568 



few characteristic lines stand out very clearly (e. g. the green Tl- 

 line; the three green Cu-lines). If tiie acid contains two kinds of 

 metalions, often only one of these kinds of ions can be detected 

 spectroscopically. An example of this is furnished by the following 

 experiment : 



A platinum anode was immersed in sulphuric acid 1 : 40, which 

 contained a sufficient quantity of sulphate of sodium and sulphate 

 of copper. First so much current was passed through that the anode 

 wire became incandescent; then gradually resistance was inserted 

 until the incandescence stopped and the characteiistic yellow lumi- 

 nescence appeared. Only a very strong Na-D-line was observed then 

 in the spectroscope. When gradually still more resistance was put 

 in, the yellow luminescence and the Na-D-line became fainter and 

 fainter, and the Cu-lines began to appear {in the green). At a definite 

 terminal voltage green sparks were also immediately to be observed 

 by the side of the yellow sparks at the anode. 



It is exceedingly diflicult to elucidate the nature of these very 

 faintly luminous phenomena experimentally. It can only be stated 

 that the luminescence appeais to be caused by numerous sparks, 

 and that there is undoubtedly a gas-envelope present also here, as 

 already Voller and Walter pointed out. Very piobably a similar 

 mechanism is to be supposed here as in Lecoq de Boisbaudran's "ful- 

 gurator". In this apparatus we have a layer of gas and vapour 

 between anode and electrolyte, through which the sparks penetrate. 



§ 7. 2. A beautiful and very intense anodic arc-spectrum can be 

 obtained with an iron rod in hot concentrated or diluted sulphuric 

 acid (sp. gr. 1.80 and H,SO< 1 -.4); less easily by means of tungsten 

 anodes in the same medium Then the temperature of tiie anodes 

 is prettj' high; the colour of the emitted light is a brilliant blue. 

 The tension in these experiments was 225 Volts. The emission did 

 not appear until the luminescence described under 3 had been 

 observed for a shorter or longer time. We shall, therefore, have to 

 return to the said phenomenon presently. 



3. A very peculiar light phenomenon is observed when the current 

 is closed by immersion of a carbon- or metal-anode in concentrated 

 or diluted sulphuric acid. The carbon then gets covered by a beautiful 

 yellow mantle of light, which continues to persist for a long time; 

 the carbon surface gets smooth, carbon powder and superficial impu- 

 rites are removed. Metal anodes present an analogous behaviour, as 

 was by observed by v. Bolton (8), to wliom we owe a method by 

 this procedure for polishing and cleansing carbon electrodes. (14). 



