and Didymium. 247 



means of warm concentrated sulphuric acid, the ignited oxide 

 of cerium is immediately rendered soluble, in consequence of 

 combining with the acid. Neutral sulphate of oxide of cerium 

 is, when dry, a beautiful yellow, becomes by heating orange 

 yellow, with a higher degree of temperature almost cinnabar 

 red, but after cooling the bright yellow colour returns. The 

 salt is soluble in a small quantity of water, but if the solution 

 be heated to boiling, the greater part of the salt is deposited 

 in the form of a tough, soft, semi-transparent, and very 

 viscid mass. If the concentrated solution, which is red yellow, 

 be diluted, it becomes lighter yellow, but begins immediately 

 to grow turbid, depositing a sulphur-yellow powder, which is 

 a basic salt requiring 2500 parts of water for its solution. 

 With sulphate of potash, sulphate of oxide of cerium gives a 

 beautiful yellow salt, which is altogether insoluble in a satu- 

 rated solution of sulphate of potash, but the double salt can- 

 not be dissolved in water without being decomposed and a 

 basic salt precipitated. Notwithstanding the oxide of cerium, 

 is nearly insoluble in diluted acids, it must be remembered 

 that intimately mixed with other easily soluble oxides, it 

 readily passes into solution : sulphuret of cerium is of a dark 

 brown-red colour. 



The oxide of lanthanium which was first obtained by me 

 was of a brown colour, but after having been heated to a 

 white heat, became a dirty white; by heating in hydrogen it 

 also lost its brown colour, although a scarcely perceptible loss 

 of weight arose therefrom : by heating in the air, the brown 

 colour returned. 



This circumstance, together with several other phaenomena 

 which presented themselves during the examination of the 

 properties of oxide of lanthanium, caused me to presume that 

 the oxide of lanthanium which had been obtained was still 

 accompanied by some unknown oxides, and it was in the be- 

 ginning of 1840 that I succeeded in freeing lanthanium from 

 that very substance which caused the brown colour. To 

 the radical of this new oxide, I gave the name of Didymium 

 (from the Greek word §»8vp,o$, whose plural &'%to» signifies 

 twins), because it was discovered in conjunction with oxide of 

 lanthanium. It is the oxide of didymium that gives to 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 



