To the unrivalled genius of Sir Isaac Newton we owe the solar spectrum, and the laws of coloured rings, by aid of which, Dr. Thomas Young proved and established the undulatory theory which forms the basis of the whole science of light. The visible part of the solar spectrum forming a band of seven colours was supposed to be continuous till the year 1802, when Dr. Wollaston looking with a prism whose axis was parallel to a narrow slit in a window shutter, at a sunbeam passing through it, discovered seven dark lines crossing the coloured band, at right angles to its length.

Twelve years afterwards, Fraunhofer of Munich, a celebrated optician, magnified the spectrum of a vertical line of light passing through an upright prism by receiving it upon the object glass of a telescope, and discovered 600 dark lines. Having ascertained that the position of the lines in the spectrum, and their distances from one another, are invariable under every circumstance, he determined their places accurately and drew the diagram known as Fraunhofer’s lines, which is universally referred to as a standard of comparison. For that purpose, the principal lines are designated by letters; thus the dark line A is in the red near the least refrangible end of the spectrum, B and C are in the orange, the very remarkable double line D is in the yellow, b and E are in the green, F is at the limit between the green and the blue, G is in the blue, and the double line H is in the violet.

The instrument used by MM. Bunsen and Kirchhoff, though more complicated, is constructed on the same principle as the preceding. A sunbeam transmitted by a very narrow vertical slit passes through four prisms, which disperse it so much, that if drawn on the scale seen with the magnifying telescope which receives it, the spectrum would extend over twenty feet. By means of a micrometer screw, the telescope can be turned round a vertical axis, and as the dark lines come successively under the cross wires in its eye-glass they are seen to pass over a graduated scale, so that the distances between two thousand of them have been measured in millimetres with unerring accuracy, but that is only a small part of the whole. When viewed through the telescope, the retina of the eye is the screen on which this wonderful spectrum falls, crossed by innumerable dark rayless lines of various breadths and intensities. Black bands given by the inferior refraction of one prism are here resolved into numerous dark lines as fine as a spider’s thread.

Mr. Glaisher during his tenth scientific balloon ascent devoted his attention for a time almost entirely to the dark lines on the solar spectrum. At a height of about four miles and a half, they were almost innumerable; all he had seen on the earth were there, and many more. The nebulous lines H were both seen, the spectrum was a good deal lengthened at the violet end, and at the red end the line A was visible. The light from the sky near the sun gave a shorter spectrum; the lines were only visible from B to G.

Besides these cosmical or permanent lines, Sir David Brewster observed that certain dark bands and lines in the red and green parts of the spectrum are only visible when the sun is near the horizon, whence he concluded that they are occasioned by the absorption of the solar light while traversing a thicker stratum of air than when the sun is in the zenith. Various groups of these absorption bands are to be seen at times on the solar spectrum, especially a remarkable one near Fraunhofer’s line D, and Dr. Miller observed that temporary dark lines appeared during a heavy shower, which vanished when the rain ceased.

When the sun was high, M. Kirchhoff mentions that he had noticed traces of lines and nebulous bands in different parts of the spectrum, which he thinks might be resolved by a greater number of prisms than those in his apparatus.

Sir David Brewster was led to his discovery of atmospheric bands by observing that the brownish red vapour of nitrous oxide has the property of absorbing solar light, resolving the spectrum into a series of bright and dark bands, alternating. Professors Daniel and Miller found that bromine, iodine, and chlorous acid do the same, and Sir John Herschel observed a multitude of similar bands in the flame of cyanogen; but Dr. W. A. Miller, who has particularly studied the phenomena of absorption bands, has proved that the colour of a vapour does not necessarily determine the position or even the existence of dark bands. He has shown that some simple substances which do not occasion dark bands produce them abundantly by the absorptive power they acquire when in composition, while lines that are produced by a simple vapour, vanish when it is in combination. Dr. W. A. Miller has proved also that none of the preceding vapours exist in the atmosphere. He computed that if free bromine constituted only one in a thousand million parts of atmospheric air, it would betray its presence by absorptive bands; nevertheless he suspects that there may be some substance in the air that occasions certain unaccountable changes. Possibly ozone, so intimately connected with atmospheric electricity, may produce some unknown effect.

The spectra from glowing solids and liquids, such as Drummond’s light, which is incandescent lime, the still more brilliant flame of the electric arc between charcoal points, glowing solid and fused metals, and coal-gas flame, are continuous; the spectra exhibit the seven colours, but they are not crossed by dark rayless lines, because such incandescent substances give off light of all refrangibility. But solids and liquids reduced to glowing vapours, and incandescent gases, only give out rays of certain refrangibilities, which cross their spectra at right angles, as bright lines of various colours and intensities. Each glowing vapour and gas has bright lines on its spectrum peculiar to itself.

In order to compare these bright lines with Fraunhofer’s dark lines, solar light is transmitted through one half of the vertical slit in Kirchhoff’s apparatus, and the light of the luminous vapour or gas through the other half. Then by prismatic refraction two spectra are seen in looking through the telescope, the gaseous one immediately below the solar one, and only divided from it by an almost imperceptible dark line. So that the bright lines appear to be continuations of the dark lines if they occupy the same position in the two spectra; if not, the deviation is at once visible. The coincidence or deviation of the bright lines on the spectra of two volatilized substances may be determined by the same method.

The coloured light that has so beautiful an effect in fire-works is owing to the combustion of the salts of different metals: as soda, or common salt, which gives a perfectly pure homogeneous yellow; potash gives a violet light, strontia red, baryta green. The colour is given out by the glowing atoms of the vaporized metals sodium, potassium, strontium, and barium in a state of violent ignition; for as the salt and its metal give the same colour and the same spectrum when ignited, it is evident that the colour is independent of the oxygen of the alkali.

