CHEMISTRY: RICHARDS AND HALL 
341 
served in quartz dishes until all the metal had been deposited from the 
solution. The crystal were further washed from time to time, and 
all washings returned to the electrolysis bath, which was gradually 
reduced in volume as the concentration of lead became smaller. The 
lead crystals were then dissolved in pure nitric acid, and the excess of 
acid was evaporated. The crystal of lead nitrate were dissolved in 
the purest water, filtered through a platinum Gooch crucible, and pre- 
cipitated by the addition of a large excess of redistilled nitric acid and 
cooling with ice. The crystal meal thus obtained was freed from ad- 
hering acid in the platinum centrifuge, washed centrifugally with 
purest concentrated acid and redissolved in the least possible amount 
of pure boiling water. 
Five precipitations with centrifugal drying were made in this way. 
The crystals from the fifth crystallization were placed in a quartz dish 
and dissolved. From this solution lead chloride was precipitated by 
passing in pure hydrochloric acid gas made by warming the purest acid 
of commerce. The chloride thus obtained was dried in the centrifuge, 
washed with cold water, and redissolved in boiling water. Four crystal- 
lizations as chloride followed. A little hydrochloric acid was added to 
the solution each time to prevent the formation of basic salt and to ren- 
der the chloride less soluble. The last crystals, after centrifugal wash- 
ing and dr3dng were preserved in an exhausted desiccator over fused 
potash. Nothing but platinum and quartz vessels were used. 
The Determination of Atomic Weight. — For these determinations the 
usual procedure was employed. About five grams of the dried crystals 
were placed in a platinum boat, which with its container had been cai;e- 
fuUy weighed. The boat was gradually heated in a quartz tube attached 
to the usual 'bottling apparatus,^ in a current of pure dry hydrochloric 
acid gas. After nearly all the residual water had been driven from the 
crystals, the temperature was raised so as to fuse the salt, which was. 
then cooled as quickly as possible in a current of pure dry nitrogen 
prepared by the Wanklyn process. : 
After the salt had cooled, and the nitrogen had been replaced by dry 
air, the boat was pushed into the weighing tube, bottled, and reweighed; 
The boat and salt were next heated at 80°-90° on an electric stove 
in a glass flask containing about a Hter of water to which enough pure 
nitric acid had been added to prevent the formation of basic salt, until 
the lead chloride had all dissolved. As usuaP a small amount of siliceous 
and carbonaceous residue had to be removed by scrupulously quantita- 
tive filtration through a tared Gooch-Monroe crucible. On the average, 
about twice as much was found in the more soluble fraction as in the 
