School of Freiberg. It also was detected by its peculiar spectrum, which consists simply of two indigo-coloured lines.1 (See Frontispiece, No. 6.) It was discovered in certain zinc-works in the Hartz, and has only been found in small quantities. Its chemical characters are still imperfectly understood; but in its properties it appears to stand about half-way between zinc and lead. Its combining weight is 378, and its specific gravity 7·277; and it forms definite compounds, which, however, have not yet been examined with a sufficient amount of attention to enable me to give you a detailed account of them.

I can here show you the indigo colour which indium compounds impart to the flame; and you now see on the screen that the spectrum of indium consists of two bright indigo-coloured lines, one situated in the blue and one in the ultra-blue, or indigo, portion of the spectrum. I need scarcely say that there is as yet no notion of any practical employment of any of these new substances, though chemists never can tell what important applications of their most recondite discoveries may not arise, even in the immediate future.

I would next endeavour to show you, that not only substances such as the metals of the alkalies and alkaline earths, when heated up to a gaseous state, give off a light peculiar to themselves, but that bodies which are invisible to the ordinary eye, such as atmospheric air, or nitrogen, oxygen, and carbonic acid, -that substances which are gaseous at the ordinary temperatures can also

1 These lines are best seen when a bead of an indium compound is held between two electrodes from which a spark passes. The lines In a and In ẞ fall respectively upon divisions 107.5 and 140 mm. of the photographic scale of the spectroscope, when Na a 50 and Sr = 100.5.

be made to give off a peculiar light when heated up by means of the electric spark.

When we pass the electric spark through a gas, it becomes heated up to a temperature far higher than anything we can obtain by means of flames; and when thus heated the gas gives off the light which is peculiar to itself. Thus we find that, according to the nature of the atmosphere which surrounds the spark, the colour of that spark varies.

If we seal up a quantity of hydrogen gas, of carbonic acid gas, of nitrogen gas, in separate tubes, and allow an electric spark to pass through these tubes (see Fig. 27), the spark which passes through the hydrogen has a red colour, and that which passes through the nitrogen has a yellow colour; while that which passes through the carbonic acid gas has a blue colour: and these differences of colour are due simply to the effect of the gas enclosed in the tube. I can vary the experiment by taking Geissler's tubes (Fig. 28) 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 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

+

FIG. 28.

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.

I have here some large tubes, in which are seen the same effects of the ignition of the small quantities of these various gases by means of the electric spark (Figs. 29, 30), and you observe the beautiful striated appearance which the light exhibits.

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 hydrogen 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 greenishblue 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

FIG. 29.

passing it over hygroscopic substances, the red line disappears. Here 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 the dark line & in the sun's spectrum. 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.

The nitrogen spectrum is more complicated than that of hydrogen, but 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 gases enclosed in these tubes undergo. They find that the spectrum of highly rarified nitrogen undergoes a change when the intensity of the electric discharge varies; and they explain this by supposing that the nitrogen exists 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 conditions of the nitrogen, and that these give rise to a variation in the appearance of the spectrum. 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. Plücker has even found that under certain conditions of increased electrical tension hydrogen 1 Plücker and Hittorf (Phil. Trans. 1865, p. 1).

FIG. 30.