Sir David Brewster appears to have been the first who analysed coloured light with a prism; and in 1822 Sir John Herschel, besides having made a series of observations on coloured flames, had determined the spectra of the muriates of strontia and lime, the chlorides and nitrate of copper and boracic acid; and observes that ‘the colours thus communicated by different bases to flame afford, in many cases, a ready and neat way of detecting extremely minute quantities of them.’^[15]

The same opinion was afterwards formed by Mr. Fox Talbot, who after many experiments on metallic salts, says in his paper,^[16] that a glance at the prismatic spectrum of a flame may show it to contain substances which it would otherwise require a laborious chemical analysis to effect. In that paper this gentleman noticed that the glowing salts of lithium and strontium give a crimson or red colour to flame so exactly of the same tint that if these metals were in combination it would be impossible to decide to which metal the colour is due. But when he passed their respective lights through a prism, he found that the bright lines on their spectra are entirely different. ‘The strontia flame,’ he observes, ‘exhibits a great number of red rays well separated from each other by dark intervals, not to mention an orange, and a very definite bright blue ray. The lithia exhibits one single red ray,’ Whence Mr. Fox Talbot observes, ‘I hesitate not to say that optical analysis can distinguish the minutest portions of these two substances from each other with as much certainty, if not more, than any other known method.’ Thus Sir John Herschel and Mr. Fox Talbot laid the foundation of a spectrum analysis of unrivalled delicacy and beauty, since carried to perfection by Messrs. Bunsen, Kirchhoff and other experimenters, presently to be mentioned.

M. Bunsen detected the characteristic crimson lithium line in the spectra of numerous substances; in granite, in the earliest geological strata, in meteoric stones, in the ashes of most land plants, in blood and other animal matter; so that instead of being one of the rarest metals, it exists in all the three kingdoms of nature. In the year 1857 Mr. Swan gave an instance of the extreme minuteness of spectrum analysis, by detecting the ¹⁄_2,300,000th part of a grain of salt by its yellow light; but by the same reaction M. Bunsen not only recognised the 180 millionth part of a grain of sodium, but found that there is hardly any substance that does not contain it. It exists in the dust on our clothes and furniture, particles of it float in the air we breathe, so that while examining the spectra of other incandescent substances, flashes of yellow light appear as these atoms are volatilized and instantly burnt up, which shows that common salt is perhaps more universally diffused than any other kind of matter.

By spectrum analysis, M. Bunsen has discovered the two new metals, rubidium and cæsium. While examining with a prism the spectrum of the hundredth part of a grain of an alkaline substance separated from the residuum of the Durckheim mineral water, he saw coloured lines, which he had never seen before on the spectrum of any other alkali, and at once concluded that they belonged to a new metal; and having obtained about 200 grains of the substance by the evaporation of forty tons of the water, he found that they contained the chlorides of the two new metals in question. Moreover he perceived that these metallic chlorides resemble the chloride of potassium so nearly in spectrum and chemical character, that a refined prismatic analysis could alone determine the difference. He thus ascertained that the spectra of all the three have two red lines in the red part of their spectrum, and two violet lines in the indigo, while the middle part is occupied by a continuous diffused light. The only difference is that the two red lines in the rubidium spectrum are less refrangible than the red lines in the potassium spectrum, and that the cæsium spectrum is distinguished by two bright blue lines in the diffuse middle part. Rubidium received its name from rubidus, on account of the dark red of its lines, and cæsium from its sky-coloured blue lines.

M. Bunsen thinks that there can hardly be a doubt of rubidium having been mistaken for potassium, but he has shown that they may be distinguished by the difference in the solubility of the double salts which the chlorides of these two metals form with the chloride of platinum. An aqueous solution of the bichloride of platinum and potassium gives an insoluble yellow precipitate, consisting of the bichlorides of platinum and potassium. An aqueous solution of the bichlorides of platinum and rubidium gives an insoluble yellow precipitate of the bichlorides of platinum and rubidium. These two precipitates are undistinguishable to the eye. Now if a solution of platinum be added to the first, no further precipitate can take place, but if a solution of rubidium be added to it, a yellow precipitate is formed consisting of the bichloride of rubidium and potassium, because the chloride of rubidium resolves the precipitate, combines with the chloride of potassium, and sets the chloride of platinum free. Thus the precipitate of the bichloride of rubidium and potassium is the least soluble of the two. The yellow colour is evidently due to the potassium. Cæsium may be distinguished from potassium by the same process. The carbonates, hydrates, and other salts of the two metals were determined; their carbonates were shown to be readily separated, because the carbonate of cæsium is soluble in alcohol, which the carbonate of rubidium is not, and finally the metal rubidium was separated. It has an extreme avidity for oxygen, and burns in water like potassium, and possesses many other analogous qualities. It melts at the temperature of 38·5° Cent., and has a specific gravity of 1·516. Rubidium is abundant in the mineral lepidolite in many parts of Europe and North America, and M. Grandeau has detected it in the ashes of beetroot, tobacco, coffee, tea, and grapes by spectrum analysis. It exists in various mineral waters, and in fact is very general. Traces of various metals are met with in the same vegetable; thus the spectrum of tobacco gives lines characteristic of lithium, potassium, rubidium, and lime.

Mr. W. Crookes discovered the new metal thallium by means of its spectrum, which differs from every other in having one bright green line upon a dark ground. He obtained its various salts, and the metal itself, which he describes as being heavy, dense, and very like lead, but of greater specific gravity. Its fresh surface has a bright metallic lustre, not so blue as that of lead, but it tarnishes more easily. It is so soft that it can be indented by the nail, yet it can be drawn into wire, and in chemical properties it resembles mercury, lead, and bismuth. Altogether it is more like a metal than a metalloid, perhaps something between the two. Thallium is completely volatilized at a temperature below red heat, whether single or in composition. If the quantity be small, the green line appears in a sudden flash, lasting but the fraction of a second. If a larger quantity of the metal be gradually put into the flame, it lasts a little longer, appearing as a single green line of extraordinary purity and intensity, sharply defined on a black ground. With respect to volatility, thallium is analogous to the non-metallic element selenium, which is so volatile that its beautiful blue light only lasts a few seconds. The green light of thallium comes out more rapidly, and with less of the substance, than the blue light of selenium, a quantitative distinction which accords with Dr. Miller’s observation that the rapidity with which a result is obtained, and the minuteness of the quantity required for the examination, gives this method a superiority over every other for the qualitative analysis of the alkalies and alkaline earths. Thallium has been detected in mineral waters, wine, treacle, tobacco, and chicory.

