he drew an accurate map, not only of these metallic lines, but of the dark lines in the spectrum of the sun; and this is a copy of one of his diagrams, to which I would now briefly allude. The dark lines here represent the dark lines in the sun. With these I have at present nothing to do. We shall devote our next lecture to a detailed discussion of this most remarkable subject. Today I would simply draw your attention to the short lines at the lower part of the diagram, which indicate to us the position of the bright metal lines with regard to the fixed dark solar lines, these latter being taken as a sort of inch rule, by which the position of the other lines are reckoned. These lines which you see joined by a horizontal line, and marked Fe (for Ferrum), are the iron lines; and I beg you to notice the very large number and the very beautifully fine nature of these iron lines. On Kirchhoff's map each line is accompanied by a letter, which gives the chemical symbol of the element to which this line belongs; here an aluminium line, here an antimony line, here a calcium line, here again a number of iron lines connected together; and so I might go through all these diagrams, showing the number of lines which Kirchhoff has mapped: and this for only a small portion of the spectrum. The one end of this diagram is in the yellow, and the other end in the green, so that we have here, on this map, only a very, very small portion of the extent of the metal lines which would be visible.

I would next illustrate this fact by showing you another beautiful drawing of these metal lines, made by our countryman, Mr. Huggins (see Plates I. and II. at the end of this Lecture). This map, which is copied from Mr. Huggins' paper in the Philosophical Transactions for 1864, will give you an idea of the very great number

of these metal lines. We have here about twenty metals, and each pair of these horizontal lines includes the spectrum of a particular metal. For instance, if we take silver, here is one line, here three, here two, here a number of other lines thus we go on through the whole spectrum, and have altogether a great number of silver lines. On the right we have the red end, on the left the blue end of the spectrum, and at the top, for the sake of comparison, are the chief lines of the solar spectrum and the air lines. Now from this table you may form an idea of the large number of metal lines existing, and you will also see that the lines of any one metal do not coincide with those of any other.

These lines are by no means all the peculiar rays which such highly heated metallic vapours emit, for Professor Stokes has shown that the bright sparks from poles of iron, aluminium, and magnesium give off light of so high a degree of refrangibility, that distinct bands are situated at a distance beyond the last visible violet ray, ten times as great as the length of the whole visible spectrum from red to violet! These bands cannot of course be seen under ordinary circumstances, but when allowed to fall on a fluorescent body, such as paper moistened by quinine solution, they can easily be rendered visible; or we may photograph them, and make them leave their impression on the sensitive film. In order that these highly refrangible rays may be seen, no glass lenses or prisms must be used, as these rays of high refrangibility cannot pass through glass: quartz on the other hand permits them to pass; hence all the lenses and prisms must be made of quartz.

In new and interesting subjects like those which now occupy our attention, the mind is very apt to be led

away into speculations, which, however engrossing they may prove, are foreign to the spirit of the exact scientific inquirer. Such speculations might in this case have special reference to the possibility or probability of arriving by the help of the observations of the bright lines which bodies give us at some more intimate knowledge of the composition of the so-called elements. We might speculate as to the connexion, for instance, between the wave-lengths of the various bright lines of the metal and the particular atomic weight of the substance; or we might ask, Can we find out any relation between the spectra of the members of some well-known chemical family, as iodine, chlorine, and bromine, or between those of the alkaline metals, potassium, sodium, cæsium, and rubidium? Such questions as these naturally occur to every one. At present, however, this subject is in such an undeveloped state, that such speculations are useless, because they are premature, and the data are insufficient; but doubtless a time will come when these matters will be fully explained, and a future Newton will place on record a mathematical theory of the bright lines of the spectrum as a striking monument of the achievements of exact science.

The next point to which I would direct your attention is one of a slightly different kind. We find that certain substances-not only gases, but liquids, and even solid bodies-exert at the ordinary temperature of the air a selective absorption power upon white light when it passes through them. In the next lecture I shall have occasion to show you, in various ways, the absorptive effect which glowing sodium vapour exerts upon the particular kind of yellow light which sodium itself gives off; but I would now consider some cases of selective

absorption occurring at the ordinary temperature, and just indicate to you a most interesting and important branch of this subject which has been, to a certain extent, worked out; but in which a rich harvest of investigation still remains open. I refer to the absorption spectra obtained by the examination of various coloured gases and liquids, especially of blood and other animal fluids. In the first place, then, it has long been known that certain bodies have at the ordinary temperature the power of selecting a kind of light and absorbing it. In Fig. 38 we have a representation of the selective absorp

FIG. 38.

tion exhibited by two coloured gases. No. 1 shows the dark bands seen when white light passes through the violet vapours of iodine; whilst No. 2 gives the bands first observed by Brewster in red nitrous fumes. Some coloured gases, such as chlorine, do not give any dark absorption bands. Perhaps the most striking instance of the formation of these absorption lines in the case of liquids is the one which I will now show you of this colourless solution of a salt of the rare metal didymium. Now this didymium salt possesses the power of absorbing from white light certain definite rays, so that if I place

the solution in the path of our continuous spectrum we get a broad absorption band by which, as Dr. Gladstone has shown, the presence of didymium can be recognised, when present even in very minute quantities. It is very remarkable that, although these didymium absorption lines are so black, and serve as such a reliable test of the presence of this metal, yet the fraction of the total light which is absorbed is so small that the solution appears colourless. From the recent experiments of Bunsen on this subject it seems that the various didymium com

pounds do not exhibit exactly the same absorption lines and if light is allowed to fall upon a crystal, the dark bands also differ according to the direction in which the light passes through (see Appendix C.).

The solutions of many other coloured metallic salts possess a similar property of yielding definite absorption lines, and Dr. Gladstone finds that with very few exceptions all the compounds of the same base, or acid, have the same effect on the rays of light: thus the chromium salts (both green and purple) exhibit the same form of absorption spectrum (Fig. 39). Fig. 40 shows the bands produced by potassium permanganate solution,