Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

240 COMPOUNDS OF THE RARE ELEMENTS.

potassium double sulphates than the yttrium earths. Cerium itself occu-

pies a characteristic position, as it is the only member of the group from

which two series of salts are derived: the cerous salts with tervalent cerium;

and the eerie salts containing quadrivalent cerium.

For the qualitative characterization of the earths, the color of their salts

is first of all of importance. Lanthanum and cerous salts are colorless, eerie

salts are yellowish-red, erbium salts pink, didymium salts violet. It marked

then the beginning of a new era in the history of the rare earths, when, in

1861, through the application of spectrum analysis, it became possible to

measure exactly the color of salt solutions by means of their absorption spec-

tra, and the color of glowing vapors by means of emission spectra. It was in

this epoch that the discovery of the periodic system of the elements was

made (1869), by the aid of which Mendelejeff first recognized the tervalency

of the rare-earth metals and predicted the existence of the element scandium,

which was later discovered by Nilson. Further investigation and the exami-

nation of new minerals has since that time added a number of elements to

the group. In the presence of didymium Lecoq de Boisbaudran discovered

samarium. In the group of the yttrium earths, which since the time of

Mosander was studied especially by Bahr and Bunsen, ytterbium was dis-

covered by Marignac, while Cleve added the elements holmium and thulium.

Marignac also discovered gadolinium, which together with terbium and europ-

ium occupies an intermediate position between the cerium and yttrium earths.

Finally, didymium was separated by von Welsbach in 1885 into praseodymium,

the salts of which are green, and neodymium, whose salts are violet.

The most recent epoch dates from the technical application by von Wels-

bach of thorium and cerium oxides in the incandescent gas-lighting industry.

The following table gives a summary of the rare earths and their atomic

weights:

Cerium earths. Terbium earths. Yttrium earths.

Lanthanum, La 138.90

Cerium, Ce 140.25

Praseodymium, Pr 140.92

Neodymium, Nd 144.27

Samarium, Sm 150.43

Europium, Eu 152.0

Gadolinium, Gd 157.26

Terbium, Tb 159.2

Scandium, Sc 45.10

Yttrium, Y 88.9

Erbium, Er 167.7

Ytterbium, Yt 173 6

, Further details concerning the chemistry of the rare earths can be found in
(the articles by R. J. Meyer which are published in Abegg's Handbuch der
anorganischen Chemie and in the monograph by R. J. Meyer and O. Hauser
" Die Analyse der seltenen Erden und der Erdsauren."
As raw material for the rare-earth preparations, the mixture of carbon-
ates obtained in working up monazite sand (No. 170) may be used. The
crude cerium carbonate, or cerium oxalate, that can be obtained on the market,
furnishes practically the same mixture of the earths, since it usually con-
tains 40 to 50% of cerium salt, 15 to 20% of lanthanum salt, 25 to 30% of
didymium salt, and 5 to 8% of yttrium earths.
The cerium is precipitated, according to the method of Witt and Theel, by
adding ammonium persulphate to a boiling solution of the nitrates; eerie sul-