Drs. Reich and Richter discovered a fourth new metal in the zinc-blende at Freiberg in Saxony, which has been called indium, from two beautiful indigo-blue lines in its spectrum, which have a greater refrangibility than the blue lines in strontium. The chemical relations of indium resemble those of zinc, with which it is associated in nature. The metal can be reduced before the blowpipe into a bead, which marks paper and has the colour of tin.

The practical importance of spectrum science has been beautifully illustrated by Professor Roscoe by its application to overcome a difficulty in Bessemer’s process for the manufacture of steel. According to that process, steel is made by sending a blast of air through a quantity of melted iron; the difficulty was when to stop the blast, for if stopped too soon, the metal retains so much carbon that it crumbles under the hammer; if continued a few minutes too long, the molten metal is so viscid that it cannot be poured into the moulds. Experience had hitherto enabled the manufacturer to judge of the right time from the appearance of the flame which issued from the mouth of the converting vessel, but now Professor Roscoe has determined the exact moment for cutting off the blast by a spectral examination of the flame, the light of which is most intense. The flame spectrum in its various phases revealed complicated masses of dark absorption bands and bright lines, showing that a variety of substances were present in the flame in a state of incandescent gas; and by a simultaneous comparison of these with well-known spectra of certain elementary bodies, Mr. Roscoe ascertained the presence of sodium, potassium, lithium, iron, carbon, phosphorus, hydrogen, and nitrogen in the flame.

Both Dr. Wollaston and Fraunhofer noticed that the spectrum of the electric spark was crossed by bright-coloured lines; and in the year 1835, Professor Wheatstone determined the spectra of the electric spark taken from fused zinc, cadmium, tin, bismuth, lead, and from mercury, and found that each is crossed by bright lines differing in number, position, and colour, but which are the same whether the electric spark be from a static, voltaic, or magneto-electric machine. Having given a plate showing the colours of these bright lines on the respective spectra, he proved that they are not owing to the electricity, but to the incandescent atoms of the metals, for by using different metals as terminals to the conducting wires, he determined the spectra of these metals in vacuo, which proved that they were due alone to the volatilization of the metallic terminals, and concluded that any one metal may be distinguished from another by the appearance of the spark.

Wheatstone discontinued his spectrum researches, for he had invented the electric telegraph, and was busy in extending the first telegraphic wire that ever carried the thought of man to man between London and Manchester. Soon after he laid the first aquatic line across the Thames, and he has lived to see his telegraphic lines spread over the surface of the earth and the bottom of the ocean.

Mr. Wheatstone had perceived that the bright lines on the spectra of the metals are different and more complicated when taken in air than in vacuo, and Professor Angström made the important remark that the electric spark gives two superposed spectra, one due to any metal that may be under examination, the other to the incandescence of the air through which the spark passes. Hence the importance of the spectrum analysis of gaseous substances, especially of those which constitute our atmosphere, a subject that has been ably and successfully investigated by Professor Plücker. For that purpose he made use of the Geissler or vacuum tubes, similar to those he used in his experiments on the stratification of electric light. When electricity was sent through a tube containing oxygen gas, the gas combined so rapidly with the platinum of the negative terminal of the battery that there was little time to examine the spectrum. The electral light in the tube was too red at first, but as the attenuated gas gradually disappeared it changed through flesh-colour to green, then through blue to reddish-violet, and at length there was too little gas to convey the electricity. However, the oxygen spectrum has a remarkably bright red band at its red extremity, two bright orange lines divided by a black one in the orange, and some bright bands in the green.

The electric light of attenuated hydrogen is red, and almost the whole light in its spectrum is concentrated into six bright bands of nearly equal breadths. There is a dazzling red band near the red end of the spectrum, which, however, does not coincide with the oxygen band; then comes a very beautiful yellow band, in which the whole of the yellow rays seem to be concentrated, followed by a grey interval which separates the yellow from three bright lines in the green, the first of which is yellowish green, the last a beautiful greenish blue; a black and a dark space separates the latter from the violet in which there is a bright line. The electric light in a tube containing highly rarefied aqueous vapour is red, the vapour is resolved into its simple elements by the electricity, the oxygen combines with the platinum of the negative or heat pole, and the spectrum is that of pure hydrogen with the three most prominent bands only.

The nitrogen spectrum is brilliant with all the seven colours; there are no broad dark spaces like those which divide the bright bands in the hydrogen spectrum, but it is crossed by numerous very fine black and grey lines. Fifteen of the latter stripe the red and orange; the green is separated from the yellow by a black narrow band; it is terminated by two bright blue lines, and very fine dark lines cross it and the rest of the spectrum. The tube light is yellowish red.

The spectrum of highly rarefied atmospheric air is chiefly that of nitrogen, for the oxygen combines with the platinum of the negative terminal, and is in too small a quantity to transmit the electricity through the tube.

The rarefied vapours of chlorine, bromine, and iodine are so rapidly combined with the platinum of the negative terminal, that it is difficult to determine their spectra; but they have peculiarities in common, which distinguish them from all other spectra. The bright lines that cross them are first at rest, but soon become flickering. In the iodine spectrum, five of those lines of flickering light of great beauty are in the green, two of them close together. The bromine spectrum shows a greater number, which extend across the colours of its middle part, accompanied by dark lines; and in the chlorine spectrum there are many lines, both of flickering light and darkness. New lines are brought out in the iodine spectrum by increase of temperature. At a low heat it is crossed by a number of dark lines, but with a higher temperature the vapour has a greenish hue, which is resolved by the prism into green lines at some distance from one another, and fainter blue light, crossed by groups of luminous bands.

