706 
MR, W. CROOKES OK RADIANT MATTER SPECTROSCOPY. 
128. The final oxalates—the ultimate cumulation of the portions least soluble in 
nitric acid—were next converted into nitrates, and the excess of acid driven off. The 
anhydrous salt was dissolved in fifty times its weight of water, and fractionally pre¬ 
cipitated by ammonia in the following manner :—A large quantity of ammonia was 
first prepared of the dilution (l to 5000) used in the previous fractionation (120), and 
500 c.c. of this was gradually added, with constant stirring, to Winchester quarts 
about three-fourths full of the dilute didymic nitrate. In about half an hour another 
500 c.c. of ammonia was again added, and this operation was repeated at intervals till 
the Winchester quarts were full. The bulk of the samaria comes down in the 
first precipitates, which are filtered off and set aside for the preparation of pure 
samaria (133). 
To the filtrate, containing didymium, with a little samarium and lanthanum, ten 
successive quantities of about 200 c.c. each of dilute ammonia were added to each 
Winchester quart at intervals of about an hour, and after violent agitation allowed to 
subside. The clear supernatant liquid was now poured off, evaporated to about half 
its bulk, and then, when cold, again poured back into the precipitate, and the operation 
of precipitating with dilute ammonia was likewise repeated. By this means the 
greater portion of the samarium present was obtained in the precipitate, whilst the 
didymium left in solution contained a less proportion of samarium. 
129. After a time a balance seemed to be established between the affinities at work, 
when the earths would appear in the same proportion in the precipitate and the 
solution. At this stage they were thrown down by ammonia, and the precipitated 
earths set aside to be worked up by the fusion of their anhydrous nitrates so as to 
alter the ratio between them, when fractionation by ammonia could be again employed. 
Samaric nitrate decomposes by heat before didymic nitrate. The nitrates were 
mixed with four times their weight of potassic nitrate, and the whole kept fused in a 
crucible till about three-fourths of the earthy nitrates were decomposed. The cooled 
mass was then dissolved in water, filtered, and the solution evaporated to dryness, and 
again submitted to fusion. This was repeated several times. 
The basic nitrates insoluble in water were dissolved in nitric acid, and put through 
the operation of fractional pi'ecipitation with ammonia, for samaria (133), in the 
manner just described above (128). 
130. To remove the last traces of samarium which might have survived this treat¬ 
ment, the solution of nitrates which had longest resisted decomposition by fusion was 
now mixed with excess of potassic sulphate. The precipitated double sulphates were 
subjected to long washing with a saturated solution of potassic sulphate, in which the 
samarium salt is more soluble than the didymium salt. They were then reconverted 
into nitrates, and the precipitation and washing with potassic sulphate repeated 
several times. Finally, the didymium salt was converted into oxalate, and re-crystal¬ 
lised many times from nitric acid, to eliminate any trace of lanthanum that might still 
contaminate it. Pure didymia is of a very deep chocolate-brown colour. 
