MM. Plücker and Hittorf, in recent experiments, proved that many non-metallic bodies, such as nitrogen and sulphur, give two distinctly different spectra on change of temperature, and that the transition from one spectrum to the other is sudden. The change is particularly striking in sulphur, for at the moment the first spectrum attains its maximum brightness, it disappears, and gives place to the second or high temperature spectrum, which is one of the richest in brilliant rays known. When the temperature is lowered the first spectrum reappears. These changes M. Plücker ascribes to the existence of the elements in two allotropic conditions. M. Plücker has also found that each metalloid possesses a peculiar and characteristic spectrum: as hydrogen, which has three bright lines, all of which are coincident with dark solar lines, and nitrogen, which exhibits a complicated series of bands.
The experiments of the Rev. Dr. Robinson on a variety of gases and vapours, inclosed in glass tubes, show that a greater change is produced by pressure than by heat. At the ordinary atmospheric pressure, the spectra show a number of bright lines on a coloured ground, the light of which is, in general, stronger towards the red than the violet end, and strongest in the green. In some the ground is so bright as to efface all but the most luminous lines. This is especially the case with hydrogen. On gradually exhausting the tube in which the vapour is contained, the spectra rather suddenly fade away, leaving only a suspicion of one or two lines, but upon exhausting the tube still more, these transition spectra become bright again, fresh lines appear, and they are changed into new spectra which are never so bright as those at ordinary pressure. Fewer lines are visible in the rarefied spectra, and of these four-tenths are not found in the spectra of atmospheric pressure. The difference between the common pressure spectra, the transition, and the rarefied spectra shows, that the character and even the existence of certain lines depend upon the mere density of the media, the chemical circumstances remaining unchanged. Dr. Robinson also observed that spectra are not superposed without a change; the spectrum of atmospheric air does not always exhibit all the lines of oxygen and nitrogen, and occasionally there are some lines not visible in either of them. It appears also that for certain lines the actions of bodies may be antagonistic.
Metals do not always give the same spectrum, whatever may be the combinations in which they are found. Among various instances M. Mitscherlich mentions that the spectra of copper and the chloride and iodide of copper present essential differences, and Mr. Roscoe has found that a similar difference prevails in the spectra of carbon compounds when in a state of incandescent gas, which have hitherto been supposed to yield the same spectrum. ‘The spectrum obtained from the flame of olefiant gas is different from that obtained by the electric discharge through a vacuum of the same gas; while the spark passing through a cyanogen vacuum produces a spectrum identical with that of the olefiant gas flame, and through the carbonic oxide vacuum a spectrum coincident with that of the spark through olefiant gas vacuum.’
The chlorides, bromides, and iodides are the most easily vaporized of all the metallic salts, and give the most brilliant flames and the most intense spectra, especially the chlorides. A small piece of the chloride of barium volatilized by a colourless gas flame tinges the flame green, and the red and green lines on the spectrum stand out with extreme brilliancy. The scattered yellow light on the spectrum of the chloride of sodium is comparatively dark by contrast with the bright lines, and upon shading off the more luminous part of it, traces of lines are visible in the more refrangible portion.
Chloride of lithium gives the red and orange lines on its spectrum; the brilliant blue band discovered by Mr. Tyndall, and another more refrangible blue line is seen when the ignition is at its greatest intensity. Chloride of calcium gives a blue band very brightly, and several other lines. The light of the chloride of copper is very vivid, and its spectrum is remarkable for changing its appearance with the decomposition of the chloride. The chlorides of lead and cadmium, also, give very bright and definite spectra, and chloride of bismuth shows numerous brilliant red and blue rays which quickly disappear. Thus the chlorides give spectra with lines, such as the blue lithium and strontium lines, hitherto only brought out by an intense electric spark.[[19]]
M. Bunsen produced a beautiful effect by vaporizing a mixture of equal parts of the chlorides of sodium, potassium, lithium, calcium, strontium, and barium, and passing the light through the slit of his apparatus. For on looking through the telescope the spectrum of each substance with its characteristic coloured lines in all their brilliancy came successively into view, and gradually faded away as each substance was volatilized and driven off. The sequence showed the time required to vaporize each metal, and by spectrum analysis each metal could be recognized, although the mixture only contained the 1⁄1000 part of a grain of each chloride.