Rarefied compound gases are resolved by the electricity into their component parts, and the result is superposed spectra, one belonging to each element. M. Seguin considers the aspect of the electric spark to be a sure indication of chemical action, for while the decomposition is in progress, the electric spark is encompassed by a halo, and the bright lines of the double spectrum are less distinct; but when the reaction is finished, the spark becomes slender, and the spectrum bands distinct. In the decomposition of highly carburetted and attenuated hydrogen gas, the spark resembles a flame, and the spectrum is like that of white light. When the gas is decomposed, the hydrogen is disengaged, and the carbon deposited on the extremities of the conducting wires; the spark becomes slender, and then the lines of the hydrogen, the lines belonging to the hydro-carbon and to carbon itself may be seen on the spectrum.^[17]

The bright and coloured lines on the spectra of the gases, and the vapours of a great number of the metals and metallic salts, were known before MM. Bunsen and Kirchhoff began their systematic researches, during which they added many more, some so difficult and analogous, that it required all their skill and experience to make them out.

Of all the spectra that have been determined, those of sodium and iron are the most important and interesting. In that of sodium, the only light is of the purest yellow condensed into a double line of intense brilliancy on a dark ground. The iron spectrum on the contrary is crossed by bright lines of all intensities and colours in such multitudes, that their number has not been ascertained. The calcium spectrum has one very bright green band in the orange, a red line in the yellow, and a well-defined yellow line in the indigo. As already mentioned, the red and orange parts of the strontium are crossed by many red lines separated by dark intervals; there is a bright blue line between the orange and yellow, and an orange line in the blue. One intense crimson band in the orange characterises the lithium spectrum. Seven broad green bands stripe the yellow and a part of the green, in the barium spectrum, and that of magnesium has many green bands and lines.

All of these were determined by the heat of white coal gas flame, which amounts to 2350° Cent., and at the time MM. Bunsen and Kirchhoff were not aware that by an increase of temperature new bright lines were added to some of the spectra. That discovery was made by Professor Tyndall, while examining the spectrum of chloride of lithium, which with the low temperature has only one crimson band in the orange, but with the hotter flame of hydrogen gas, amounting to 3259° Cent., an orange line appeared in the yellow, and when Mr. Tyndall employed the electric lamp,^[18] the spectrum acquired a broad brilliant blue band between the orange and yellow, while the crimson band remained unchanged. Professors Roscoe and Clifton confirmed Tyndall’s discovery, and upon comparing the spectra of strontium and lithium, they found where only one prism was employed that the blue line of lithium appeared to coincide with the blue line, delta, of strontium; but with an apparatus having several prisms like that of Kirchhoff, they saw that the two blue lines differed by one division of the measuring scale, the lithium line being the most refrangible. A great change was produced on the strontium spectrum by increased electric temperature: three of the red bands vanished, and new bright lines appeared, that were not coincident with those they replaced; the blue line was not affected, but four new violet lines were added. With the intense heat of the electric spark, the broad green band of the calcium spectrum is replaced by five green lines of less refrangibility, the well-defined yellow line vanishes, and instead of the red band three red or orange lines appear, of greater refrangibility than those that have vanished. Six of the bright green bands in the spectrum of barium entirely vanish, and bright new non-coincident lines appear. Thus, not only new lines appear at very high temperature, but the broad bands, characteristic of the metal or metallic compound at a low temperature of the flame or a weak spark, totally disappear at the higher temperature. The new bright lines, which supply the part of the broad bands, are generally not coincident with any part of the band, sometimes being less and sometimes being more refrangible. The gentlemen who made these experiments add, that possibly the cause of the disappearance of the broad bands and the production of the bright lines may be, that at the lower temperature of the flame or weak spark, the spectrum observed is produced by the glowing vapour of some compound, probably the oxide of the difficultly reducible metal, whereas, at the enormously high temperature of the intense electric spark, these compounds are split up, and the true spectrum is obtained, namely, the narrow bright lines. No such changes take place in the easily reducible metals, potassium, sodium, or lithium, which remain unaltered by change of temperature. In these experiments, a bead of the metallic salt on a platinum wire was placed between the platinum terminals, from which the spark of a powerful inductive coil could be passed, but in order to have a more intensely hot spark the coating of a Leyden jar was placed in communication with the terminals of the secondary current respectively. By this addition of static electricity, the intensity of the current was increased four-fold, and must have been beyond estimation.

By high temperature the cæsium spectrum has been so changed, that for number, colour and distinctness of its lines, it is the most beautiful of those of the alkaline and earthy metals, for besides its characteristic blue lines, it has six red and an orange-red line in the red part of its spectrum, a fine yellow line, and nine green lines, the last coinciding with Fraunhofer’s E. The thallium spectrum also acquires more lines when evaporated by electricity, for besides the remarkable green line in the green, it acquires a faint one in the orange, two of nearly equal intensity in the green, a third fainter, and a fifth in the blue.

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]

As early as the year 1849, M. Foucault discovered that the sun’s light when shining through the electric light gives black bands on that part of the spectrum where the electric light alone would have produced bright bands, so that the black and bright bands could be produced alternately by admitting or excluding the solar light; whence he concluded that the electric arc emits the same lines which it absorbs when they come from another luminous source. M. Angström also observed that the bright lines on the spectra of volatilized metals could be reversed by a stronger light shining through their flames. Neither of these gentlemen was aware of the importance of a discovery which enabled M. Kirchhoff to apply his delicate and refined analysis of terrestrial matter to the sun and stars.

He had already determined the coincidence of the double yellow sodium line with Fraunhofer’s dark line D, but while looking with a prism at a bright solar beam passing through a yellow sodium flame, he was surprised to see a strong and well-defined double dark line instead of the double yellow sodium line which he expected. He obtained the very same result, more strongly, with Drummond’s lime light, which is brighter than the flame of any volatilized metal, and as he found that he could produce the dark and yellow lines alternately, by admitting and shutting out the brighter light, he concluded that the sodium flame is subject to the law of exchange, in consequence of which it absorbs rays of the same refrangibility with those that it emits. In fact, the soda flame is pervious to all the rays in solar light and Drummond’s flame, except those of the same refrangibility with its own; these it absorbs and it may be supposed changes them into heat. Hence M. Kirchhoff came finally to the conclusion, that the double dark line in the solar spectrum is the reverse or negative of the double yellow line seen on the spectrum of the sodium flame.

