and Didymium. 249 



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 pris- 

 matic crystals accompany it. The remaining large red cry- 

 stals 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 toWallmark's mea- 

 surement, 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 resinous lustre, sometimes nearly black, with 

 the lustre and appearance of dark orthite, at the same time 

 particles are obtained of all the most dissimilar colours, so 

 that they represent together a pattern map of all the most 

 dissimilar kinds which are obtained of the mineral orthite, 

 from the light red brown to the nearly black. The powder 

 becomes light brown. If this oxide be heated to a white heat, 

 it assumes a dirty white colour approaching gray green. Oxide 

 of didymium is a weaker basis than oxide of lanthanium : it 

 has no alkaline reaction, and appears not to absorb water after 

 having been heated. It is, however, tolerably easily dissolved 

 even in diluted acids, and the brown oxide with a development 

 of gas. It is insoluble in carbonate of ammonia; its salts are 

 amethyst-red, as well as the solutions of the salt, which forms 

 no precipitate with hydrosulphuret of ammonia, unless a large 

 quantity be added, or the liquid be heated, when the sulphu- 

 retted hydrogen isdeveloped,andabasicsaltprecipitated having 

 a slight tint of red. If the oxide be dissolved in phosphoric 



