium gives an absorption band of 6910 to 6860 and transmits the green and blue more freelythan the double salt. The tartrate onlyshows the absorp-tion band in the red very weakly and absorbs more red than the previously mentioned solutions. The acetate transmits more yellow than the other salts and has some broad absorption bands near a wave-length of 7170. When the solution is heated it becomes green, absorbing the red more than when cold, but leaving the green and blue absorption unchanged. The absorption phenomena shown by uranium salts are more complicated than those of the chromium salts, but they are at the same time more characteristic, as the spectra are more definitely broken up into bands. According to Vogel, the uranic and uranous salts behave differently (Praktische Spectral-Analyse, p. 247), but a more careful investigation is de-sirable. Sorby finds that a mixture of zirconium and uranium dissolved in a borax bead shows characteristic bands, which are visible neither with uranium nor with zirconium alone.
There is little to be said as regards the remaining groups of metals (tin, antimony, gold). Their spectra are best obtained by taking the spark from metallic electrodes or by volatilization in the voltaic arc.
Influence of Temperature and Pressure on Spectra of Gases.
If the spectrum of an element is examined under different conditions of temperature or pressure, it is often found to differ considerably. The change may be smallthat is to say, the lines or bands may only show a different distribution of relative intensity or it may be so large that no relationship at all can be discovered between the spectra. It has been pointed out by Kirchhoff that a change in the thickness of the luminous layer may produce a change in the appearance of the spectrum, and Zollner and Wiillner have endeavoured to explain in this way a number of important varia-tions of spectra. But their explanation does not stand the test of close examination. The thickness of layer cannot be neglected in the discussion of solar and stellar spectra, or in the comparison of absorption spectra of liquids; but none of the phenomena which we shall notice here are affected by it.
Widening of Lines.The lines of a spectrum are found to widen under certain conditions, and, although probably all spectra are subject to this change, some are much more affected by it than others. The lines of hydrogen and sodium, for instance, widen so easily that it is sometimes difficult to obtain them quite sharp. "When a system of lines widens it is generally found that the most refrangible lines widen most easily. A line my expand equally towards both sides or chiefly towards one side ; in the latter case the expansion towards the less refrangible side preponderates pretty nearly in every case. It is the almost unanimous opinion of spectro-scopists that the widening is produced by an increase of pressure. If sparks are passed through gases, the lines are always broader at high than at low pressures, and the metallic lines are also broader when a spark is taken from them at higher pressures. "Without altering the pressure, we may often produce a widening of lines by an increase in the intensity of the discharge, but here the pressure is indirectly increased by the rise of temperature. According to the molecular theory of gases, the following explanation might be given for the widening of lines. As long as a molecule vibrates by itself uninfluenced by any other molecule, its vibrations will take place in regular periods. The lines of its spectrum will conse-quently be sharp. But, if the molecule is placed in proximity with others, its vibrations will be disturbed by occasional encounters. During each encounter forces may be supposed to act between the molecules, and these forces will affect the regularity of the vibra-tion. The question arises, whether for a given temperature and pressure a line may be of different width according as the molecule is placed in an atmosphere of similar or dissimilar molecules. Such a difference exists in all probability. If gases are mixed in different proportions, the lines are sharper when an element is present in small quantities, although the total pressure may be the same. There is one cause which limits the sharpness of spectroscopic lines : the molecules of a gas have a translatory motion. Those molecules which are moving towards us will send us light which is slightly more refrangible than those which move away from us ; hence each line ought to appear as a band. In reality the width of lines generally is greater than that due to this cause.