Quite recently, M. Fizeau has discovered that the spectrum of sodium burning in air is reversed during the combustion. At first it is black, with the usual double yellow line; at last, when the light is at its maximum, the double yellow line becomes black on a continuous spectrum with all the seven colours.

After M. Kirchhoff had ascertained that the bright lines in the spectra of calcium, chromium, magnesium, iron and nickel coincide with dark lines in the solar spectrum, he reversed them by sending Drummond’s light through their respective flames, thus proving that the coloured flames of these six metals are subject, like the sodium light, to the law of exchanges.

M. Kirchhoff infers by analogy that the vapours of all these six metals exist in the luminous atmosphere of the sun, and that they absorb and change into heat such rays of the continuous light of the incandescent solar globe as have the same refrangibility with their own, so that the corresponding dark rayless lines on the solar spectrum are the reverses of the bright lines in the spectra which these vapours would give were it not for the brighter light of the sun shining through his luminous atmosphere.

The dazzling white light of the incandescent body of the sun containing rays of all refrangibilities would give a continuous spectrum shaded with all the seven colours, but for his luminous absorbent atmosphere, which comes like a veil between him and the earth, and crosses his spectrum with thousands of dark lines, which are the reverses or negatives of the bright lines in the spectra of the innumerable vapours it contains, all of which must doubtless be the gases of substances existing in the solar mass itself and vaporized by his intense heat.

Every metal, and almost every elementary substance in a state of gaseous combustion, gives its own peculiar luminous lines to its spectrum, but no volatilized matter can be proved to exist in the sun’s atmosphere except such as have bright lines in their spectra coincident with some of its dark lines.

The bright lines in the spectrum of iron, coincident with the dark lines of the solar spectrum, are so numerous that many yet remain unknown. M. Kirchhoff counted seventy in the small space between Fraunhofer’s lines D and F, in which the coincidence extends even to shade, the deepest dark lines corresponding to the most brilliant bright ones, and he computed that the chances are as 1 to the ninth power of 10, that the coincidence of these seventy lines is not fortuitous, but owing to a definite cause, whence he concluded that the presence of iron vapour in the solar atmosphere is proved with as much certainty as can be attained in any question of natural science.

In a later publication, M. Angström observes that, although the coincident iron lines between D and F are not so numerous as M. Kirchhoff affirmed, they are quite sufficient to establish beyond a doubt the presence of iron in the solar atmosphere. The iron lines are the most characteristic in the whole solar spectrum, and if a magnifying power be used, or if the light be refracted through several prisms, these lines, or at any rate the stronger ones among them, appear to be perfectly black. M. Angström noticed that on a careful examination of the solar spectrum, certain lines can be discovered, imbedded in a mass of fainter ones, which, with increased illumination, seem to withdraw themselves and disappear, while the first mentioned lines, on the contrary, only stand out in a stronger relief. These are metallic lines of high fusion temperature; the most remarkable among them almost invariably belong to iron.

The substances common on earth that have their vapours in the atmosphere of the sun, though they have fewer bright lines in their spectra than that of iron, are quite as characteristic, and quite as distinctly coincident with their reverses, whether they be single, in groups, or double, as the sodium line, which is brighter and its reverses darker than that of any other substance, because volatilized sodium gives out a greater quantity of light, and consequently absorbs a greater quantity.

M. Angström has added aluminium and manganese to the seven metals whose vapours M. Kirchhoff has shown to exist in the atmosphere of the sun, but he thinks it doubtful whether barium, zinc, or copper are solar metals, for although their brighter lines correspond with distinct dark solar lines, their weaker lines do not. Strontium is doubtful also, for one of its strongest bright lines is not coincident with any dark line. Though both iron and nickel are decidedly solar metals, yet as cobalt is doubtful, it cannot be presumed that meteorites are of solar origin.

The spectrum of luminous magnesium has many green lines perfectly coincident with those in the solar spectrum, so there is no doubt of that metal being a constituent of the sun’s atmosphere. But there are magnesium rays as well as some of iron of such high refrangibility that in Mr. Stokes’s long spectrum they are situated ten times as far from H as the whole length of the visible spectrum from A to H. These highly refrangible rays only become visible at the exalted temperature of the electric spark, and as they are not found in the solar spectrum, it is inferred that the heat of the sun is inferior to that of the electric spark.^[21] Mr. Roscoe observes that this conclusion would only be legitimate if we knew that these rays of high refrangibility are not absorbed in passing through the atmosphere.

These are some of the most striking results of the numerous investigations that have been made since M. Kirchhoff published his discoveries, for the subject is anything but exhausted.

The intensely vivid light of a magnesium flame is rich in violet and extra-violet rays, partly due to the incandescent vapour of magnesium, and partly to the intensely heated magnesia formed by the combustion. The properties of this light having been examined and compared with those of the sun by Professors Roscoe and Bunsen, with a view to photographic purposes, they came to the conclusion that ‘the steady and equable light evolved by magnesium wire, burning in the air, and the immense chemical action thus produced, render this source of light valuable as a simple means of obtaining a given amount of chemical illumination, and that the combustion of this metal constitutes a definite and simple source of light for the purpose of photochemical measurement.’

Bright lines of two different metals sometimes coincide with the same black line, that is, they appear to have the same reverse as an iron and a magnesian line, an iron and a nickel line, and some others; but it is not known whether the coincidence be real or apparent.

M. Kirchhoff has proved that neither gold, silver, tin, lead, antimony, arsenic, mercury, lithium, cadmium, and some others are constituents of the sun, because none of their bright lines are coincident with any of the dark lines of the solar spectrum. This negative discovery does no less honour to M. Kirchhoff than the proof of so many substances being common to the earth and sun.

