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

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

Ha coincident with Fraunhofer's C in the red.

The lines Ha, HB, 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 Hß, though fainter, remains well defined. Plücker finds that when the intensity of the spark is increased the bands Hẞ 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.

Nitrogen. In the spectrum of the electric spark when taken in a current of pure nitrogen, under the ordinary pressure, a few of the lines of common air are wanting, but no new lines appear. The lines of the air-spectrum which remain in nitrogen preserve their relative brightness and their distinctive character. In the Tables these lines are distinguished by the letter N (pp. 142-149). Plücker and Hittorf have observed some remarkable changes which the nitrogen spectrum undergoes when the current is intensified. Nitrogen, like other gases, does not allow the induction current to pass when it is in an extreme state of rarefaction; but when its tension is only a fraction of a

1 Pogg. Ann. cxxxv. p. 174.

millimetre the current passes, and the gas becomes luminous. At a comparatively low temperature nitrogen thus ignited emits a golden coloured light, giving a series of bands (see Chromolith. No. 9, facing Lecture VI.); above this point the colour becomes bluish, and a new spectrum of bands appears. If a Leyden jar be enclosed in the circuit, the temperature again rises, and a brilliant white light is emitted, the spectrum again changing to one of bright lines on a dark ground. These lines do not change their position with alteration of temperature, though the brilliancy of all does not increase in the same ratio. Plücker designates the spectra consisting of broad bands "spectra of the first order;" whereas those composed of fine bright lines on a dark background are termed "spectra of the second order." The nitrogen spectrum of the second order is doubtless that of the air-spectrum. The differences thus observed are attributed by Plücker to the existence of allotropic conditions of nitrogen which decompose at high temperatures (for analogous phenomena, see Appendix B, Lecture V.). According to Kundt, the spectrum of lightning varies with the nature of the discharge, the difference being due to the appearance of the two nitrogen spectra; one of these (viz. the second spectrum of the first kind) is also seen when the discharge of electricity from a point is observed. The discharge of forked lightning gives a spectrum consisting of bright lines, being the nitrogen spectrum of the second order.

Oxygen. The lines given by this gas are given in Huggins' Tables, and designated by the letter O. The same experimenter found that some few lines appeared in the spectra of both nitrogen and oxygen. On further examination he finds that the phenomenon is produced by the superposition in the airspectrum of lines of oxygen and nitrogen. Plücker, operating as with nitrogen, obtained only one "secondary" spectrum of oxygen, but the lines appeared to expand so as to form a continuous spectrum at a higher temperature.

Sulphur.-When sulphur burns in the air, or when carbon disulphide burns in nitric oxide, a continuous spectrum is observed. If a little sulphur be introduced into a narrow

Geissler's tube, and the air withdrawn, a band spectrum of the first order is seen upon warming the tube and passing the spark through. On continuing to heat the tube, these bands change to bright lines. A figure of these two spectra is given in Plücker and Hittorf's memoir.

Selenium likewise yields a characteristic spectrum.

Phosphorus yields a spectrum of the second order when treated like sulphur. The characteristic lines are three bright bands in the green, having the positions 58, 70, and 74 to 75, on the scale of the spectroscope when Na = 50. The green line Pẞ appears with one prism to be coincident with the green barium line Bad. The green bands may be seen by observing the spectrum of the green spot which makes its appearance in the interior of a hydrogen flame when the slightest trace of a phosphorus compound is placed in contact with the dissolving zinc (Cristofle and Beilstein, Annales de Chimie et de Physique, 4 Sér. iii. 280).

Chlorine, Bromine, and Iodine.-When enclosed in Geissler's tubes, each gives a peculiar spectrum of bright lines, which expand, and ultimately form continuous spectra when the temperature is increased. Figures of these spectra are given in the memoir above referred to.

Carbon. The complicated question of the carbon spectra has been carefully investigated by W. M. Watts (Phil. Mag. Oct. 1869). He finds that there are four distinct modifications of the spectrum of carbon, or, at any rate, of the spectrum obtained from carbon compounds.