The position, colour, and nature of the bright lines on the spectra of more than thirty metals have been determined, besides those of the elementary gases and that of the electric spark. To these M. Louis Grandeau has added the spectrum of lightning. By a particular arrangement the light passed at once through the slit in the instrument, and a glass tube containing nitrogen and the vapour of water. The general appearance of the lightning spectrum at first recalled that of the electric spark, but on a closer examination, M. Grandeau noticed in the spectrum of almost every flash the coincidence of a certain number of the rays of the lightning spectrum with those of the spectra of nitrogen and hydrogen. M. Grandeau remarks that this result is not surprising, since all admit the production of ammonia and nitric acid under the influence of electrical discharges. Besides the rays of nitrogen and hydrogen, the lightning spectrum contains the ubiquitous yellow ray of sodium.
Fraunhofer had noticed a coincidence between the double yellow sodium line and the double dark line D of the solar spectrum, though he was not aware to what it was due. This coincidence, observed by M. Kirchhoff many years afterwards, was fully appreciated by him, and became the foundation of one of the most brilliant discoveries of modern times. During a systematic comparison between the spectra of volatilized substances and the solar spectrum, he discovered a perfect coincidence between Fraunhofer’s dark lines and all the bright and coloured lines on the spectra of the volatilized substances, sodium, calcium, magnesium, chromium, iron, and nickel. To these M. Angström has added aluminium and manganese, and M. Plücker has very recently found that all the three bright lines in the hydrogen spectrum are coincident with dark solar lines, and that none of the potassium lines correspond with any solar lines.
Drawings have been made of Fraunhofer’s spectrum placed above the spectra of the principal metals and metallic salts, in which the coincidence of the bright and dark lines is shown from the line A in the extreme red to the line G in the indigo, and as the length of an undulation of the extreme violet light of the solar spectrum is the 17⁄1,000,000 of an inch, and the length of an undulation of the extreme red is the 26⁄1,000,000 of an inch, the length of the undulations of the intermediate rays can be computed by the undulatory theory of light. The length of the waves corresponding to Fraunhofer’s seven principal lines and many of the intermediate ones have been computed, so that when a bright or coloured line is coincident with any of these, the length of its waves is at once known. There are other tables of Fraunhofer’s lines, and the coincident bright ones in which each dark line is marked by its own number, as the two principal lines in the double line D, which are expressed by the numbers 1002·8 and 1006·8, and so with the others; thus the coincidence of the spectra of volatilized substances with the solar line forms a regular system.
Professor J. P. Cooke, junior, has recently constructed a spectroscope which shows that the lines of the solar spectrum are as innumerable as the stars of heaven, that at least ten times as many are distinctly seen as are given by Kirchhoff in his chart, besides an infinitude of nebulous bands just on the point of being resolved. Yet even with this greatly increased power, the coincidences between the bright lines of the metallic spectra and the dark lines of the solar spectrum remain perfect. M. Kirchhoff had seen a fine yellow line between the double lines D of the sodium spectrum. M. Merz of Munich found four additional lines, but Professor Cooke has discovered that there are in all seven intermediate lines and a nebulous band. Although the two members of the sodium line D could be spread so far apart that the 1⁄2000 part of the intermediate space could be readily distinguished, yet the coincidence with the two dark Fraunhofer lines was absolute. The spectroscope ‘shows that many of the bands of the metallic spectra are broad coloured spaces crossed themselves by bright lines. This is the case with the orange band of the strontium spectrum, and with the whole of the calcium and barium spectra to a remarkable extent.’[[20]]