Since all incandescent solid and liquid bodies give a continuous spectrum which exhibits no dark lines, M. Kirchhoff conceives that the sun consists of a solid or liquid nucleus, heated to the temperature of the most dazzling whiteness, and that it is surrounded by a luminous gaseous atmosphere of somewhat lower temperature, endowed with the law of exchanges. The spectra of Arcturus, Capella, and many other fixed stars are crossed by dark lines similar to, and often coincident with, the dark lines in the solar spectrum; therefore, it may be concluded that their structure is to a certain extent the same with that of the sun.

Numerous observations have been made on the spectra of the fixed stars, both in Britain and on the Continent. In England, Mr. Huggins and Professor W. A. Miller have published tables of the measures of about ninety dark lines in the spectrum of Aldebaran, nearly eighty in that of α Orionis or Betelgeux, and fifteen in that of β Pegasi, with diagrams of the two first which include the results of a comparison of the spectra of various terrestrial elements with those of the stars. Thus coloured lines of sodium, magnesium, calcium, hydrogen, iron, bismuth, tellurium, antimony and mercury were found to be coincident with some of the dark lines in the spectrum of Aldebaran, and besides these there are numerous lines in the spectrum of this star which are probably due to forms of matter unknown to us. Coloured lines of sodium, magnesium, calcium, iron and bismuth, coincided with dark lines in the spectrum of α Orionis; and β Pegasi had a spectrum closely resembling that of α Orionis, but much fainter.

Between forty and fifty stars were examined, and it was observed that the solar lines C and F corresponding to hydrogen, which are present in the spectra of nearly all the stars, are wanting in those of α Orionis and β Pegasi. With a few exceptions, the terrestrial elements hydrogen, sodium, magnesium, and iron, which appear to be most widely diffused through the stars, are precisely those which with the exception of magnesium are essential to life as it exists upon the earth. Besides, the elements hydrogen, sodium, and magnesium, represent the ocean, which is an essential part of a world similar to the earth. Should any planets revolve round α Orionis and β Pegasi, they probably would have no hydrogen, consequently, no ocean and no water: therefore, they could not be inhabited by beings constituted as we are.

Padre Secchi, the Roman astronomer, divides the stars into three types; the first and most dominant type includes Sirius, α Lyræ, and other white stars, which invariably contain hydrogen of high temperature, and are denoted by a black line in their spectra, which coincides with the solar line F; and there is another band also probably due to hydrogen in the violet half of the stars visible to the naked eye belonging to this group. A singular modification of this group, however, occurs in the stars of the constellation Orion, which so rarely show any deviation from one type, that, with the exception of α Orionis or Betelgeux, they may be said to form a family distinguished from all the other stars in the sky; their spectra are crossed by fine lines, faint in the violet, with a band more or less visible in F. γ Cassiopeiæ and β Lyræ differ from the stars of the first type in having a bright band near the solar line F, instead of a black one.^[22]

Padre Secchi’s second type includes α Orionis, α Tauri, Antares, β Pegasi, &c., which have coloured bands in the red and orange. According to M. Secchi, the most remarkable star in this section is α Herculis. It gives a spectrum which has the appearance of columns illuminated on one side; ‘the stereoscopic effect of the convexity of these bands due to the shading is so surprising, that it cannot be beheld without astonishment.’ The spectrum of the star δ² Lyræ has a similar appearance, only instead of convex it has concave bands.

The third type consists of stars whose spectra are crossed by fine lines, as Arcturus, Capella and our own sun.

The colours of the stars are produced by vapours existing in their atmospheres, one colour predominating over the others, which are absorbed by the number of dark lines.

Messrs. Huggins and Miller obtained extraordinary results from the examination of temporary and periodic stars. Temporary stars suddenly shine forth with great brilliancy and soon vanish or nearly vanish. A temporary star which suddenly appeared on the night of May 12, 1866, when examined with a spectroscope, had two spectra, showing that its light emanated from two distinct sources. One spectrum, analogous to that of the sun, was formed by the light of an incandescent solid or liquid photosphere, which suffered absorption by the vapours of an envelope cooler than itself. The second spectrum consisted of a few bright lines, indicating that the light by which it was formed was emitted by luminous gas: the position of some of the lines denoted hydrogen; whence the observers believed the phenomena to result from the burning of hydrogen with some other element, and that the photosphere was heated to incandescence by the resulting temperature.

The variation in the brightness of periodic stars has by some been supposed to be due to an opaque body periodically obscuring the light. Should that body be surrounded by an atmosphere like our planet’s, its presence would be revealed by the absence or presence of additional lines of absorption in the spectrum of the star. Now three lines determined in the spectrum of Betelgeux were no longer found when the star arrived at its maximum of brightness, indicating it may be the presence of an atmosphere round the opaque body.

With regard to our own planets, Jupiter has lines in his spectrum which indicate the existence of an absorptive atmosphere; one band indicates the presence of vapours similar to those existing in our atmosphere, another band has no counterpart among the lines of absorption of the earth’s atmosphere, and tells of some gas which it does not contain.

In the feeble spectrum of Saturn there are lines similar to those in the spectrum of Jupiter. These lines are less strongly marked in the ansæ of the rings, and show that the absorptive power of the atmosphere about the rings is less than that of the atmosphere which surrounds the ball.

M. Jansen has found lines denoting aqueous vapour in the atmospheres of both Jupiter and Saturn. Some very remarkable lines have been seen in the more refrangible part of the spectrum of Mars supposed to be connected with his red colour. Though the spectrum of Venus is brilliant, and the dark lines distinct, no additional lines indicate the existence of an atmosphere differing from our own.

The phenomena resulting from an examination of the nebulæ are most wonderful; their light is very feeble, even that of the brightest. ‘The total light of the whole nebula in Orion, the largest and brightest of them, makes so small an impression on the naked eye, that you may look twenty times at its place and not perceive any nebulous light at all.’^[23] Besides, the brightness of a surface cannot be increased by a telescope, however good. Notwithstanding difficulties which seem to be almost insurmountable, Mr. Huggins in England, and Padre Secchi at Rome, have been, and still are, engaged in these researches.