Spectra of Different Orders.-Spectra may be classified according to their general appearance. The different classes have been called orders by Pliicker and Hittorf. At the highest temperature we always obtain spectra of lines which need no further description. At a lower temperature we often get spectra of channelled spaces or fluted bands. When seen in spectroscopes of small resolving power these seem made of bands which have a sharp boundary on one side and gradually fade away on the other. With the help of more perfect instruments it is found that each band is made up of a number of lines which lie closer and closer together as the sharp edge is approached. Occasionally the bands do not present a sharp edge at all, but are made up of a number of lines of equal intensity at nearly equal distances from each other. Continuous spectra, which need not necessarily extend through the whole range of the spectrum, form a third order, and appear generally at a lower temperature than either band or line spectrum. One and the same element may at different temperatures possess spectra of different orders. A discussion has naturally arisen as to the cause of these remarkable changes of spectra, and it is generally believed that they are due to differences of molecular structure. Thus sulphur vapour when volatilized shows by absorption a continuous spectrum until its temperature is raised to 1000°, when the continuous spectrum gives way to a spectrum of bands. We know that the molecule of sulphur is decomposed as the temperature is raised, and we are thus justified in saying that the band spectrum belongs to the molecule containing two atoms, while the continuous spectrum belongs to the more complex molecule which first appears on volatilization. When a strong electric spark is passed through the vapour of sulphur a bright line spectrum is seen, and this is believed to be due to a further splitting up of the molecule into single atoms.
Long and Short Lines.If the spectrum of a metal is taken by passing the spark between two poles in air the pressure of which is made to vary, the relative intensity of some of the lines is often seen to change. Similar variations take place if the intensity of the discharge is altered, as, for instance, by interposing or taking out a Leyden jar. It is a matter of importance to be able to use a method which in the great majority of cases will give at once a sure indication how each line will behave under different circum-stances. This method we now proceed to describe. It has often been remarked, even by the earliest observers, that the metallic lines when seen in a spectroscope do not always stretch across the field of view, but are sometimes confined to the neighbourhood of the metallic poles. Some observations which Loekyer made jointly with Professor Frankland led him to conclude that the distance which each metallic line stretched away from the pole could give some clue to the behaviour of that line in the sun. In 1872 Loekyer worked out his idea. An image of the spark was formed on the slit of the spectroscope, so that the spectrum of each section of the spark could be examined. Some of the metallic lines were then seen to be confined altogether to the neighbour-hood of the poles, wdiile others stretched nearly across the whole field. The relative length of all the lines was estimated. Tables and maps are added to the memoir. The longest lines (that is, those which stretch away farthest from the pole) are by no means always the strongest; and there are many instances where a faint line is seen to stretch nearly across the whole field of view, while a strong line may be confined to the neighbourhood of the pole, or is reduced sometimes to a brilliant point only. We give a few conspicuous examples of lines which are long and weak or short and strong. In lithium the blue line (46027) is brilliant but short. In lead 4062-5, one of the longest lines, is faint and according to Loekyer difficult to observe. In tin 5630'0 is the longest line, but it is faint, while the stronger lines near it (5588'5 and 5562-5) are shorter. The zinc lines 4923'8, 4911-2, 48097, 4721-4, 4679'5 are given by Thalen as of equal intensity, but the three most refrangible ones are longer. On reduction of pressure Loekyer found that some of the shorter lines rapidly decreased in length, while the longer lines remained visible and were some-times hardly affected. When the spark was taken from a metallic salt instead of from the metal the short lines could not be seen, but only the long lines remained. An alloy behaves in the same manner as a compound, and by gradually reducing one constituent of an alloy we may gradually reduce the number of lines, which disappear in the inverse order of their length. Subsequent work has shown that the longest lines are also generally those which are most persistent on reduction of temperature, so that in the voltaic arc the longest lines seen in the spark are absent. In order to explain these facts it seems necessary in the first place to assume that the short lines are lines coming out at a high temperature only ; but this explanation is not sufficient. Why should a mixture of different elements only show the longest lines of that constituent which is present in small quantities ? In the case of chemical combinations we might assume that, the spark having to do the work of decomposition, the temperature of the metal is lowered, and that therefore the short lines are absent. But this cannot be if a chemical compound is replaced by a mechani-cal mixture. All these facts would be explained, however, if we assume that the spectrum of a molecule that is excited by molecules of another kind consists of those lines chiefly which a molecule of the same kind is already capable of bringing out at a lower temperature. It would follow from this that the effects of dilution are the same as those of a reduction of temperature, which is the case.
