These are some of the most striking results of the numerous investigations that have been made since M. Kirchhoff published his discoveries, for the subject is anything but exhausted.
The intensely vivid light of a magnesium flame is rich in violet and extra-violet rays, partly due to the incandescent vapour of magnesium, and partly to the intensely heated magnesia formed by the combustion. The properties of this light having been examined and compared with those of the sun by Professors Roscoe and Bunsen, with a view to photographic purposes, they came to the conclusion that ‘the steady and equable light evolved by magnesium wire, burning in the air, and the immense chemical action thus produced, render this source of light valuable as a simple means of obtaining a given amount of chemical illumination, and that the combustion of this metal constitutes a definite and simple source of light for the purpose of photochemical measurement.’
Bright lines of two different metals sometimes coincide with the same black line, that is, they appear to have the same reverse as an iron and a magnesian line, an iron and a nickel line, and some others; but it is not known whether the coincidence be real or apparent.
M. Kirchhoff has proved that neither gold, silver, tin, lead, antimony, arsenic, mercury, lithium, cadmium, and some others are constituents of the sun, because none of their bright lines are coincident with any of the dark lines of the solar spectrum. This negative discovery does no less honour to M. Kirchhoff than the proof of so many substances being common to the earth and sun.
Since all incandescent solid and liquid bodies give a continuous spectrum which exhibits no dark lines, M. Kirchhoff conceives that the sun consists of a solid or liquid nucleus, heated to the temperature of the most dazzling whiteness, and that it is surrounded by a luminous gaseous atmosphere of somewhat lower temperature, endowed with the law of exchanges. The spectra of Arcturus, Capella, and many other fixed stars are crossed by dark lines similar to, and often coincident with, the dark lines in the solar spectrum; therefore, it may be concluded that their structure is to a certain extent the same with that of the sun.