SEPARATION OF THE RARE EARTHS. 239
little water, then with alcohol, and dry them at the room
temperature.
Atomic Weight Determination of Thorium. In order to illus-
trate the principle of an atomic weight determination by the sul-
phate method, prepare first some anhydrous thorium sulphate
from about 2 g. of the above hydrated salt. Heat the crystallized
salt to 400° in a weighed platinum crucible which is supported by
a platinum triangle within a larger crucible. When the weight
has become constant, the amount of the anhydrous thorium sul-
phate, a, is given by subtraction. Then ignite the crucible over the
blast lamp until the weight has again become constant. This gives
the weight of thorium dioxide, b. If the atomic weight of oxygen
is taken at 16.00 and that of sulphur as 32.06, the atomic weight
of thorium is obtained by solving for x in the expression:
x +2 '32.06 + 8 -16.00 a
' x + 2 • 16.00 ~ I"
- Separation of the Rare Earths.
The rare earths form a group of very closely related sesquioxides, the
separation and characterization of which for a long time offered considerable
difficulties. The properties of the analogous compounds of these earths do
not differ sharply enough from each other to permit a complete separation
to be made in a single operation; but the slight gradations in the properties
may in general be used to effect a satisfactory separation if a given process
is systematically repeated again and again. When these slight gradations
are taken into account, the rare earths fall into a classification which corre-
sponds closely to their occurrence in nature.
The first distinction is made between the cerium earths and the yttrium
earths.. The latter were discovered by Gadolin at the end of the eighteenth
century in a mineral found in a feldspar quarry near Ytterby, in Sweden, and
afterwards named after him gadolinite. The cerium earths were discovered
at the beginning of the last century by Berzelius and Hisinger in the mineral
cerite which had previously been investigated by Scheele without success.
Soon afterwards lanthanum and didymium were discovered in the first half of
the nineteenth century by Mosander, a pupil and friend of Berzelius. This
same investigator found that yttrium is accompanied by terbium and erbium.
The separation of the earths was accomplished by two different processes
which in a more perfected form serve even to-day as the standard methods.
Of these processes, one rests upon differences in the basicity of the earths, as
manifested in the varying hydrolytic and thermic dissociation of their salts;
the second process rests upon differences in the solubility of the double salts.
The cerium earths are more weakly basic and form more difficultly soluble