Other Changes in Relative Intensity of Lines. Besides the changes we have noticed, there are others which have not been brought under any rule as yet. Lines appear sometimes at a low temperature which behave differently from the proper low-tem-perature lines. These require further investigation. They may, in some cases at least, be due to some compound of the metal with other elements present. We give some examples. If a spark is taken from lead without the condenser the line 5005 appears, and Huggins has found it to be sensibly coincident with the chief line of the nebulae. It is given as a strong line by Lecoq de Bois-baudran, who used feeble sparks, and in many cases it seems to behave as a low-temperature line ; it ought to be a long line therefore, but it is in reality short. In line 6100 of tin, Salet noticed that when a hydrogen flame contains a compound of tin an orange line appears, which is apparently coincident with the orange line of lithium. This line does not figure on any of the maps of the tin spectrum. Loekyer found that zinc, volatilized in an iron tube, showed by absorption a green line. It is very likely the line 5184 seen by Lecoq de Boisbaudran in sparks taken from solution of zinc salts. In the absorption spectra of sodium and potassium lines appear in the green which were shown by Liveing and Dewar not to be coincident with any known line of these metals. It was suggested by them that they are due to hydrogen compounds. The wave-length of the sodium line is 5510 and that of the potassium line 5730. Lecoq de Boisbaudran mentions that an increase of temperature is often accompanied by a relatively greater increase in the bril-liancy of the more refrangible rays. It is often said that such an increase is a direct consequence of the formula established by Kirchhoff. If the absorbing power of a molecule remains the same while the temperature is increased, it follows that the blue rays gain more quickly in intensity than the red ones, but the less refrangible rays ought never to decrease in intensity, the quantity of luminous matter remaining the same. Now such a decrease is actually observed in many cases when there is no reason to suppose that the quantity of luminous matter has been reduced. We must conclude, therefore, that the observed differences in the spectra are not solely regulated by Kirchhoff's law ; but it is a perfectly plausible hypothesis that a higher temperature is in general accompanied by a decrease in the absorbing power of the less refrangible rays. As a stronger impact often brings out higher tones, stronger molecular shocks may bring out waves of smaller length. There are several instances of a regular increase in the relative intensity of the blue rays which may be ascribed to this cause. The most remarkable instance is perhaps seen in the spectrum of phosphoretted hydrogen. If a little phosphorus is intro-duced into an apparatus generating hydrogen, the name will show a series of bands chiefly in the green. The spectrum gets more brilliant if the flame is cooled. This can be done, according to Salet, by pressing the flame against a surface kept cool by means of a stream of water or by surrounding the tube, at the orifice of which the gas is lighted, by a wider tube through which cold air is blown. The process of cooling the flame, according to Lecoq, changes the relative intensity of the bands in a perfectly regular manner. The almost invisible least refrangible band becomes strong, and the second band, which was weaker than the fourth, now becomes stronger. Another example of a similar change is the spectrum shown by a Bunsen burner. By charging the burner with an indifferent gas (N, HC1, C02) the flame takes a greenish colour, and, though the spectrum is not altered, the least refran-gible of the bands are increased in intensity. While in these instances the changes are perfectly regular, the more refrangible rays gaining in relative intensity as the temperature is increased, there are other cases, some of which have already been mentioned, in which the changes are very irregular ; such are those which take place in the spectra of tin, lithium, and magnesium. In the case of zinc the less refrangible of the group of blue rays gains in relative intensity. We cannot, therefore, formulate any general law.
Numerical Relations between the Wave-lengths of Lines belonging to the Spectrum of a Body.
