BASQUIN. — THE ARC SPECTRUM OP HYDROGEN. 163 



Observations of Hydrogen Lines. 



The arc spectra of the following metals in hydrogen have been ex- 

 amined : Aluminium, copper, magnesium, coin-silver, sodium, tin, and 

 zinc. With the exception of sodium the arc of each metal shows to the 

 eye very clearly the H a and ILj lines of hydrogen, and in most of them 

 the H ? line comes out with the small instrument very clearly, and in- 

 distinctly with the large one. The H 5 line shows only rarely, and then 

 to the eye rather indistinctly. The II a line is quite sharp and well de- 

 fined, unless the electric current through the arc is unusually great ; it 

 has much the same appearance as the zinc line at 6363. The other three 

 are always broad, hazy, and ill-defined. 



On the photographs taken with the large spectroscope H^ and II V 

 usually show very plainly, always excepting the spectrum of metallic 

 sodium, while H5 shows in spectra of tin, silver, and copper. On 

 photographs taken with the small spectroscope -these lines show more 

 sharply, on account of the very much smaller dispersion, and the photo- 

 graphs of tin show the next hydrogen line, H e quite clearly. Not hav- 

 ing found the hydrogen lines in the metallic sodium arc (using copper as 

 stationary electrode), I tried it in dry hydrogen also, thinking that in 

 some way the water vapor might have affected the appearance of the 

 hydrogen lines, but I have been unable to detect any of the hydrogen 

 lines in that arc in any way. 



None of these lines excepting H a is sharply defined. A wide space 

 in the middle of each line has fairly uniform intensity, shading off gradu- 

 ally and uniformly to each side. The following table gives a rough 

 estimate of widths, in Angstrom units, of these lines as they appear on 

 the photographic plates, the middle of the shading being taken as the 

 edfre of the line. 



Line. 



H a 



H y 



Hs 



II e faint, same general width. 



It will be noticed that these lines, with the exception of TT ai are exces- 

 sively wide, and I think it is for this reason alone that I have been 

 unable to photograph the still weaker hydrogen lines of Balmer's series. 



