In 1859, Kirchhoff, while engaged in observing the solar spectrum, lighted on the discovery that a certain double dark line, which had already been found to correspond exactly in position with the double bright line forming the spectrum of the glowing vapor of sodium, was intensified when the light of the sun was allowed to pass through that vapor. This at once suggested the idea that the presence of this dark line (or, rather, pair of dark lines) in the spectrum of the sun is due to the existence of the vapor of sodium in the solar atmosphere, and that this vapor has the power of absorbing the same order of light-waves as it emits. It would of course follow from this that the other dark lines in the solar spectrum are due to the presence of other absorbent vapors in its atmosphere, and that the identity of these would admit of being established in the same way, supposing this general law to hold, that a vapor emits the same light-waves that it is capable of absorbing. Kirchhoff was soon able to confirm his views by a variety of experiments. The general principles to which his researches led—in other words, the principles which form the basis of spectrum analysis—are as follows: 1. An incandescent solid or liquid gives a continuous spectrum. 2. A glowing vapor gives a spectrum of white lines, each vapor having its own set of bright lines, so that, from the appearance of a bright-line spectrum, one can tell the nature of the vapor or vapors whose light forms the spectrum. 3. An incandescent solid or liquid shining through absorbent vapors gives a rainbow-tinted spectrum crossed by dark lines, these dark lines having the same position as the bright lines belonging to the spectra of the vapors; so that, from the arrangement of the dark lines in such a spectrum, one can tell the nature of the vapor or vapors which surround the source of light.[24] The application of the new method of research to the study of the solar spectrum quickly led to a number of most interesting discoveries. It was found that, besides sodium, the sun’s atmosphere contains the vapors of iron, calcium, magnesium, chromium, and other metals. The dark lines corresponding to these elements appear unmistakably in the solar spectrum. There are other metals, such as copper and zinc, which seem to exist in the sun, though some of the corresponding dark lines have not yet been recognized. As yet it has not been proved that gold, silver, mercury, tin, lead, arsenic, antimony, or aluminium exist in the sun—though we can by no means conclude, nor indeed is it at all probable, that they are absent from his substance. The dark lines belonging to hydrogen are very well marked indeed in solar spectrum, and, as we shall see presently, the study of these lines has afforded most interesting information respecting the physical constitution of the sun.