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



In solutions the spectrum is often free from any con- 

 tinuous background ; on the other hand, the background is 

 occasionally for a few seconds very bright, especially in the 

 case of the negative brush. During the " bubble formation " 

 at the negative point a continuous background is always 

 developed. 



Hydrogen. 



The hydrogen spectra appear best in distilled water. H a , 

 Hp, H v are very bright, while Hs and H e , though readily 

 discernible, are much fainter. Relative to the series lines the 

 secondary is much fainter than is the case for the spectrum 

 of dry hydrogen in a Geissler tube. For the same current 

 through the brush, the hydrogen lines are a little weaker 

 at the positive than at the negative point (fig. 4 a, b). In 

 this case, the potential is much greater for the negative brush 

 than for the positive. If the potential is the same in each 

 ease, the current through the positive brush is much the 

 larger and the hydrogen lines brighter than in the negative 

 brush (fig. 3). The intensity of the series lines falls off 

 more rapidly towards the violet in the case of the negative 

 brush than in the case of the positive. 



As the water increases in conductivity the potential falls 

 rapidly, even with a large increase in current, and the 

 hydrogen lines decrease rapidly in intensity. In solutions, 

 the spectrum is generally too weak to show any trace of the 

 secondary spectrum. When each brush has changed over to 

 the solution type, it is found that for the same current 

 through the brush the hydrogen lines are of about equal 

 intensity in each spectrum, although the potential is greater 

 for the positive brush. This is the case no matter whether 

 the solution is acid, alkaline, or contains some neutral salt. 

 In very weak solutions, where one brush retains the form 

 characteristic of the brush in distilled water, then, no matter 

 what the sign of the brush, the hydrogen lines are stronger 

 in it than in the brush which has changed over to the 

 solution type. 



In PL XI. fig. 7 are given reproductions of three spectra of 

 the negative brush, taken on the same plate and given the 

 same exposure. The brush was produced in distilled water, 

 dilute sulphuric acid, and a solution of caustic potash. The 

 hydrogen lines are of about equal intensity in each. In the 

 two solutions, it will be seen that about the same current and 

 a low potential are required for each ; while in distilled water 

 a much smaller current with a larger potential gave the same 

 intensity of spectrum. 