The planetary nebulæ are beautiful objects; they are like planets with a round or oval disc, equable, slightly mottled and of enormous magnitude; one near γ Aquarii is twenty seconds, and another is twelve seconds in diameter. Sir John Herschel computed that if these objects be as far from us as the nearest of the fixed stars, their magnitude, on the lowest estimation, would fill the orbit of Uranus. He discovered twenty-eight or twenty-nine of them, some of a beautiful blue tint, in the southern hemisphere; and from the uniformity of the discs in both hemispheres, and their apparent want of condensation, he presumed that they may be hollow shells emitting a feeble light from their surfaces only. The spectrum analysis of that light, by Mr. Huggins, in six of the planetary nebulæ, showed that their structure is utterly unlike anything else in creation,^[24] for instead of an ordinary spectrum he found, to his infinite surprise, that the spectra of the feeble light of these bodies consist only of three bright lines, such as those which proceed from an intensely heated gas, and that the lines exhibited some of those of the hydrogen and nitrogen spectra and an unknown gaseous substance: whence he draws the astounding conclusion, that planetary nebulæ are probably composed of hydrogen, nitrogen, and some unknown gas, without any solid nucleus whatever.

The annular nebula in Lyra, which is probably nearest to the earth, and the dumb-bell nebula, gave a spectrum indicating matter in a gaseous form. The annular nebula appears to be a hollow elliptical ring of nebulous matter of enormous magnitude. The interior opening of the ring is not entirely dark, but filled with a faint hazy light, like fine gauze stretched over a hoop. The dumb-bell nebula in the constellation Vulpecula is like an hour-glass of bright matter surrounded by a thin hazy atmosphere, which gives the whole the form of an oblate spheroid. Both of these nebulæ when viewed with a very high telescopic power seem to consist of minute clustering stars, but the spectra of these two nebulæ have one bright line, the structure of both being of the same gaseous constitution.

The great nebula on the sword handle of Orion was then examined. The spectrum of the light from the brightest parts of this nebula, near the trapezium, was crossed by three bright lines, in all respects similar to those on the spectra of the planetary and other nebulæ. Other portions of the great nebula were then brought successively under examination, but the spectra of the whole of those portions which still were sufficiently bright for this method of observation remained unchanged, and exhibited the three bright lines only. The whole of the great nebula, as far as it lay within the power of Mr. Huggins’ instrument, emits light which is identical in its characters; the light from one part differs from the light from another part in intensity alone. The brighter portions of this nebula have been to a certain extent resolved into stars, by the powerful telescopes of Lord Rosse and Professor Bond, of the United States of America; the whole, or the greater part, of the light from that portion of the nebula must therefore be regarded as the united radiation of numerous stellar points. The spectrum of this radiation being crossed by the three bright lines reveals its gaseous source; Mr. Huggins therefore infers that at least some of these stellar points are merely denser parts of a gaseous matter, and that the nebulæ which he examined are enormous gaseous systems.

The spectrum of the great nebula in Orion was subsequently examined by Padre Secchi. He describes the light of the spectrum as of a uniform green, crossed by three bright lines; one tolerably wide and perfectly sharp, a very slender one close to it, and the third at a little distance from the latter. This spectrum afforded a striking contrast to the spectra of the small stars in the brighter parts of the nebula. As soon as the light from one of these stars entered the slit of the instrument, its continuous spectrum was seen to flash across the field of vision in a long coloured band. This shows that the mass of matter in this immense nebula is in a different state from that of the stars themselves, as Mr. Huggins had already observed. Padre Secchi does not draw any inference from his observations as to the structure of nebulæ in general, probably thinking it premature, but he expresses astonishment at their results.

Since the preceding lines were written, Mr. Huggins and Professor W. A. Miller have continued their researches on the constitution of the celestial bodies by a method of direct simultaneous comparison of the lines in their spectra with the lines in the spectra of many of the terrestrial elements. The spectra for comparison were obtained from the spark of the induction coil taken between points of various metals; and sometimes a platinum wire was used, surrounded with cotton, moistened with a solution of the substance required. The telescope of the instrument was mounted equatorially, and followed the star by clockwork. By this arrangement the spectrum of the star, and the spectrum of the metal compared with it, are seen in juxtaposition; and the coincidence or relative position of a dark line in the stellar spectrum with a bright line in the metallic spectrum can be determined with great precision.

It was found that Jupiter’s atmosphere has a much greater absorptive power than the terrestrial atmosphere; that they have some gases or vapours in common, but that they are not identical.

Some of the lines seen in the atmosphere of Saturn appear to be identical with those seen in the spectrum of Jupiter.

‘The lines characterizing the atmospheres of Jupiter and Saturn are not present in the spectrum of Mars. Groups of lines appear in the blue portion of the spectrum; and these, by causing the predominance of the red rays, may be the cause of the red colour which distinguishes the light of this planet.’^[25]

All the stronger lines of the solar spectrum were seen in the brilliant light of Venus; but no additional lines indicating an absorptive action of the planet’s atmosphere.

The authors are of the opinion that in most of the planets the light is probably reflected from clouds floating at some distance from the surface, so that it is not subject to the strong absorptive action of the lower and denser strata of the planet’s atmosphere, which, like our own, are most effective in producing atmospheric lines.

The results of the observations on the fixed stars are exceedingly interesting, for they show that their elementary constituents are similar, but not identical; and that although they contain many of the sixty-five terrestrial elements, there are probably new unknown substances also.

