minerals, as is the case, for example, with strontium and chromium, and tungsten. Of course it is difficult to appraise the relative abundance in Nature of different elements; more especially from the circumstance of those which are put to commercial uses being everywhere sought for, and those not put to commercial uses being habitually neglected—save indeed by the man of science, to whom the peculiar properties of some of the less familiarly known elements, as palladium, osmium, erbium, didymium, uranium, and thallium, render them objects of the highest interest.
A very notable point with regard to the last-discovered four elements, namely, rubidium, cæsium, thallium, and indium, is their successive discovery within a few years of each other, by one and the same process, namely, that of spectrum analysis. This process, invented and made available as a means of chemical research by Bunsen and Kirchhoff in 1859, consists simply in allowing the light given off by different ignited gases and vapors, limited by means of a fine slit, to pass through a prism or succession of prisms; and in observing the so-produced, brightly-colored, widely-extended image of the slit. It has been known from the days of Newton, that, by the passage of heterogeneous light through a prismatic, highly-dispersive medium, its differently refrangible constituents become widely separated from each other, so as to furnish an elongated, colored spectrum. But, whereas the spectra of incandescent solid and liquid bodies are continuous, and not distinctive of the particular luminous bodies yielding them, the spectra of incandescent, gaseous, or vaporized bodies, are found to be discontinuous, and to consist of one or more bright lines of different color, thickness, and position, according to the nature of the particular incandescent gases or vapors from which the light through the slit is proceeding. In this way it is found that the spectra of the different chemical elements, alike when free and in combination, are perfectly definite, and characteristic of the particular elements vaporized and made incandescent.[1] And, in many cases, the spectra, or portions of the spectra of particular elements, even when present in the most minute proportion, are so extremely well marked and distinctive, that the presence or absence of these elements is determinable with the greatest ease and certainty, by a mere inspection of the emission spectra yielded by the incandescent gases or vapors under examination. Moreover, gases and vapors are further capable of affecting heterogeneous light which is passed through them; and of thus yielding absorption spectra, in which the characteristic lines of the above-described emission spectra are reversed, so as to appear, unaltered in position, as black lines or intervals in an otherwise continuous band of color.
Now, the salts of the alkali-metals, lithium, sodium, and potassium,
- ↑ For some qualifications of this statement, vide Roscoe's "Spectrum Analysis."
