Mr. Wheatstone had perceived that the bright lines on the spectra of the metals are different and more complicated when taken in air than in vacuo, and Professor Angström made the important remark that the electric spark gives two superposed spectra, one due to any metal that may be under examination, the other to the incandescence of the air through which the spark passes. Hence the importance of the spectrum analysis of gaseous substances, especially of those which constitute our atmosphere, a subject that has been ably and successfully investigated by Professor Plücker. For that purpose he made use of the Geissler or vacuum tubes, similar to those he used in his experiments on the stratification of electric light. When electricity was sent through a tube containing oxygen gas, the gas combined so rapidly with the platinum of the negative terminal of the battery that there was little time to examine the spectrum. The electral light in the tube was too red at first, but as the attenuated gas gradually disappeared it changed through flesh-colour to green, then through blue to reddish-violet, and at length there was too little gas to convey the electricity. However, the oxygen spectrum has a remarkably bright red band at its red extremity, two bright orange lines divided by a black one in the orange, and some bright bands in the green. The electric light of attenuated hydrogen is red, and almost the whole light in its spectrum is concentrated into six bright bands of nearly equal breadths. There is a dazzling red band near the red end of the spectrum, which, however, does not coincide with the oxygen band; then comes a very beautiful yellow band, in which the whole of the yellow rays seem to be concentrated, followed by a grey interval which separates the yellow from three bright lines in the green, the first of which is yellowish green, the last a beautiful greenish blue; a black and a dark space separates the latter from the violet in which there is a bright line. The electric light in a tube containing highly rarefied aqueous vapour is red, the vapour is resolved into its simple elements by the electricity, the oxygen combines with the platinum of the negative or heat pole, and the spectrum is that of pure hydrogen with the three most prominent bands only. The nitrogen spectrum is brilliant with all the seven colours; there are no broad dark spaces like those which divide the bright bands in the hydrogen spectrum, but it is crossed by numerous very fine black and grey lines. Fifteen of the latter stripe the red and orange; the green is separated from the yellow by a black narrow band; it is terminated by two bright blue lines, and very fine dark lines cross it and the rest of the spectrum. The tube light is yellowish red.