When seventy dark lines on the spectrum of the star Aldebaran, and eighty on that of α Orionis (Betelgeux) were compared with the bright lines on the spectra of the vapours of a variety of the terrestrial simple elements, it was found that Aldebaran contained nine terrestrial substances and α Orionis five: that is, there were only nine out of seventy of the dark lines of Aldebaran coincident with bright lines, and five out of eighty of those of α Orionis. Yet the seventy and eighty dark lines that were compared represented some of the strongest only of the numerous lines which were seen on the spectra of these stars. Some of those remaining were probably due to the vapours of other terrestrial elements which were not compared with these stars, but Mr. Huggins concludes that many of those dark lines are due to new unknown elements existing in these stars, and that we cannot assume that the sixty-five simple terrestrial elements constitute the entire primary material of the universe. A community of matter, however, exists throughout the visible creation; for the stars contain many of the elements common to the sun and earth. ‘It is remarkable that the elements most widely diffused through the host of stars are some of those most closely connected with the living organism of our globe, including hydrogen, sodium, magnesium and iron. May it not be that, at least, the brighter stars are like our sun, the upholding and energizing centres of systems of worlds adapted to the abode of living beings?’

With regard to the nebulæ Mr. Huggins’s observations show that nine are gaseous, the spectra of six exhibiting three bright lines, one shows an additional faint line also, while the spectra of the dumb-bell nebula and the annular nebula in Lyra show the brightness of three green lines only. The spectra of eight other nebulæ were continuous, showing that their light has not undergone any modification on its way to us.

Mr. Huggins has been able to discriminate between the light of the nucleus of a comet and that of its tail. The nucleus is self-luminous, and its substance is in the form of ignited gas. The coma shines by reflected light as clouds do, and observations of the spectra give reason to believe that comets chiefly consist of nitrogen and another elementary body different from nitrogen combined with it.

The terrestrial elements found in the fixed stars show that, like the sun, they have an intensely luminous nucleus: but if it be taken for granted that highly heated gases are non-luminous internally, the planetary nebula and the great nebula in Orion itself being thus considered to be gaseous, must emit their feeble light from their surfaces alone. All the true clusters of stars which are resolved by the telescope into distinct bright points of light, give a spectrum which does not consist of separate bright lines, but is apparently continuous in its light. The great nebula in Andromeda, which is visible to the naked eye, has an apparently continuous spectrum, but the whole of the red and orange part is wanting, and the brighter parts have a mottled appearance. The easily resolvable cluster in Hercules has a similar spectrum; Lord Rosse discovered dark streaks or lines in both.

There is a striking correspondence between the results of prismatic and telescopic observations; half of the nebulæ which have a continuous spectrum have been resolved into stars, while none of the gaseous nebulæ have been resolved even by Lord Rosse’s telescope. Thus it appears probable that primordial nebulous matter does exist, according to the theories of Sir William Herschel and La Place.

The structure of the sun himself, which forms one amidst the multitude of stars which constitute the Milky Way; and the maintenance of his light and heat without apparent waste, are still in various respects involved in mystery.

The luminous gaseous atmosphere of the sun is of great extent and of lower temperature, at least in its upper regions, than the photosphere on which it rests. Mr. De la Rue’s photographs of the sun show that the light from the border of the solar disc is less intense than that from the equator, on account of the greater depth of solar atmosphere it has to pass through before it reaches the earth, by which a larger portion of the light is absorbed.

The photosphere of the sun has a mottled appearance, exhibiting minute masses, which must be of enormous magnitude to be visible at such a distance. They have been examined with a very high telescopic power by Mr. Nasmyth, who describes them as lens-shaped bodies of wonderful uniformity, and likens them to willow leaves crossing each other in all directions, and moving irregularly among themselves. Mr. De la Rue and Padre Secchi say they have seen something similar, and others liken them to rice grains. Sir John Herschel^[26] is of opinion that they consist of incandescent matter sustained at a level corresponding to their density in the solar atmosphere, an atmosphere which he considers as varying from a liquid state below to the highest tenuity of a rarefied gas above. In a memoir read at the Institute of Paris,^[27] by M. Faye, something of the same kind is suggested.

There are comparatively brighter waves of the sun’s disc, called faculæ, which are portions of the sun’s photosphere thrown up into the higher regions of his atmosphere; for Mr. De la Rue took a stereoscopic impression of a solar spot and some faculæ, in which the spot appeared to be a hollow and the faculæ elevated ridges. Being elevated above the photosphere, their light is less absorbed by the sun’s atmosphere, and by contrast they are brighter at the less luminous border of the solar disc than at the equator.

It appears that the red flames and protuberances seen round the edge of the sun during a total eclipse are gaseous or vaporous luminous bodies which certainly belong to the sun; for during the total eclipse in 1860 it was observed, that as the moon moved over the sun’s disc, the red flames and part of the corona discovered themselves at the side which she had left, and were covered by her disc at the side towards which she was approaching. Besides, the illuminating effect of the red light of these flames is so inferior to its photographic power, that Mr. De la Rue photographed one of the protuberances, although it was invisible to the naked eye.

The sun spots which are situated in that region of the sun which lies below the photosphere consist of a central darkness or umbra, surrounded by a penumbra which is less dark. Professor Wilson, of Glasgow, proved that the spots are cavities, of which the umbra or darkest part forms the bottom, and the penumbra the sloping sides, by observing that the umbra encroaches on that side of the penumbra which is next to the visual centre of the sun. Hence the umbra of a spot is at a lower level than the penumbra; and since luminous ridges and sometimes detached portions of luminous matter cross over the spots, it is concluded that the whole phenomenon is below the surface. The spots have an apparent motion from east to west, due to the rotation of the sun; and Mr. Carrington discovered that they have a proper motion also from east to west, those nearest the solar equator moving fastest. They are confined to the equatorial regions.

No reason has yet been assigned for the periodicity of the spots, which go through a cycle of maxima and minima every ten years nearly. They are singularly connected with terrestrial magnetism; the maximum of the spots coincides with the period of the greatest disturbance of terrestrial magnetism. The spots seem to be influenced by the planet Venus in such a manner that when a spot comes round by rotation to the ecliptical neighbourhood of this planet, it has a tendency to dissolve; and, on the other hand, as the sun’s surface recedes from the planet it has a tendency to break out into spots.^[28]