It seems a priori probable that there is a numerical relation between the different periods of the same vibrating system. In certain sounding systems, as an organ-pipe or a stretched string, the relation is a simple one, these periods being a submultiple of one which is called the fundamental period. The harmony of a com-pound sound depends on the fact that the different times of vibra-tion are in the ratio of small integer numbers, and hence two vibrations are said to be in harmonic relation when their periods are in the ratio of integers. We may with advantage extend the expression " harmonic relation " to the case of light, although the so-called harmony of colours has nothing to do with such connexions. We shall therefore define an " harmonic relation " between different lines of a spectrum to be a relation such that the wave-lengths or wave-numbers are in the ratio of integers, the integers being suffi-ciently small to suggest a real connexion. Some writers use the word in a wider sense and call a group of lines harmonics when they show a certain regularity in their disposition, giving evidence of some law, that law not being in general the harmonic law. We shall here use the expression in its stricter sense only. We begin by discussing the question whether there are any well-ascertained cases of harmonic relationship between the different vibrations of the same molecule. The most important set of lines exhibiting such a relationship are three of the hydrogen lines which, when pro-perly corrected for atmospheric refraction, are, as pointed out by Johnstone Stoney, very accurately in the ratio of 20 :27 : 32 (Phil. Mag., xli. p. 291, 1871). Other elements also show such ratios ; but when a spectrum has many lines pure accident will cause several to exhibit whatever numerical relations we may wish to impose on them. If we calculate the number of harmonic ratios which, with an assumed limit of accuracy, we may expect in a spectrum like that of iron, we find that there are in reality fewer than we should have if they were distributed quite at random (Proc. Roy. Soc, xxxi. p. 337, 1881). With fractions having a denominator smaller than seventy the excess of the calculated over the observed values is very marked, while there are rather more coincidences than we should expect on the theory of probability if we take fractions having a denominator between seventy and a hundred. The cause of this, probably, is to be sought in the fact that the lines of an element are liable to form groups and are not spread over the whole spectrum, as they would be if they were distributed at random. This increases the probability of coincidence with fractions between high numbers, and diminishes the probability of coincidence with fractions between lower numbers. There is one point which deserves renewed investigation. When the limits of agreement between which a coincidence is assumed to exist are taken narrower, there is an increased number of observed as compared with calculated coincidences in the iron spectrum ; and this would seem to point to the existence of some true harmonic ratios. With the solar maps and gratings put at our disposal by Professor Rowland, we may hope to obtain more accurate measurements, and therefore more definite information. Even if the wave-lengths of two lines are found to be occasionally in the ratio of small integer numbers, it does not follow that the vibrations of molecules are regulated by the same laws as those of an organ-pipe or of a stretched string. E. J. Balmer has indeed lately suggested a law which differs in an important manner from the laws of vibration of the organ-pipe and which still leaves the ratios of the periods of vibration integer numbers. According to him, the hydrogen spectrum can be represented by the equation
== IMAGE ==
where X0 is some wave-length and m an integer number greater than 2. The following table (I.) shows the agreement between the calculated and observed hydrogen lines. And the agreement is a very remarkable one, for the whole of the hydrogen spectrum is represented by giving to m successive integer values up to sixteen.
== TABLE ==
The differences between the observed and the calculated numbers show a regular increase towards the ultra-violet. It might be thought that a better agreement could be obtained by taking a number slightly different from four in the denominator ; but this is not the case. On the contrary, the agreement in the visible part is at once destroyed if we make the ultra-violet lines fit better. The agreement is not improved but rendered slightly worse if we take account of atmospheric refraction.
As a first approximation Balmer's expression gives a very good account of the hydrogen spectrum. If the law was general we should find that in the iron spectrum, for instance, which is the only spectrum carefully examined, those fractions would occur more frequently than others which can be put into the form m-j(m ?i ), that is to say, f and f for fractions made up of numbers smaller than 10. A reference to the table in Proc. Roy. Soc, vol. xxxi. p. 337, shows that those fractions do not occur more frequently than others. But, if we change the sign of n- in the denominator, we find | and ^ as the only fractions falling within the range of spectrum examined, and these two fractions are indeed those which occur more frequently than any others made up of numbers smaller than 10.
