178 PROCEEDINGS OF THE AMERICAN ACADEMY. 



tiveness of the method could have been increased still further if 

 necessary. Since even 0.05 percent of neodymium would raise the 

 atomic weight of praseodymium by only 0.002 unit, it is obvious that 

 the purity of the material so far as neodymium is concerned is amply 

 sufficient. 



Attempts were made also to detect neodymium by means of the 

 spark spectrum between copper electrodes with the quartz specto- 

 graph, but the absence of strong neodymium emission lines at points 

 in the praseodymium spectrum which are comparatively free from 

 lines prevents this method from being at all satisfactory. 



At the other end of the series the detection of cerium was under- 

 taken. Since lanthanum ammonium nitrate is even less soluble than 

 the corresponding cerium salt, the absence of cerium in the praseo- 

 dymium material is sufficient proof of the absence of lanthanum also. 

 In the spark spectrum of cerium fortunately there is a strong line of 

 wave length 306 located at a point in the praseodymium spectrum 

 which is comparatively free from lines. By photographing the spec- 

 trum of the spark, between copper electrodes, of praseodymium mate- 

 rial originally free from cerium (fraction 4380) but diluted with known 

 percentages of cerium, it was found that the limit of detection of 

 cerium in this way lay between 0.5 and 1.0 percent. Assuming the 

 atomic weights of cerium and praseodymium to be 140.3 and 140.9 

 respectively, one percent of cerium, if in the trivalent condition, would 

 lower the atomic weight of praseodymium by 0.006 unit. Such a 

 difference is difficult to detect by the method which we are using for 

 the determination of the atomic weight of praseodymium. Upon 

 photographing the spark spectra of the extreme fractions 4361, 4364 

 and 4368 (the most impure analyzed), it was found by comparison 

 that the first fraction of the three was rich in cerium, the second con- 

 tained much less, and fraction 4368 contained no more at any rate 

 than 1 percent. 



Because of the uncertainty in estimating proportions of impurity 

 from the intensity of the spectrum lines, fractions 4365, 4368, and 

 4371 were further tested for cerium as follows: The solution was 

 precipitated with an excess of sodium hydroxide and the precipitated 

 hydroxides were washed several times. Carefully scrubbed chlorine 

 gas was next passed into the solution in order to dissolve the prase- 

 odymium hydroxide. The residual eerie hydroxide was dissolved 

 and the process was repeated. The second residue was collected 

 upon a filter paper, washed and ignited. Then the dissolved praseo- 

 dymium was precipitated as oxalate and ignited to oxide which was 



