Now, other kinds of light besides sunlight can be analysed. Light from any substance which has been made incandescent may be observed with the spectroscope in the same way, and each element can be thus separated. It is found that each substance (in the same conditions of pressure, etc.) gives a constant spectrum of its own. _Each metal displays its own distinctive colour. It is obvious, therefore, that the spectrum provides the means for identifying a particular substance._ It was by this method that we discovered in the sun the presence of such well-known elements as sodium, iron, copper, zinc, and magnesium. [Illustration: _Yerkes Observatory._ FIG. 9.--THE GREAT SUN-SPOT OF JULY 17, 1905] [Illustration: _From photographs taken at the Yerkes Observatory._ FIG. 10.--SOLAR PROMINENCES These are about 60,000 miles in height. The two photographs show the vast changes occurring in ten minutes. October 10, 1910.] [Illustration: _Photo: Mount Wilson Observatory._ FIG. 11.--MARS, October 5, 1909 Showing the dark markings and the Polar Cap.] [Illustration: FIG. 12.--JUPITER Showing the belts which are probably cloud formations.] [Illustration: _Photo: Professor E. E. Barnard, Yerkes Observatory._ FIG. 13.--SATURN, November 19, 1911 Showing the rings, mighty swarms of meteorites.] Every chemical element known, then, has a distinctive spectrum of its own when it is raised to incandescence, and this distinctive spectrum is as reliable a means of identification for the element as a human face is for its owner. Whether it is a substance glowing in the laboratory or in a remote star makes no difference to the spectroscope; if the light of any substance reaches it, that substance will be recognised and identified by the characteristic set of waves. The spectrum of a glowing mass of gas will consist in a number of bright lines of various colours, and at various intervals; corresponding to each kind of gas, there will be a peculiar and distinctive arrangement of bright lines. But if the light from such a mass of glowing gas be made to pass through a cool mass of the _same_ gas it will be found that dark lines replace the bright lines in the spectrum, the reason for this being that the cool gas absorbs the rays of light emitted by the hot gas. Experiments of this kind enable us to reach the important general statement that every gas, when cold, absorbs the same rays of light which it emits when hot.