It might be worth trying to see whether the wave-lengths of lines making up a fluted band can be put into the form maj.Ma \> > according to the sign chosen in the denominator, the band would shade off towards the blue or red. The form of expression seems at first sight well adapted, for it shows how by giving m gradually in-creasing numbers the lines come closer and closer together towards what appears in the spectrum as the sharp edge of the band. If we take periods of vibration instead of wave-lengths Balmer's expression would reduce to
== IMAGE ==
where T0 is a fixed period of vibration, n a constant integer, and m an integer to which successive values are given from n upwards.
It is often observed, and has already been mentioned, that the spectrum of some elements contains in close proximity two or three lines forming a characteristic group. Such doublets or triplets are often repeated, and if the harmonic law was a general one we should expect the wave-lengths of these groups to be ruled by it; but such is not the ease. The sodium lines which lie in the visible part of the spectrum are all double, the components being the closer to-gether the more refrangible the group. But neither are the lines themselves in any simple ratios of integers, nor do the distances between the lines show much regularity. The ultra-violet lines of sodium as photographed by Liveing and Dewar are single, with the exception of the least refrangible of them (3301). But this line is a very close double, and it may be that the others will ultimately be resolved. Some elements, such as magnesium, calcium, zinc, cadmium, show remarkable series of triplets; and the relative dis-tances of the three lines seem well maintained in each of them. Even the distances when mapped on the wave-number scale are so nearly the same for each element that it would be a matter of great importance to settle definitively whether the slight variations which are found to exist are real or due to errors of measurement. In the following table (II.) we give the position of the least refrangible line of each triplet together with the distances between the first and second (column B) and between the second and third line of each triplet (column C). The figures in column A represent the number of waves in one millimetre. For the zinc and calcium triplets the measurements of Liveing and Dewar are given; the magnesium triplets are put down as measured by Cornu as well as by Hartley and Adeney. The differences in these measurements will give an idea of the degree of uncertainty. The triplets of cadmium are farther apart and are mixed up with a greater number of single lines.
== TABLE ==
Relation between Spectrum of a Body and Spectra of its Compounds.
The spectrum of a body is due to periodic motion within the molecules. If we are justified in believing that the molecule of mercury vapour contains a single atom, it follows that atoms are capable of vibration under the action of internal forces, for mercury vapour has a definite spec-trum. We may consider, then, the spectrum to be de-termined in the first place by forces within the atom, but to be affected by the forces which hold together the different atoms within the molecule. The closer the bond of union the greater the dependence of the vibrations on the forces acting between the different atoms. Experimental evidence seems to favour these views, for we observe that whenever elements are loosely bound together we can recognize the influence of each constituent, while in the compounds which are sufficiently stable to resist the temperature of incandes-cence the spectrum of the compound is perfectly distinct from the spectra of the elements. The oxides and haloid salts of the alkaline earths, for instance, have spectra in which we cannot trace the vibrations of the component atoms ; but the spectra of the different salts of the same metal show a great resemblance, the bands being similar and similarly placed. The spectrum seems displaced towards the red as the atomic weight of the haloid increases. No satisfactory numerical relationship has, however, been traced between the bands. The number of compounds which will endure incandescence without decomposition is very small, and this renders an exhaustive investigation of the relationship between their spectra very difficult.
