242 COMPOUNDS OP THE RARE ELEMENTS.
the solution and precipitate the cerium as before by adding mag-
nesium carbonate.
Bring the cerium-free filtrate to boiling in an evaporating dish,
and, while stirring as before with the mechanical device, add
powdered potassium sulphate (about 35 g. in all) until the didy-
mium absorption bands can scarcely be detected spectroscopically
in a filtered sample of the solution. In making this test, use a
pocket spectroscope; fill a test-tube of 2 to 3 cm. diameter — or
still better a parallel-walled vessel of the same thickness — with
the solution and place it between the slit of the spectroscope and
a Welsbach light. The most easily recognized of the absorption
bands of didymium are those to the right and left of the sodium
line. If the solution is almost free from didymium, drain the pre-
cipitate of the double sulphates, M 2 (SO 4 ) 3 «3 K 2 SO 4 , of didymium
and lanthanum, and wash it with a dilute potassium sulphate
solution.
Add ammonium oxalate to the filtrate, and after some hours
wash the precipitate with cold water; this precipitate contains the
remainder of the rare earths that were present in the monazite —
chiefly the yttrium earths; these are not to be further separated.
(a) Cerium Compounds.
Working Up of the Cerium Precipitate.
Cerium may be readily purified from other metals by forming eerie ammo-
nium nitrate, (NH 4 ) 2 Ce(NO 3 ) 6 , which is difficultly soluble in nitric acid.
Weigh the dried cerium precipitate approximately, and boil it
in an evaporating dish for 30 minutes with five times its weight of
a 10% solution of sodium hydroxide. After letting the solution
settle, decant off the liquid as completely as possible, and boil
the residue with some fresh caustic soda solution. Pour off the
solution again, and wash the residue by decantation with hot
water, pouring all the liquid through a filter. Finally, collect the
entire residue on the filter, wash it until free from soluble sul-
phate, and dry it in the hot closet.
Treat the eerie hydroxide thus obtained with 2.65 parts by
weight of concentrated nitric acid (sp. gr. 1.4), filter the solu-
tion through an asbestos felt in a Gooch crucible, and add to
the filtrate a hot solution of 0.39 parts of ammonium nitrate in