After the position of the spectral lines of most of the elements had been discovered, Messrs. Bunsen and Kirchhoff were one day examining the saline deposit of a spring which issues from the earth near Durkheim, in the Palatinate, and were surprised to find that a blue line belonging to no known metal made its appearance in addition to the potassium, sodium, and other lines produced by the saline ingredients of the water. These philosophers immediately concluded that the unknown line was caused by an unknown metal, and they at once set to work to obtain a larger quantity of the saline residue from the spring. They evaporated down no less than forty tons of water, and succeeded in isolating the new substance, which turned out to be a metal resembling potassium. While examining the residue more carefully, a new, dark _red_ line, beyond that belonging to potassium, was discovered, pointing to the existence of a second new element, which was also afterwards obtained in the pure state. These two new metals, which closely resemble potassium in their properties, were named in accordance with the lines given by them when converted into luminous gas. The first was called cæsium, from _cœsius_, Lat. light blue; and the other, rubidium, from _rubidus_, Lat. dark red. Since the publication of MM. Bunsen and Kirchhoff’s experiments, these two elements have been found in comparatively large quantities in various minerals, and these properties have been closely studied. Spectrum analysis has yielded us two more new metals since first these philosophers applied the prism to the determination of the chemical composition of various bodies. Mr. W. Crookes, F.R.S., an English chemist of eminence, while examining the flame of a deposit obtained during the manufacture of sulphuric acid from a certain sulphur mineral found in the Hartz mountains, perceived a brilliant green line with which he was previously unacquainted, which quickly flashed into view, and then disappeared. After numerous experiments on various other minerals (for the deposit he had first experimented upon only yielded him a few grains of the new body), Mr. Crookes succeeded in discovering a comparatively large quantity of it in a sulphur mineral found in Belgium. The new element was found to be a heavy metal, closely resembling lead in its properties. It was named by the discoverer, thallium, from the Greek word _thallos_, a green twig, from the brilliancy of the single green line that indicates its presence. In like manner, Messrs. Reich and Richter have discovered a fourth new metal, which has been named _indium_, from its principal lines being found in the centre of the _indigo_ of the spectrum.