BAXTER AND STEWART. — PRASEODYMIUM CHLORIDE. 177 



520 being faintly visible. The proportion of neodymium in this frac- 

 tion we estimate to be at least as small as 0.05 percent. In fraction 

 3476 the same neodymium band could scarcely be detected. At this 

 point fraction 3474 was removed for analysis, since it seemed likely 

 that this fraction was as pure as any in the series 174. The fractions 

 3475-3480 were rejected. Fractional crystallization was then con- 

 tinued in a similar way with the remaining fractions, except that 

 while the extreme crystal fractions were occasionally rejected to re- 

 move cerium and lanthanum, no fractions were rejected from the 

 more soluble end. When the extreme fraction at this end became very 

 small, it was set aside to be added to a subsequent similar one, and 

 the fractionation then continued. After 41 more series of crystalliza- 

 tions the process was discontinued because very careful spectroscopic 

 examination of the extreme mother liquor, fraction 4383, in saturated 

 solution with a 10 cm. layer showed no sign of the neodymium absorp- 

 tion band X 520. The quantitative examination of selected fractions 

 was then undertaken. Those chosen were 3474, 4383, 4381, 4379, 

 4377, 4374, 4371 and 4368. Since the less soluble fractions beyond 

 4368 were believed to contain cerium, they were rejected. In fact, 

 an analysis of fraction 4368 showed it to contain about 0.4 of one 

 percent of this element, a proportion, however, which is hardly per- 

 ceptible in the atomic weight. 



The Purity of the Fractions of Praseodymium Material. 



The purity of the fractions in the final series was determined as 

 follows: In neither fraction 3474 nor the more soluble fractions of the 

 last series, 4383 and 4381, could any of the neodymium absorption 

 lines be detected, either visually with a Hilger wave length spectro- 

 scope or by photography with a Hilger quartz spectrograph of the 

 Fery type. With the latter instrument photographs were made with 

 various depths of solution and with widely varying exposures, but the 

 results were less satisfactory than the visual ones. In order to find 

 out what proportion of neodymium could be detected in praseodymium 

 material, measured portions of a standard solution of neodymium 

 ammonium nitrate were added to weighed portions of the double 

 ammonium nitrate of fraction 4367, which was as free from neody- 

 mium as any. By using concentrated solutions and a 10 cm. layer it 

 was found that 0.05 percent of neodymium could be detected with ease 

 through the absorption band X 520, and it was evident that the sensi- 



