340 
CHEMISTRY: RICHARDS AND HALL 
The lead from Australian carnotite, with which the kindness of Mr. 
S. Radcliff and Mr. E. R. Bubb had supplied us and which had been 
used in previous atomic weight and density determinations in this labora- 
tory ,2 seemed well suited to our purpose. 
About 1 kilogram of this lead was converted into nitrate and used for 
the fractional crystallization. Lead nitrate crystallizes from aqueous 
solution in compact, large octahedra which can be very completely freed 
from mother Kquor merely by drainage, and come rapidly to equilibrium 
with the mother Hquor. 
The whole of the nitrate was dissolved in distilled water and evapo- 
rated until a crust began to form. This was called fraction 1 . After cool- 
ing to room temperature the mother Hquor was poured off and formed 
fraction 2. To the crystals from fraction 1 was added sufficient water 
to dissolve them, and the solution was called fraction 3. 
Fractions 2 and 3 were evaporated to saturation and cooled as frac- 
tion 1 had been, the liquor from 2 forming fraction 4. Fraction 5 was 
made by adding the Hquor from 3 to the crystals from 2, and water was 
added to the crystals from 2 to form fraction 6. This process was con- 
tinued until, as occurred first with fraction 16, one of the end fractions 
became too small to work with conveniently and was consequently added 
entire to the mother liquor from fraction 17 to form fraction 22. This 
expedient was resorted to wherever the end fractions became incon- 
veniently small. In aU, seventy-five courses or tiers of fractions were 
obtained, most of the last of which contained eighteen fractions to a 
tier. The number of effective crystalHzations was 904. 
The crystalHzation was carried out entirely in porcelain, at first in 
large evaporating dishes, then, as the fractions became smaller, in cas- 
seroles covered with watch glasses. The hot casseroles were usuaUy 
set in evaporating dishes full of cold water, which was frequently changed 
to hasten cooling. 
Centrifugal drainage was not used because, as determined by calc?u- 
lation, the gain in effectiveness of separation would have been less than 
the disadvantage caused by the increased time necessary to treat the 
precipitates centrifugally. 
Purification. — Two samples, one from the more soluble and one from 
the less soluble end of the final row of crystalHzations were treated in 
an identical manner. Chiefly to remove the large amount of silica which 
had dissolved from the casseroles during the fractionation, the crystals 
were first electrolyzed in warm concentrated solution between platinum 
wires. The well formed flat leaflets of lead were removed from time to 
time, with glass rod, rinsed with a stream from a wash bottle, and pre- 