The compounds whose absorption spectra have been investigated have often been of a more unstable nature, and, moreover, dissociation seems going on in liquid solutions to a large extent ; the influence of the component radicals in the molecule is more marked in consequence. Dr Gladstone, at an early period in the history of spectrum analysis, examined the absorption spectra of the solu-tion of salts, each constituent of which was coloured. He concluded that generally, but not invariably, the following law held good : " When an acid and a base combine each of which has a different influence on the rays of light a solution of the resulting salt will transmit only those rays which are not absorbed by either, or, in other words, which are transmitted by both." He mentions as an important exception the case of ferric ferro-cyanide, which, when dissolved in oxalic acid, transmits blue rays in great abundance, though the same rays are absorbed both by ferro-cyanides and by ferric salts. Soret has confirmed, for the ultra-violet rays, Dr Gladstone's conclusions with regard to the identity of the absorption spectra of different chromâtes. The chromâtes of sodium, potassium, and ammonia, as well as the bichromates of potassium and ammonia, were found to give the same absorption spectrum. Nor is the effect of these chromâtes confined to the blocking out simply of one end of the spectrum, as in the visible part, but two distinct absorption bands are seen, which seem unchanged in position if one of the above-mentioned chromâtes is replaced by another. Chromic acid itself showed the bands, but less distinctly, and Soret does not consider the purity of the acid sufficiently proved to allow him to draw any certain conclusion from this observation. Erhard's work on the absorption spectra of the salts in which chromium plays the part of base has already been mentioned. Nitric acid and the nitrates of transparent bases, such as potassium, sodium, and ammonia, show spectra, according to Soret, which are not only qualitatively but also quantitatively identical ; that is to say, a given quantity of nitric acid in solution gives a characteristic absorption band of exactly the same width and darkness, whether by itself alone or combined with a transparent base. It also shows a continuous absorption at the most refrangible side, beginning with each of the salts mentioned at exactly the same point. The ethereal nitrates, however, give different results. In 1872 Hartley and Huntington examined by photographic methods the absorption spectra of a great number of organic compounds. The normal alcohols were found to be transparent to the ultra-violet rays, the normal fatty acids less so. In both cases an increased number of carbon atoms increases the absorption at the most refrangible end. The fact that benzene and its derivatives are remarkable for their powerful absorption of the most refrangible rays, and for some characteristic absorption bands appearing on dilution, led Hartley to a more extended examination of some of the more complicated organic substances. He determined that definite absorption bands are only produced by substances in which three pairs of carbon atoms are doubly linked together, as in the benzene ring. More recently he has subjected the ultra-violet absorption of the alkaloids to a careful investigation, and has arrived at the conclusion that the spectra are sufficiently characteristic to "offer a ready and valuable means of ascertaining the purity of the alkaloids and particularly of establishing their identity. " " In comparing the spectra of substances of similar constitution it is observed that in such as are derived from bases by the substitution of an alkyl radical for hydrogen, or of an acid radical for hydroxyl, the curve is not altered in character, but may vary in length when equal weights are examined. This is explained by the absorption bands being caused by the compactness of structure of the nucleus of the molecule, and that equal weights are not molecular weights, so that by substituting for the hydrogen of the nucleus radicals which exert no selective absorption the result is a reduction in the ab-sorptive power of a given weight of the substance. . . . Bases which contain oxydized radicals, as hydroxyl, methoxyl, and carboxyl, increase in absorptive, power in proportion to the amount of oxygen they contain."
