igoS.] NEODYMIUM AND PRASEODYMIUM. 277 



The Nernst filament was found to be the most satisfactory 

 source of Hght from the extreme red to the beginning of the ultra- 

 violet. It is sufficiently brilliant to require an exposure of only a 

 minute, but practically ceases at about A 3200. For wave-lengths 

 shorter than this some spark spectrum must be used. 



The cadmium-zinc spark used by Jones and Uhler was fairly 

 satisfactory, especially in the extreme ultra-violet, but has the draw- 

 back that there are present a limited number of very intense lines, 

 on a rather faint continuous background. We tried to obtain a 

 spark spectrum having a very large number of lines, but with no 

 lines of very great intensity. We found that tungsten, molybdenum 

 and uranium all satisfied these requirements. 



The terminals finally used were prepared by dipping pieces of 

 carbon in a concentrated solution of ammonium molybdatc, and 

 then heating in a bunsen burner. They were then dipped into a 

 solution of uranium nitrate and similarly heated. 



The coil used to produce the spark was a large Rontgen X-ray 

 coil. 



Making a Spectrogram. 



In making a spectrogram consisting of seven photographic strips, 

 the following mode of procedure was adopted : Seven separate solu- 

 tions were made up of the desired strengths. The celF to be used 

 was filled to the required depth with the most concentrated solution 

 of the series, and the quartz plates determining the depth of the 

 solution adjusted to parallelism. The exposure to the Nernst lamp 

 was then made, being usually one minute long. An opaque screen 

 covering up the visible spectrum as far down as A 4000 was then 

 interposed between the grating and the photographic film, and the 

 exposure to the light of the spark in the ultra-violet made. The 

 duration of this exposure was usually about two minutes. The 

 photographic film was then moved into the proper position for the 

 next exposure. The above series of operations was then repeated 

 for each of the succeeding strips. 



After the film had been exposed for each solution and the spark 

 spectrum impressed, it was necessary to make a similar series of 



" See Carnegie Publication No. 60. 



