placed behind the box for a series of very short spaces of time, and will also be seen by you through the front hole for a series of equally short moments. If I turn the handle slowly, after darkening the room, you do not see the fluor-spar; but if I increase the speed of rotation so that the times of illumination and of observation do not exceed the... th part of a second, you will observe that the crystal glows with a very perceptible amount of light, or in other words it becomes phosphorescent. This crystal of nitrate of uranium produces a more brilliant appearance. But many substances, such as sulphur, quartz, the metals, and liquids, cannot be made thus to phosphoresce.

1000

Becquerel has carefully examined the spectra of these phosphorescent bodies, and he has found that the light emitted by many of them is of a peculiar kind; that they, in fact, give broken spectra, or bands of differently coloured rays. Thus on Fig. 53 we have a representation of the phosphorescent spectra of several substances : alumina, when phosphorescent, emits a red light, and its spectrum (No. 1) exhibits four bands between the lines c and H in the solar spectrum; diamond (No. 3) emits, when phosphorescent, light of many degrees of refrangibility, giving an almost continuous spectrum stretching from B in the red to beyond & in the indigo; aragonite (4) also gives a continuous spectrum; whilst native phosphate of lime (5), fluor-spar (6), and nitrate of uranium (7), each phosphoresces with the emission of a peculiar light, as is seen in the varying character of the above spectra.

The different rays of the solar spectra possess a very different power of producing phosphorescence. By far the most powerful in this respect are the more refrangible rays. Phosphorescent bodies exposed to the chemically

active portion of the spectrum emit light which, as we have seen, varies from red to violet, and as a rule depends only upon the nature of the substance.

Another interesting property which certain substances possess when exposed especially to these blue rays is that of fluorescence. This piece of uranium glass appears self

luminous when held in the scarcely visible violet rays of our electric lamp; and this is produced by a change of refrangibility of the light, the emitted rays being always of lower refrangibility than the exciting rays. This phenomenon of fluorescence may be used, thanks to the researches of Stokes, for the purpose of identifying certain substances, such as quinine, or the very interesting substance resembling quinine lately discovered in animal fluids and tissues by Dr. Bence Jones; but the spectra which fluorescent bodies emit are generally continuous, and in this direction it does not therefore seem likely that spectrum analysis will give us much help.

In the next lecture I hope to bring before you the simple facts upon which Professor Kirchhoff founded his discovery of the chemical composition of the solar atmosphere.

LECTURE IV.-APPENDIX A.

DESCRIPTION OF THE SPECTRA OF THE GASES AND NON-METALLIC ELEMENTS, AND ON THE EXISTENCE OF MORE THAN ONE SPECTRUM FOR EACH ELEMENT.

THE spectra of the gases are obtained, (1) by passing the electric spark from poles of certain metals whose lines are known, through the gas under the ordinary atmospheric pressure; or (2) by observing the electric discharge passed through a capillary tube (Geissler's) containing the gas in a rarefied state. Kirchhoff and Huggins have adopted the first, Plücker and Hittorf (Phil. Trans. 1865, p. 1) the second method.

The Air Spectrum.-"The lines given in this spectrum are present with all the electrodes when the spark is taken in air at the common pressure. The lines thus obtained between one set of electrodes of platinum and the other of gold were observed simultaneously. The lines common to both these spectra were measured as those due to the components of the air. The spectrum thus obtained remains invariably constant, with reference to the position and relative characteristics of its lines, with all the metals which have been employed. The air spectrum varies as a whole, however, in distinctness according to the metal employed as electrodes, owing to the difference in the volatility of the metals, the air in and around the electrodes being more or less replaced by the metallic vapours." The air spectrum is made up of the spectra of the following components -nitrogen, oxygen, and hydrogen. Grandeau1 and Kundt2 have observed the spectrum of lightning; and, in addition to the nitrogen and hydrogen spectra, have seen the bright yellow sodium line.

1 Chemical News, ix. 66.

Pogg. Ann. cxxxv. p. 315.

Huggins has employed the air lines (seen on Plates and in the Tables, Appendix C, Lecture III.) as a scale of reference for recognizing the bright lines of the metals.

Hydrogen.-The spectrum of hydrogen seen under the ordinary pressure consists of four bright lines (see Chromolith. No. 8, facing Lecture VI.).

Ha coincident with Fraunhofer's C in the red.

The lines Ha, Hẞ, and Hy are seen fine and very bright when the gas is rarefied; but if the reduction of pressure be continued, the red line Ha gradually disappears, whilst H3, though fainter, remains well defined. Plücker finds that when the intensity of the spark is increased the bands HS and Hy begin to broaden ; and when the tension of the gas is increased to 360 mm. and a Leyden jar introduced into the circuit to raise the temperature of the discharge, the bright lines are found to give way to a continuous spectrum. This change from lines to a continuous spectrum is not observed under the ordinary atmospheric pressure. Wüllner has recently shown' that by intensifying the discharge through a Geissler's tube containing hydrogen, the tube and the abraded particles of the glass become highly heated, so that first the sodium line and afterwards the calcium lines make their appearance, whilst at last the spectrum becomes continuous, and the sodium line is reversed, giving a dark absorption line.

In a paper recently published, Wüllner (Phil. Mag. [4] xxxvii.) fully describes the variation of the hydrogen spectrum with the pressure when the tension of the gas is 135 mm. the tube shines with a white light of insufficient intensity to yield a visible spectrum; when the pressure is reduced to 100 mm. the light emitted is bluish-white, and gives a continuous spectrum, in which the lines Ha (C) and He (F) stand out. Under a pressure of 70 mm. the light is reddish-white, and the spectrum is continuous with H, a, ẞ, and y visible, and also a series of beautifully shaded bands in the greenish and reddish-yellow. Pog. Ann. cxxxv. p. 174.