1. The carbon spectrum No. 1 is obtained when olefiant gas and oxygen are burnt together in an oxyhydrogen jet. This spectrum was first described by Swan, and afterwards by Attfield. It can be obtained from each of the following carbon compounds: olefiant gas, cyanogen,1 carbonic oxide, carbon disulphide, carbon tetrachloride, amyl alcohol, marsh gas, and naphthalin (No. 10 of the Chromolith. facing Lecture VI.), and must, therefore, be produced by carbon vapour.

1 The cyanogen spectrum varies according to its mode of production. No. 11 Chromolith. Plate facing Lecture VI. shows the spectrum of the flame of cyanogen burning in air.

2. Carbon spectrum No. 2 is obtained when carbonic oxide or olefiant gases are heated in a Geissler's vacuum-tube, when the pressure on the gas does not exceed 12 mm. of mercury.

3. Carbon spectrum No. 3 is seen in the Bessemer-flame, observed not only in the flame during the process of conversion, but also in the "Spiegel-flame," and in the coke-flame of the converter and of other furnaces. This is not identical with the carbonic oxide spectrum.

4. The fourth modification of the carbon spectrum is obtained from the induced spark, either from carbonic acid or carbonic oxide, when a Leyden jar is introduced into the circuit. The spectrum thus obtained consists of sharply-defined lines, and not of bands as seen in the former modifications.

For the exact description and maps of these carbon spectra the memoir above mentioned must be consulted.

APPENDIX B.

ON THE EFFECT OF INCREASED TEMPERATURE UPON THE NATURE OF THE LIGHT EMITTED BY THE VAPOUR OF CERTAIN METALS OR METALLIC COMPOUNDS.

BY H. E. ROSCOE AND R. B. CLIFTON.1

In a letter communicated to the Philosophical Magazine for January last we stated that, in examining, with Steinheil's form of Kirchhoff and Bunsen's apparatus, the spectra produced by passing the induction spark over beads of the chlorides and carbonates of lithium and strontium, we had observed an apparent coincidence between the blue lithium line, which is seen only when the vapour of this metal is intensely heated, and the common blue strontium line called Sr d. We further stated that on investigating the subject more narrowly by the application of several prisms and a magnifying power of 40, we came to the conclusion that the lithium blue line was some1 Proc. Lit. Phil. Soc. Manchester, read April 1, 1862.

what more refrangible than the strontium 8, but that two other more refrangible lines were observed to be coincident in both spectra. Having constructed a much more perfect instrument than we at that time possessed, we are now able to express a definite opinion on the subject, and beg to lay a short notice of our observations before the Society. Our instrument is in all essential respects similar to the magnificent apparatus employed by Kirchhoff in his recent investigations on the solar spectrum and the spectra of the chemical elements. It consists of a horizontal plane cast-iron plate, upon which three of Steinheil's Munich prisms, each having a refracting angle of 60°, are placed; and of two tubes fixed into the plate, one being a telescope having a magnifying power of 40, moveable with a slow-motion screw about a vertical axis placed in the centre of the plate, and the other being a tube carrying at one end the slit, furnished with micrometer screw, through which the beam of light passed, and at the other end an object-glass for the purpose of rendering the rays parallel. The luminous vapours of the metals under examination were obtained by placing a bead of the chloride or other salt of the metal on a platinum wire, between two platinum electrodes, from which the spark of a powerful induction coil could be passed. In order to obtain a more intense, and therefore a hotter, spark than can be got from the coil alone, the coatings of a Leyden jar were placed in connexion with electrodes of the secondary current respectively. When this arrangement was carefully adjusted, the two yellow sodium lines were observed to be separated by an apparent interval of two millimetres, as seen at the least distance of distinct vision.

The position of the blue line, or rather blue band, of lithium was then determined with reference to the fixed reflecting scale of Steinheil's instrument, by volatilizing the carbonate of lithium in the first place on a platinum wire between platinum electrodes, and secondly on a copper wire between copper electrodes. A bead of pure chloride of strontium was then placed on new platinum and copper wires between two new platinum and copper electrodes, and the position of the blue line Sr & read off