Of all the various spectra which the progress of experimental science has enabled man to observe in the present day, none are so rich, varied, and important, as those of gases. And no wonder! for it is only when matter has been reduced to the gaseous condition, that it is able to specialise itself and write its character with much of its history in any otherwise smooth, undefined, continuous spectrum; while, if in former times, men would have found it an impossibility to drive many of the more refractory substances into the state of incandescent vapour, what is there now anywhere on the surface of this earth which, in small quantity, can resist the action of a powerful and condensed induction spark of electricity; and what application of that spark is so neat, elegant, convenient, and economical, as when it is employed in conjunction with so-called gas-vacuum tubes.