704 
MR, W. CROOKES OK RADIANT MATTER SPECTROSCOPY. 
121. The separation of the last traces of didymium from the samarium can be 
accomplished only by fractional precipitation, an operation so tedious that probably few 
chemists will be inclined to undertake it. The second portion of hydrates, consisting 
chiefly of didymium, is purified from the small quantities of samarium and lanthanum 
by fusing with potassic nitrate, as explained above for the traces of cerium (118); to 
separate the lanthanum the oxalates are dissolved in warm strong nitric acid and 
allowed to cool, when didymic oxalate nearly free from lanthanum is obtained; after 
repeating several times, the last trace of lanthanum remains in the solution. 
122. To separate the small quantity of didymium from the lanthanum obtained in 
the final precipitates with ammonia, the only method is to continue the process of 
fractionation; the lanthanic oxide finally obtained should be pure white, any trace of 
yellow showing that didymium is still present. 
123. As cerite contains small quantities of the yttria earths, these must be sepa¬ 
rated from cerium, didymium, &c., by making a cold solution of the sulphates and 
adding finely-powdered potassic sulphate in quantity more than sufficient to saturate 
the solution, allowing the mixture to stand (with frequent agitation) for a few days ; 
filtering, and washing the filtrate with a, saturated solution of potassic sulphate. 
Tire filtrate contains the yttria earths, and for their complete separation it is 
advisable to repeat the operation with potassic sulphate three or four times. The 
insoluble residues, consisting of a double potassic sulphate with either cerium, 
didymium, or other member of this group, are boiled with sodic hydrate, filtered, 
well washed, re-dissolved in nitric acid, precipitated with oxalic acid, and the oxalates 
ignited, leaving the earths lanthana, didymia, or samaria to be finally purified as 
described further on. 
124. The ceric oxide obtained in the manner just described was white. A con¬ 
siderable thickness of a strong solution did not show a trace of absorption spectrum. 
The atomic weight of the metal was taken and yielded the number =141T 
The number given by Buhrig . ..=141 "2 
„ „ Robinson t.=140-2 
Many older determinations| range from 138 to 139. 
This ceric oxide gave no orange-band spectrum in the radiant matter tube, either 
with or without the addition of lime. 
Purification of Lanthana . 
125. The lanthana obtained in the manner described above (120, 122) was more 
difficult to purify than ceria. Long after the lanthana appeared pure, it gave in the 
* Journ. prakt. Cliem., (2), xii., 209. 
f ‘ Chemical News,’ vol. 1., p. 251, Nov. 28, 1884. 
* rind., vol. xlix., p. 282, .Tone 27, 1384. 
