Then if a hydrocarbon is present, the passage of a spark from a Ruhmkorff's coil will cause the appearance of a sky-blue light. Viewed with the spectroscope, this presents the spectrum of carbon, and generally so brilliant as to mask totally the spectra of other gases present. The rare metals cerium, lauthanum, and didymium have been lately investigated by Drs.

It tarnishes more easily and inflames less easily than cerium. Didymium resembles lauthanum. The metals were all obtained by electrolysis of the chlorides.