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of light rays, fall on the spectrum. The correct statement is that those rays (or rather light-waves) which reach this part of the spectrum are capable of exciting heat to a certain extent, and light to a somewhat greater extent.

We see, then, that in addition to what spectroscopic analysis has already taught us, we must add this interesting fact, that the three forms of action which sun-light is capable of exerting are associated with different parts of the spectrum-heat with the red end and parts beyond; chemical action with the violet end and parts beyond that; and, finally, light more particularly with the yellow part of the spectrum, though of course, as the very term visible spectrum implies, more or less light is received from all parts of the coloured spectrum.

And the intimate real association which exists between the three forms of action is shown by nothing more distinctly than by this, that when a solid or fluid body is gradually raised to a white heat, all the forms of action are generated :-first, heat; then heat and light; then heat, and light, and actinism-as the formation of the continuous spectrum of such bodies serves abundantly to prove.

Only one solid substance, the earth erbia, gives a non-continuous spectrum when heated. So that it has come to be regarded, as a characteristic peculiarity of incandescent solids and fluids, that they present a rainbow-tinted streak of light crossed by no dark lines or

gaps.

The

With glowing vapours the case is altogether different. Although there are exceptions to the rule, it may be stated as a general characteristic of the spectra of such vapours that they consist of coloured lines or bands. Sir David Brewster, Sir John Herschel, and Talbot, were among the first who examined such spectra. In 1822 Sir John Herschel called attention to the importance of the study of the lines and bands seen in the spectra of the vapours of different elements. pure earths,' he said, when violently heated, yield from their surfaces lights of extraordinary splendour, which when examined by prismatic analysis are found to possess the peculiar definite rays in excess which characterise the tints of the flames coloured by them; so that there can be no doubt that these tints arise from the molecules of the colouring matter reduced to vapour, and held in a state of violent ignition.'

It would be interesting to trace the history of those laborious researches by which men of science-Herschel, Brewster, Tyndall, the Millers, Huggins, Gladstone, Frankland, Plücker, Hittorf, and many other-have determined the spectra of gases and vapours by various methods and under various conditions. But as in this treatise I feel bound to deal only with those parts of the history of spectroscopic analysis which are associated with the study of the Sun, or which serve to elucidate solar physics, I must here content myself with indicating results without describing the processes by which they were obtained, or assigning to each of the eminent men above-named, and to their fellow

workers their exact share in the noble series of labours

referred to.

At an early stage of the inquiry, though at the time the phenomenon was not correctly interpreted, a means was found of obtaining the spectra of the vapours of elements which cannot be vaporised by ordinary methods. It was noticed that the electric spark has a spectrum consisting of bright lines. But, unlike all the sources of light whose spectra have hitherto been considered, the electric spark yields a

variable spectrum. place between conductors of the same nature, and through any given and unchanged medium, the same spectrum is always seen; but when either condition is departed from, a different spectrum is obtained.

When the electric discharge takes

It was presently recognised that the spectrum obtained from the electric spark is twofold in its nature. It includes the spectrum of the gas or vapour through which the discharge takes place, and also the spectrum of the vaporised substance of the conductors.

Here, then, was a ready means of determining the spectra of the metallic and other elements* not easily volatilised in other ways, as also of such gases as nitrogen and oxygen; while conditions of pressure, temperature and the like, could be introduced, which would be wholly impracticable if the method of vaporising elements in the flame of an ordinary lamp had alone been available.

* It seems unnecessary to speak of the spectra of the vapours of such, and such elements, since in reality it is only as vapours that iron, sodium, and the rest have characteristic spectra at all.

I

In this way a large number of elements had their spectra assigned to them, while in several instances physicists began to recognise peculiarities in the spectrum of the same element when examined under different conditions.

But an important series of researches yet remains to be considered. Hitherto we have dealt with the actual spectrum of luminous objects; we have now to inquire into the effects produced on these spectra, when the light which forms them is allowed to pass through absorbing media. We owe chiefly to Sir David Brewster the initiation of this branch of research,-though in this, as in so many other departments of spectroscopic inquiry, we find a host of distinguished physicists joining in the work. Brewster found that when ordinary solar light is transmitted through the thick vapour of nitrous gas, a number of new dark lines are seen, parallel to the Fraunhofer lines, and congregated in a remarkable degree towards the violet end of the spectrum. He further proved that these lines were seen whatever the source of light might be. Professors Miller and Daniel made further researches into the effects of vapours in causing dark lines to appear in the spectrum of solar or white light. Some of Professor Miller's results are worth quoting, because they show how closely a physicist may approach a great discovery without actually effecting it. First,' he says, 'colourless gases in no case give additional lines, or lines differing from those of Fraunhofer. Secondly the mere presence of colour is not a secu

rity that new lines will be produced; for instance, of two vapours undistinguishable by the eye, one, bromine, gives a great number of new lines, while the other, chloride of tungsten, exhibits none. Thirdly: the position of the new lines has no connection with the colour of the gas; with green perchloride of manganese, the new lines abound in the green of the spectrum; with red nitrous acid they increase in number and density as we approach the spectrum's blue extremity.' Some of these results, rightly understood, contain the germ of the great discovery afterwards effected by Kirchhoff; since in some of the cases actually experimented on by Professor Miller, the absence of new lines meant simply that the absorption lines, corresponding to the element he was dealing with, were coincident with some of the Fraunhofer lines. Others, however, approached the solution of the great problem even more nearly, since they actually touched on the principle of the reversal of the spectral lines, which affords the explanation of the coincidences detected but unnoticed by Professor Miller. None of these distinguished men,' says Professor Tyndall, betrayed the least knowledge of the connection between the bright bands of the metals and the dark lines of the solar spectrum. The man who came nearest to the philosophy of the subject was Ångström. In a paper translated from Poggendorff's "Annalen" by myself, and published in the "Philosophical Magazine" for 1855, he indicates that the rays which a body receives are precisely those which it can emit when rendered luminous. In another place he

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