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TRANSACTIONS OF THE SECTIONS. 99 
the salts of lanthanium and cerium the amethyst colour which is attributed to these 
salts ; also the brown colour which the oxides of the same metals assume when heated 
to a red heat in contact with the air. Notwithstanding all possible care, I have not 
yet succeeded in obtaining the oxide in a state of purity ; and I have only arrived so 
far as to ascertain that a constant compound with sulphuric acid can be produced 
by different means, but from the quantity of water of crystallization, and other cir- 
cumstances, the conclusion may be drawn that the salt is really a double salt, although 
I cannot at present say whether the other accompanying oxide is oxide of lanthanium 
or some other. That which I now thus briefly describe as oxide of didymium is the 
basis in combination with sulphuric acid in that salt whose properties I will now 
communicate, as well as a method of obtaining it. The sulphate of oxide of didy- 
mium, prepared in different ways, is much more soluble in water than the sulphate 
of oxide of lanthanium. This circumstance induced me to try whether by treating 
the mixture of the anhydrous salts in great excess with water in small proportions, 
solutions could not be obtained, which, in the order they had been procured, should 
be richer in salts, and particularly in sulphate of oxide of didymium, while, on the 
contrary, what remained undissolved, should be nearly pure sulphate of oxide of 
lanthanium ; but after having examined five successive saturated solutions, obtained 
from the same mixture of anhydrous salts, it was found that one part of anhydrous 
salt had in the first experiment been dissolved in 7°64 parts of water; in the 2nd 
experiment in 8°48 parts ; in the 3rd experiment in 7:8 parts; in the 4th experiment 
in 5 parts, and in the 6th experiment in 7:44 parts of water. These remarkable pro- 
portions of salt dissolved I thus explained: during the dissimilar degrees of tempera- 
ture which accidentally arise under the development of heat which takes place when, 
by the addition of water to the anhydrous salt, this takes up water of crystallization, 
salts containing unlike portions of water of crystallization, and of unlike solubility had 
been formed, and it was for the purpose of ascertaining the correctness of this suppo- 
sition that I afterwards prepared the solution of the salts in the manner which I 
have already stated in describing the sulphate of oxide of lanthanium, the dissimilar 
solubility of which salt with different degrees of heat was in this manner discovered. 
If therefore the mixed salts, with a temperature which should not exceed 48°2 Fahr., 
be dissolved in 6 parts cf water, and the solution thus obtained afterwards heated to 
104° Fahr., a quantity of light amethyst-coloured salt of lanthanium is deposited, 
which, by a repetition of the same treatment, after ten to fifteen operations, becomes 
colourless and nearly pure. The amethyst-coloured solution separated from the 
salt of lanthanium is evaporated to dryness, and the salt is freed from water; it is 
again dissolved in the before-mentioned manner, but the solution is now heated to 
122° Fahr., and filtered after no more salt is deposited. The solution, now red, is 
diluted with an equal weight of water, acidulated with a portion of sulphuric acid, 
and is evaporated in a warm place. Several kinds of crystals are now formed, many 
of which assume a larger size, and fall to the bottom; when only a sixth part of the 
liquid, which is generally yellow, remains, it is poured off, the salt crust which lies at 
the bottom is separated, and the collection of crystals is shaken in boiling water, 
which is suddenly poured off, when a number of smaller prismatic crystals accom- 
pany it. The remaining large red crystals are again dissolved in water, the solution 
is acidulated with sulphuric acid, evaporated in the before-named manner, and the 
large red crystals taken separately, when it will be found on a nearer examination 
that they form a mixture of two kinds: the one, which appear in the form of long, 
narrow rhomboidal prisms, is taken out, and the remaining large red crystals with 
many planes, which, according to Wallmark’s measurement, appear to belong to the 
triklinometric system, form the salt which I call sulphate of oxide of didymium. 
From a solution of a salt of didymium hydrate of oxide of didymium is precipitated 
with hydrate of potash in excess, and collected on a filter; it has a bluish-violet 
colour, absorbs during washing carbonic acid from the air, and the residuum, for the 
Most part formed of carbonate of oxide of didymium, is, after drying, light reddish 
violet. If this be heated to redness, the water passes off and carbonic acid is easily 
expelled. The oxide produced in this manner is obtained in the form of small 
lumps, dark brown on the surface, sometimes light brown in the fracture, of a resi- 
‘nous lustre, sometimes nearly black, with the lustre and appearance of dark orthite, 
