DIDYMIUM, a metallic triad element, symbol Di"', atomic weight 144. It occurs along with cerium and lantha num in the mineral cerite. It is sepa rated from cerium by igniting the oxa late, and treating the resulting oxides with very dilute nitric acid, which does not dissolve the cerium oxide. The fil tered solution is mixed with sulphuric acid, concentrated by evaporation, and then a hot solution of potassium sul phate' is added, which precipitates the lanthanum and didymium as double sul phates. Didymium can be separated from lanthanum by precipitating half the oxide with ammonia, and leaving the precipitate in contact with the solution; the lanthanum, being the stronger base, then passes into solution in predominant quantity. By repeating the process, the oxides being again dissolved and precipi tated, the didymium oxide is obtained nearly pure. Didymium is a white met al with a tinge of yellow; specific gravi ty, 6.5. It tarnishes in dry air; it burns with great brilliancy when thrown into a flame. Its oxide, is a dirty bluish color; the nitrate is obtained in large violet crystals by dissolving the oxide in nitric acid. The sulphate, Di2(SO4) 3 611,0, forms rose-red crystals. The ox alate is a crystalline powder. The spec trum of a solution of a salt of didymium contains characteristic dark bands.
In botany, the word is applied to a genus of gasteromycetous fungi, consist ing of minute plants growing upon leaves, bark, rotten wood, etc., distin
guished by its double peridium.
DIE, a word with various applications. (1) In punching-machines, a bed-piece which has an opening the size of the punch, and through which the piece is driven. This piece may be a planchet or blank, or it may be merely a plug driven out of the object to form a bolt or rivet hole. In nut-machines the nut blanks may be made by one die and punched by another. (2) In forging, a device consisting of two parts which co-act to give to the piece swaged be tween them the desired form. (3) In sheet-metal work, a former and punch or a cameo and intaglio die between which a piece of sheet-metal is pressed into shape by a blow or simple pressure. In coining, both dies are intaglio, so as to make a cameo or raised impression upon each face of the planchet. The upper die has the obverse, the face, which is often the bust of the sovereign or national emblem. The lower die has the reverse, with an effigy, legend, value, escutcheon, as the case may be. Owing to the random way in which ornaments are disposed on coins, any general defi nition will no longer meet all cases. A die will sometimes deliver 250,000 im pressions before it is necessary to re move it from the coining-press; and sometimes a die will crack at the first impression.