[ 202 ] 



XV. 



ON THE VACUUM TUBE SPECTRA OF TPIE VAPOURS OF 

 SOME METALS AND METALLIC CHLORIDES. Paht L— 

 CADMIUM, ZINC, THALLIUM, MERCURY, TIN, BISMUTH, 

 COPPER, ARSENIC, ANTIMONY, AND ALUMINIUM. 



By JAMES H. POLLOK, D.Sc, 

 Royal College of Science, Dublin. 



(Plates XV. and XVI.) 



[PutUsted February 21, 1912]. 



Since the researches of Dr. J. Pliicker, of Bonn, and Dr. J. W. Hittorf, of 

 Miinster,' in 1864, very little work has been done in the examination of the 

 vacuum tube spectra of the vapours of metals and their compounds ; and 

 writers on the subject invariably refer to the difficulties and uncertainties 

 of observations on vacuum tube spectra, for impurities show their spectra 

 with surprising persistency, and many unexpected phenomena manifest 

 themselves. 



In view of the growing importance of this class of spectra, in the 

 discovery and identification of new gases, and the investigation of the pro- 

 ducts of the action of radium and radio-activity, it seemed desirable to make 

 a systematic examination of the vacuum tube spectra of as many substances as 

 possible. Some years ago I began this work by photographing and examining 

 the spectra emitted by a large number of old tubes in the collection of the Royal 

 College of Science. These tubes were, I believe, made by Mr. C. F. Cassalla 

 many years ago, and are, no doubt, precisely the same as those used by Professor 

 C. Piazzi Smyth in his investigations " On the Measurements of Gaseous 

 Spectra " in 1884, and were very likely made at the same time — the gases 

 being prepared and purified in the manner described in the above paper. 

 The results were very unsatisfactory, most of the spectra showing 

 the banded spectrum of nitrogen, the bands attributed to cyanogen, 

 or the lines of hydrogen, with or without the bands of water-vapour, together 

 with all, or some of, the lines or bands due to the elements contained in 

 the compound in the tube ; but I did not find marked indications of definite 



' Phil. Trans., vol. civ, pp. 1-29. 1864, 



