Equally characteristic and well-defined spectra, the bands in which have each an invariable and fixed position in the spectrum, are also produced when the coloured flames arising from heating to the requisite point the remaining salts of the alkalies and alkaline earths are examined by the prism. On the opposite page the first spectrum shows some of the fixed dark lines that are always observed when a solar beam is examined by the spectroscope. These lines are compared with the position of some of the more important bright lines furnished by the spectra of the metals of the alkalies and alkaline earths, when their chlorides are heated upon a loop of platinum wire introduced into the flame of a Bunsen gas-burner. The characteristic bright lines given by each metal are denoted by the letters of the Greek alphabet, the earliest letter indicating the most strongly marked lines.

In the potassium spectrum the most characteristic bright lines are the red line K α, and violet line K β. In the case of sodium nearly the whole of the light is concentrated on the intense yellow double line Na α. In the lithium spectrum a crimson band, Li α, is the prominent line; Li β is seldom visible, but at the elevated temperature of the voltaic arc an additional blue line becomes very intense. In the spectrum of cæsium two lines in the blue, Cs α and Cs β, are strongly marked. In rubidium the lines Rb α and Rb β in the blue, and Rb γ in the red are almost equally specific. Thallium is recognised by the intense green line Il α. The spectra of the metals of the alkaline earths are equally definite, though more complicated.

By means of the spectroscope quantities so inconceivably minute as the 33,000th of a grain of chloride of rubidium, the 170,000th of a grain of chloride of cæsium, the 2,500,000th of a grain of sodium, and the 6,000,000th of a grain of lithium, have been detected, and have revealed themselves to the sight by their characteristic bands in the spectrum. Hence it is that in making use of this branch of analysis the chemist has been enabled to show the universality of many elements hitherto regarded as being very sparingly distributed throughout the globe.

Thus lithium, which until lately was supposed to be one of the rare elements, has been found as a constituent of tea, tobacco, milk, blood, and in almost all spring waters. Furthermore, the prodigiously sensitive reactions afforded by the spectroscope have not only revealed the presence of infinitesimal quantities of known elements, but have led to the discovery of new ones which had escaped detection by the older and less delicate processes of analysis. It was by means of spectrum analysis that the two alkali metals, cæsium and rubidium, were discovered by Bunsen and Kirchkoff in 1860 in a mineral water at Durkheim, and that Mr Crookes in 1861 discovered the metal thallium in the deposit found in the flue of a pyrites furnace; whilst still more recently Messrs Reich and Richter, in a spectrum examination of a zinc ore from Freiberg, discovered the metal indium.

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The most brilliant spectra are given by those salts which are the most easily volatilised, such as the chlorides, iodides, and bromides

of the different metals. But it is only the metals of the alkalies and alkaline earths that give spectra that are characteristic. When it is desired to obtain the spectra of the other metals, they may be raised to the requisite temperature by means of the electric spark, which in passing through the two points of the metal operated upon volatilises a minute quantity of it, and thus enables it to emit its particular light. The electric sparks are best obtained by means of Ruhmkorff’s coil. Thus each metal may be made to yield a spectrum which specially belongs to it, and to it alone. When the electric discharge is sent through a compound gas or vapour, owing to the intense temperature generated separation of its constituents must take place, since the spectra produced are those of the elementary components of the gas. The permanent gases give each their peculiar spectrum when they are strongly heated, by which they may be recognised; thus the spectrum of hydrogen is composed of three bands, one being bright red, one green, and the other blue. Nitrogen gives a very complicated spectrum.

The accompanying figure exhibits a very complete form of the spectroscope adapted to a single prism.