in the tube. I can vary the experiment by taking Geissler's tubes (Fig. 36), containing these gases only in very minute quantities, so that the electric discharge can pass through a longer capillary column of gas: we then find that the small quantity of gas in the exhausted tubes becomes heated up to incandescence, and gives off its peculiar rays in a line of brilliantly coloured light.

I have here a hydrogen vacuum tube, next a tube containing a carbonic acid vacuum, then one containing nitrogen, then one containing chlorine, then one containing iodine. I have only to connect these with the induction coil, and the discharge will pass through the whole of these tubes; and at once you see the variety

of bright colours obtained, entirely due to the small traces of the various gases which are here present in the tubes. If we examine the character of these lights by means of the spectroscope, we shall obtain the peculiar and characteristic spectra of each of these gases.

Here are some large tubes, in which we can see the same effects of the ignition of the small quantities of these various gases by means of the electric spark (Figs. 37, 38); and you observe the beautiful striated appearance which the light exhibits-a phenomenon which physicists are at present quite unable to explain.

I regret that it is impossible to exhibit the spectra of these luminous gases on the screen, owing to the slight

intensity of the light which they emit. I must ask you to be content with my references to diagrams to explain to you the exact character of the light which these gases give off.

Thus, when we examine the peculiar red colour which this hydrogen-tube exhibits, we find that the spectrum consists of three distinct bright lines; one bright red line so intense as almost to overpower the others, one bright greenish-blue line, and one dark blue or indigo line. These are exhibited to you in the diagram. (See fig. of hydrogen spectrum, No. 8 on the chromolith. plate facing Lecture VI.) The bright red hydrogen line is always seen when an electric spark is passed through moist air:

this is due to the decomposition of the aqueous vapour which the air contains. If the air be carefully dried by passing it over hygroscopic substances, the red line disappears. Hence the spectroscope can be made a means of testing the presence of moisture.

A very remarkable fact, and one to which I shall have frequently to refer in the subsequent lectures, is that these three lines of hydrogen are found to be coincident with three well-known dark lines in the sun, of which I spoke to you in the first lecture. This red hydrogen line possesses exactly the same degree of refrangibility as the dark line c in the solar spectrum; the green hydrogen line corresponds to the well-known solar line F; whilst

the blue hydrogen line is identical in position with a dark line near G in the sun's spectrum.1 We shall see in a subsequent lecture how such coincidences point out to us the existence of hydrogen and other elements in the solar atmosphere.

FIG. 38.

When a spark is passed through the air, the lines of both nitrogen and oxygen are seen. This air spectrum has been carefully mapped by Dr. Huggins, who employed it as a scale to which to refer the metal lines in his drawings. He observed the lines simultaneously given off from two sets of poles, one set being of gold and the other set of platinum (in order to eliminate any confusion arising from the presence of metal lines): and he took those lines which were common to both these spectra as being those due to the components of the air. The spectrum thus obtained remains perfectly constant with reference to the position and relative characteristics of its lines when other metals are employed as electrodes. It is, however, found that the air spectrum varies as a whole in distinctness according to the volatility of the

1 Ångström maps a dark line in the violet portion of the solar spectrum, and termed by him (h), as coincident with a fourth hydrogen line, which is not seen unless the gas be heated to a very high temperature.

metal used as poles, the air being more or less replaced by metallic vapours in the neighbourhood of the electrodes. The bright hydrogen lines due to aqueous vapour are, as I have said, seen when the spark is passed through moist air, whilst the spectrum of lightning, as examined by Grandeau and Kundt, has been found to exhibit in addition the nitrogen and hydrogen spectra, also the bright yellow sodium line.

The nitrogen spectrum is more complicated than that of hydrogen, but still perfectly definite and characteristic. (See No. 9 on the chromolith. plate in Lecture VI.)

Some very singular observations have been made by Plücker and Hittorf1 upon certain changes which the spectra of highly rarefied gases undergo. They find that the spectrum of highly rarefied nitrogen undergoes a change when the intensity of the electric discharge varies ; and they explain this by supposing that nitrogen can exist in various allotropic conditions, resembling for instance oxygen and ozone, their idea being that the changes in the intensity of the electric discharge may cause changes in the allotropic condition of the nitrogen, and that thus a variation in the appearance of the spectrum may be produced. These variations, however, it is important to observe, are not noticed in nitrogen gas when under the pressure of the atmosphere, however much we may increase the intensity of the spark; and from some experiments to which I shall have again to refer, it would appear that the band spectrum of nitrogen is only seen when traces of oxygen are present, so that pure nitrogen has in reality only one spectrum of bright lines, the band spectrum being due to the oxides of nitrogen (see Appendix A). Plücker has 1 Plücker and Hittorf, Phil. Trans. 1865, p. 1.

M

also noticed that under certain conditions of increased electrical tension the fine lines of hydrogen are seen to become broader and broader, until at last the hydrogen gas may be made to emit light of every degree of refrangibility, so that its spectrum becomes continuous.

These variations which the hydrogen spectrum, as obtained in Geissler's tubes, undergoes when the density of the gas on the one hand and the intensity of the spark on the other are altered, have been carefully examined by Huggins, as well as by Lockyer and Frankland, and by Wüllner. Still the explanation of the so-called double spectrum of nitrogen renders it possible that these differently constituted hydrogen spectra may be caused by some differences, as yet unperceived, in the chemical nature of the gas which is operated upon, and we shall see in the sequel that this possibility is rendered a probability by independent experiments. It is, however, not improbable that the fact of this broadening of the hydrogen lines may be found to possess important bearings upon the conclusions which spectrum analysis enables us to draw concerning the physical condition of the sun and fixed stars. To this point I will, however, direct your attention on a future occasion.

In the same way each of the non-metallic elements yields a characteristic spectrum when its vapour is heated to incandescence; but in the case of some of the elements, such as silicon, the difficulty of obtaining the spectrum is very great.

In his original memoir on the spectra of the chemical elements Kirchhoff plainly points out that under varying conditions of density, temperature, and thickness of the layer of incandescent gas, the spectrum of the same body must vary in its appearance, some of the lines coming