It would seem, however, by comparing the above results with those obtained by Captain Abney and Colonel Festing that the absorption of a great number of organic substances is more char-acteristic in the infra-red than in the ultra-violet. Some of the conclusions arrived at by these experimentalists are of great im-portance, as the following quotations will show :"Regarding the general absorption we have nothing very noteworthy to remark, beyond the fact that, as a rule, in the hydrocarbons of the same series those of heavier molecular constitution seem to have less than those of lighter." This effect agrees with the observations made by Hartley and Huntington in the ultra-violet, in so far as a general shifting of the absorption towards the red seems to take place as the number of carbon atoms is increased. Such a shifting would increase the general absorption in the ultra-violet as observed by Hartley and Huntington, and decrease it in the infra-red as observed by Abney and Festing. Turning their atten-tion next to the sharply defined lines, the last named, by a series of systematic experiments, concluded that these must be due to the hydrogen atoms in the molecule. "A crucial test was to observe spectra containing hydrogen and chlorine, hydrogen and oxygen, and hydrogen and nitrogen. We therefore tried hydrochloric acid and obtained a spectrum containing some few lines. Water gave lines, together with bands, two lines being coincident with those in the spectrum of hydrochloric acid. In ammonia, nitric acid, and sulphuric acid we also obtained sharply marked lines, coincidences in the different spectra being observed, and nearly every line mapped found its analogue in the chloroform spectrum, and usually in that of ethyl iodide. Benzene, again, gave a spectrum consisting prin-cipally of lines, and these were coincident with some lines also to be found in chloroform. It seems, then, that the hydrogen, which is common to all these different compounds, must be the cause of the linear spectrum. In what manner the hydrogen annihilates the waves of radiation at these particular points is a question which is, at present at all events, an open one, but, that the linear absorp-tions, common to the hydrocarbons and to those bodies in which hydrogen is in combination with other elements, such as oxygen and nitrogen, are due to hydrogen, there can be no manner of doubt. The next point that required solution was the effect of the presence of oxygen on the body under examination. ... It appears that in every case where oxygen is present, otherwise than as a part of the radical, it is attached to some hydrogen atom in such a way that it obliterates the radiation between two of the lines which are due to that hydrogen. ... If more than one hydroxyl group be present, we doubt if any direct effect is produced beyond that produced by one hydroxyl group, except a possible greater general absorption; a good example of this will be found in cinnamic alcohol and phenyl-propyl alcohol, which give the same spectra as far as the special absorptions are concerned. . . . Hitherto we have only taken into account oxygen which is not contained in the radical; when it is so contained it appears to act differently, always supposing hydrogen to be present as well. We need only refer to the spectrum of aldehyde, which is inclined to be linear rather than banded, or rather the bands are bounded by absolute lines, and are more defined than when oxygen is more loosely bonded."
"An inspection of our maps will show that the radical of a body is represented by certain well-marked bands, some differing in position according as it is bonded with hydrogen, or a halogen, or with carbon, oxygen, or nitrogen. There seem to be characteristic bands, however, of any one series of radicals between 1000 and about 1100, which would indicate what may be called the central hydrocarbon group, to which other radicals may be bonded. The clue to the composition of a body, however, would seem to lie between X 700 and X 1000. Certain radicals have a distinctive absorption about X 700 together with others about X 900, and if the first be visible it almost follows that the distinctive mark of the radical with which it is connected will be found. Thus in the ethyl series we find an absorption at 740, and a characteristic band, one edge of which is at 892 and the other at 920. If we find a body containing the 740 absorption and a band with the most refrangible edge commencing at 892, or with the least refrangible edge terminating at 920, we may be pretty sure that we have an ethyl radical present. So with any of the aromatic group ; the crucial line is at 867. If that line be connected with a band we may feel certain that some derivative of benzine is present. The benzyl group show this remarkably well, since we see that phenyl is present, as is also methyl. It will be advantageous if the spectra of ammonia, benzine, aniline, and dimethyl aniline be com-pared, when the remarkable coincidences will at once become apparent, as also the different weighting of the molecule. The spectrum of nitro-benzine is also worth comparing with benzine and nitric acid. ... In our own minds there lingers no doubt as to the easy detection of any radical which we have examined, . . . and it seems highly probable by this delicate mode of analysis that the hypothetical position of any hydrogen which is replaced may be identified, a point which is of prime importance in organic chemistry. The detection of the presence of chlorine or bromine or iodine in a compound is at present undecided, and it may well be that we may have to look for its effects in a different part of the spectrum. The only trace we can find at present is in ethyl bromide, in which the radical band about 900 is curtailed in one wing. The difference between amyl iodide and amyl bromide is not sufficiently marked to be of any value."
