78 
CHEMISTRY: BAXTER AND STEWART 
soluble end, until 174 series of crystallizations had been made. The 
number of fractions in each series rapidly increased to a fairly constant 
number between 20 and 25. In series 174 an examination of the absorp- 
tion spectra of the more soluble fractions showed that the extreme 
fraction contained a very small quantity of neodymlum, not more than 
0.05%, and that samarium was absent. At this point the seventh 
fraction from the more soluble end, 3474, was removed for analysis, since 
it seemed likely that this fraction was as pure as any in the series, and 
the more soluble fractions were rejected Fractional crystallization was 
then continued for 41 more series, without, however, rejecting any 
material at the more soluble end, although the least soluble fraction was 
occasionally discarded. The most soluble fraction of the 215th series, 
4383, upon careful examination of the absorption spectrum, was still 
found to be free from neodymium, although it was found possible by the 
absorption spectrum to detect less than 0.05% of the latter element. 
This result indicates that the other fractions also were free from both 
neodymium and samarium. The least soluble fraction, 4368, was 
analyzed for cerium by precipitating the hydroxide and treatment with 
chlorine, and was found to contain 0.4%. The atomic weight of cerium 
is so nearly like that of praseodymium that even this proportion would 
lower the observed atomic weight of praseodymium less than 0.003 unit. 
Fraction 4371 was found to contain only 0.1% of cerium. 
The quantitative examination of selected fractions of the last series 
was then undertaken. First, the double nitrate was converted into 
chloride. This was done by precipitating praseodymium oxalate, 
igniting the washed and dried product to oxide, dissolving the oxide in 
nitric acid, reprecipitating and igniting the oxalate, dissolving the oxide 
in hydrochloric acid in a quartz dish, and crystallizing the product from 
concentrated hydrochloric acid solution. The salt was then superficially 
dried in a vacuum desiccator 
In order to dry the salt for weighing, it was carefully dehydrated by 
efHorescence at gradually increasing temperatures in a current of dry 
hydrochloric acid gas. The salt was contained "n a weighed platinum 
boat, and this was placed n a quartz tube forming part of a Richards 
'bottling apparatus.'^ When essentially all the water had been expelled 
the temperature was raised rapidly until the salt was fused. After the 
salt had been allowed to cool the hydrochloric acid was displaced by dry 
nitrogen and this in turn by dry air. Then the boat was transferred to 
the weighing bottle without exposure to moisture, and it was weighed. 
Upon treating salt thus prepared with pure water, sometimes a very 
small proportion of insoluble matter is found. The proportion of this 
material depends upon the care with which the salt is first dehydrated 
