28 REPORT—1843. 
nium, together with all the above-named accompanying substances. I have not been 
able to find any good method of obtaining pure oxide of cerium; the salts of pro- 
toxide of cerium are like those of oxide of lanthanium, perfectly colourless, and with 
sulphate of potash the protoxide of cerium is precipitated completely from the solu- 
tion. If hydrate of protoxide of cerium, precipitated by caustic potash, be collected 
on a filter, it immediately begins to grow yellow, and after the oxidation has pro- 
ceeded as much as possible in this manner in the air, there remains after drying, 
opake light yellow lumps, which contain water; this being expelled by heat, leaves 
so-called oxide of cerium, which has not the least trace of brown, but after an hour’s 
heating at a white heat, has a slight tinge of red. If the oxide of cerium formed in 
the manner stated has the slightest tinge of brown, or becomes dark after drying or 
heating, it proceeds from foreign substances. ‘This yellow oxide, however, always 
contains protoxide of cerium, and I have not succeeded in obtaining oxide of cerium 
free from protoxide. The bright yellow oxide which is formed when hydrate of pro- 
toxide of cerium, either alone or mixed with hydrate of oxide of lanthanium, &c., is 
exposed to the action of chlorine, contains not only chlorine but even protoxide of 
cerium. If nitrate of protoxide of cerium be heated, a light yellow powder is ob- 
tained, from which much salt of protoxide of cerium may be extracted with nitric 
acid, and if this solution be again evaporated, and the dried mass heated, salt of prot- 
oxide of cerium is again obtained, and this continues even after the operation has 
been five times repeated. What I call oxide of cerium is, therefore, really a combi- 
nation of oxide of cerium with protoxide. The ignited oxide of cerium is scarcely 
affected by boiling concentrated muriatic acid, still less by other weaker acids; the 
hydrate, on the other hand, is easily dissolved in muriatic acid, with the development 
of chlorine, but even after a long boiling the solution retains a yellow colour. 
Scarcely a trace of the hydrate of oxide of cerium is dissolved by weaker diluted 
acids, but it assumes a darker yellow colour, and combines with a portion of the acid 
employed. In the solutions of carbonated alkalies, particularly carbonate of ammo- 
nia, the hydrate of oxide of cerium is dissolved in large quantities, and the solution 
assumes a bright yellow colour. Peroxide of cerium in solutions which are heated 
to boiling, is immediately reduced by oxalic acid to protoxide of cerium, while car- 
bonic acid is developed. By 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 cin- 
nabar 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 saturated solution of sulphate of potash, but the double salt 
cannot be dissolved in water without being decomposed and a basic salt precipi- 
tated. 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 Janthanium 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 phenomena which presented them- 
selves 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 accom- 
panied by some unknown oxides, and it was in the beginning of 1840 that I suc- 
ceeded 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 d/dvj0s, whose plural d/dv~or signifies twins), because it was discovered 
in conjunction with oxide of lanthanium. It is the oxide of didymium that gives to 

