CHEMISTRY: BAXTER AND STEWART 
77 
between specimens of lead of different geographical origin, and (3) that 
the atomic weight of lead referred to silver as 107.880 is very nearly 207.20. 
We are very greatly indebted to the Carnegie Institution of Wash- 
ington for generous assistance in the pursuit of this investigation.^ 
^ Proc.Amer. Acad., 4:i,2>6S {1907); J. Amer. Chem.Soc.,30, 187 ;ZeiL anorg. Chem., 57, 174. 
2 See, Richards and Lembert, /. Amer. Chem. Soc, 36, 1329 (1914); M. Curie, C. R. Paris 
Acad. ScL, 158, 1676 (1914); Honigschmid and St. Horowitz, Ibid., 158, 1796 (1914); Soddy 
and Hyman, /. Amer. Chem. Soc, 105, 1402 (1914). 
3 This work will be described in detail before long in Proc. Amer. Acad. Arts S:i., 
Boston, and in /. Amer. Chem. Soc. 
A REVISION OF THE ATOMIC WEIGHT OF PRASEODYMIUM 
By Gregory Paul Baxter and Olus Jesse Stewart 
COOUDGE MEMORIAL LABOFIATORY, HARVARD UNIVERSITY 
Presented to the Academy, December 16, 1914 
Some years ago the atomic weight of neodymium was investigated in 
the Harvard Chemical Laboratory by the analysis of the chloride and 
the success met in obtaining pure material and in the method used, led 
us to investigate in a similar fashion the atomic weight of the twin 
element praseodymium. The International Committee upon Atomic 
Weights has chosen the value 140.6 for this constant, yet some of the 
recent investigations, notably those of Brauner, have yielded consider- 
ably higher results, in the neighborhood of 141. 
Naturally, the first problem in the investigation was to prepare pure 
praseodymium salt. Through the great kindness of Dr. H. S. Miner of 
the Welsbach Light Company we were fortunate enough to secure as a 
starting point about 10 kg. of crude praseodymium ammonium nitrate, 
containing about 50% of the corresponding lanthanum and cerium salts 
as well as a small amount of neodymium. Since one of the most rapid 
and effective methods of freeing praseodymium salts from the closely 
related elements, lanthanum, cerium, neodymium, and samarium is the 
fractional crystallization of the double ammonium nitrate we chose this 
method of purification. In this crystallization the bases separate in the 
order lanthanum, cerium, praseodymium, neodymium, samarium, and 
yttrium earths. The usual method of fractional crystallization was 
followed ;2 that is, the mother liquor of a less soluble fraction was used for 
dissolving the crystals of the nex more soluble fraction. The crystal- 
lization proceeded with the occasional rejection of fairly large fractions, 
chiefly lanthanum and cerium, at the less soluble end of each series, and 
the rejection of much smaller fractions, partly neodymium, at the more 
