822 Mr. H. Smith on the Spectroscopy of the Electric 



negative brush only. Calcium and sodium are strongest 

 quite close to the point. In the case o£ a decayed point the 

 calcium lines (notably H, K, and g) and the sodium D line 

 are the only lines that occur right up to the point (see fig. 6 b). 

 They are probably due, to some extent at least, to the calcium 

 and sodium in the glass which is used in the construction of 

 the point. The silicon lines, which occur chiefly in the far 

 ultra-violet, are often much stronger than the neighbouring 

 platinum lines and occasionally much weaker. It is im- 

 possible to predict when and in what strength they will 

 appear, beyond the facts that they are weakest in distilled 

 water and that they never occur at the positive point. In 

 one case, where a quartz platinum instead of a glass platinum 

 point was used, the ultra-violet band spectrum of silicon, dis- 

 covered by de Gramont & Wattville *, was observed. Only 

 a few of the fainter bands were missing. The lead spectrum, 

 nearly all the lines being in the ultra-violet, was observed in 

 the case of acids. The lines present were those shown by 

 Exner & Haschek to be very powerful in the arc f. They 

 also occur in the flame spectrum, but the bands characteristic 

 of the flame spectrum were absent in that of the brush 

 (PL XI. fig. 6). 



Acids. 



Of the acids I have tried, hydrochloric and nitric acids 

 have no effect on the spectrum as far as their constituents are 

 concerned. In sulphuric and phosphoric acids there occurs 

 in the discharge a luminous bubble which yields a spectrum 

 connected in all probability with sulphur and phosphorus 

 respectively. In chromic and molybdic acids the spectra of 

 chromium and molybdenum occur at both points. The same 

 remarks apply to the salts of these acids as far as the acid 

 radicle is concerned. 



Summary. 



The series and secondary spectrum of hydrogen are a great 

 deal brighter in distilled water than in solutions, for the same 

 current value. In distilled water they are stronger in the 

 negative brush than in the positive for the same current. When 

 the potential drop through the brush is the same in each case, 



* Comptes Rendus, vol. cxrvii. p. 239 (1908). 

 t Kayser's H. der Spectroscopic, yol. vi. p. 268. 