The absorption spectra of the didymium and cobalt salts afford many striking examples of the complicated effects of solution and combination in the spectra. It is impossible to explain these with-out the help of illustrations, and we must refer the reader, therefore, to the original papers. Some very interesting changes have been noticed in the position of absorption bands when certain colouring matters are dissolved in different liquids. Characteristic absorp-tion bands appear for each colouring matter in slightly different positions according to the solvent. Hagenbach, Kraus, Kundt, and Claes have studied the question. In a preliminary examina-tion Professor Kundt had come to the conclusion that solvents displaced absorption bands towards the red in the order of their dispersive powers; but the examination of a greater number of cases has led him to recognize that no generally valid rule can be laid down. At the same time highly dispersive media, like bisul-phide of carbon, always displace a band most towards the red end, while with liquids of small dispersion, like water, alcohol, and ether, the band always appears more refrangible than with other solvents ; and as a general rule the order of displacement is approximately that of dispersive power.
Relations of the Spectra of Different Elements.
Various efforts have been made to connect together the spectra of different elements. In these attempts it is generally assumed that certain lines in one spectrum corre-spond to certain lines in another spectrum, and the ques-tion is raised whether the atom with the higher atomic weight has its corresponding lines more or less refrangible.
No definite judgment can as yet be given as to the success of these efforts. Lecoq de Boisbaudran has led the way in these speculations, and some of the similarities in different spectra pointed out by him are certainly of value. But whether his conclusion, that "the spectra of the alkalis and alkaline earths when classed according to their refran-gibilities are placed as their chemical properties in the order of their atomic weight," will stand the test of further research remains to be seen. Ciamician has also published a number of suggestive speculations on the question, and Hartley has extended the comparison to the ultra-violet rays.
When metallic spectra are examined it is often found
that some line appears to belong to more than one metal.
This is often due to a common impurity of the metals;
but such impurities do not account for all coincidences.
The question has been raised whether these coincidences
do not point to a common constituent in the different
elements which show the same line. If this view is correct,
we should have to assume that the electric spark decom-
poses the metals, and that the spectrum we observe is
not the spectrum of the metal but that of its constituents.
Further investigation has shown, however, that in nearly
all cases the assumed coincidences were apparent only.
With higher resolving powers it was found that the lines
did not occupy exactly the same place. With the large
numbers of lines shown by the spectra of most of the
metals some very close coincidences must be expected by
the doctrine of chances. The few coincidences which our
most powerful spectroscopes have not been able to resolve
are in all probability accidental only. (A. S*.)
375-1 Phil. Trans., clxxv. p. 49 (1884).
375-2 We may refer for all to Watts, Index of Spectra, for a list of wave-lengths of the different spectra.
378-1 Phil. Trans., clxiii. p. 253 (1873).
379-1 Ann. Chim. Phys., xxviii. p. 57 (1873).
379-2 Spectre Lumineux, p. 188 (1874).
379-3 Op. cit., p. 43 (1874).
379-4 Wied. Ann., xxv. p. 80 (1885).
380-1 Measured by Thalén.
380-2 Measured by Liveing and Dewar.
380-3 Phil. Mag., xiv. p. 418 (1857).
380-4 Phil. Trans., part ii. (1885)
380-5 Phil. Trans., iii. p. 887 (1881).
381-1 Bunsen, "On the Inversion of the Bands in the Didymium Absorption Spectra,' Phil. Mag., xxviii. p. 246 (1864), and xxxii. p. 177 (1866); Russell, "On the Absorption Spectra of Cobalt Salts," Proc. Roy. Soc., xxxii. p. 258 (1881).
381-2 Wied. Ann., iv. p. 34 (1878).
381-3 Wied. Ann., iii. p. 389 (1878).
381-4 Wien. Ber., lxxviii. (1878).
381-5 Journal Chem. Soc., September 1883.
The above article was written by: Arthur Schuster, Ph.D., F.R.S., Professor of Applied Mathematics, Owens College, Manchester.