462 OSMIUM 



but sometimes in crystals, which are oblique rhombic prisms ; these crystals are 

 flexible, of a yellow colour, and possess a brilliant lustre. See ARSENIC. 



Native orpiment has a specific gravity of about 3'48. Orpiment is also prepared 

 artificially, chiefly in Saxony, by subliming in cast-iron cucurbits, surmounted by 

 conical cast-iron capitals, a mixture in due proportions of sulphur and arsenious acid. 

 As thus obtained, it is in yellow compact opaque masses, of a glassy aspect ; yielding 

 a powder of a pale yellow colour. 



Artificial orpiment seems to be a substance of uncertain composition, it containing 

 sometimes, according to Guibourt, 94 per cent, of arsenious acid, and only 6 per cent, 

 of the tersulphide of arsenic. On this account it is much more soluble in water than 

 the native orpiment, and consequently a much more powerful poison. It has been 

 administered several times with criminal intentions, and in many of the cases proved 

 fatal. Orpiment is the colouring-matter of the pigment called king's yellow, whiuh 

 is a mixture of arsenious acid with a little tersulphide of arsenic, just as the sample 

 analysed by Guibourt. See KING'S YELLOW. 



A proper tersulphide of arsenic may be obtained by passing a stream of sulphuretted 

 hydrogen gas through a solution of arsenious acid in hydrochloric acid. It falls as 

 a brilliant yellow amorphous powder. 



Tersulphide of arsenic is insoluble in water and dilute acids, but is decomposed 

 by nitric acid and aqua regia. It fuses easily, and when heated in air burns with 

 a pale blue flame, generating arsenious and sulphurous acids. In close vessels it 

 sublimes unchanged. It is dissolved by ammonia, and the caustic fixed alkalis 

 forming colourless solutions, from which it is again precipitated by the addition of 

 an acid. The alkaline sulphides also dissolve it, forming double salts, from which 

 solutions it is precipitated even more completely than from the former, by the addi- 

 tion of an acid. 



According to Dr. Paris, Delcroix's depilatory, called poudre subtile, consists of 

 quicklime, orpiment, and some vegetable powder. 



Orpiment is used by pyrotechnists, and as a pigment : the best kinds of native orpi- 

 ment being reserved for artists. 



ORRIS-ROOT. The dried rhizomes of several species of Iris. 



ORTHOCXiASIB. See FKLSPAB. 



OSIER. Salix. The Willow-Tree. About 300 species of these plants have been 

 described. The common white willow is a native of Great Britain and many parts of 

 Europe, and is extensively cultivated. The wood is soft, and is employed in making 

 various small articles. The charcoal prepared from it is considered superior to any 

 other for the manufacture of gunpowder. The bark is very bitter, and from it a 

 bitter principle, salioine, is obtained, which has been used with some advantage as 

 a substitute for quinine, and it is sometimes used for adulterating that drug, See 

 SALICINB. 



OSMIV1VX is one of the rare metals, most generally found in the ores of platinum, 

 in which it was discovered by Mr. Tennant in 1803. These ores generally contain 

 the metals palladium, rhodium, osmium, ruthenium, and iridium, mixed with the 

 platinum. 



The process for obtaining osmium from these ores has been much simplified by 

 M. Fremy. After the exhaustion of the ores by aqua regia there remains a residue, 

 which often contains titaniferous iron and chrome iron ; but the most important con- 

 stituent is an alloy existing in flat plates or scales, of a white colour and metallic 

 lustre, and which was formerly thought to contain only osmium and iridium, but later 

 experiments have proved the presence of ruthenium, and a little rhodium. Fremy 

 takes advantage of the oxidability of osmium and of the volatility of its peroxide. 

 His process consists in roasting the alloy in a current of dry air ; for this purpose 

 the residue above mentioned is placed in a porcelain or platinum tube, and heated to 

 redness. 



The equivalent of osmium is 99 - 6 ; and its symbol, Os. 



Five compounds of osmium and oxygon exist, viz. : Protoxide, OsO ; it is a dark 

 green powder, slowly soluble in acids. Sesquioxide, Os 2 3 , has never been obtained 

 pure ; it is formed by heating a solution of osmate of ammonia, when a brown powder 

 falls, which is this compound mixed with some ammonia, which explodes feebly when 

 heated. Binoxide, OsO-, is a black powder, insoluble in acids, and burning to osinic 

 acid when heated in the air. Osmious acid, OsO 3 ; this only exists in combination ; 

 it forms a rose-red crystalline powder with potassa (KO,Os0 3 ,2HO) ; this salt is ob- 

 tained by adding alcohol to a solution of osinato of potassa ; the osmic acid is reduced 

 by the alcohol, and this salt is precipitated ; on attempting to separate this acid, 

 it is decomposed into binoxide and osmic acid. Osmic acid, OsO 4 ; the prepara- 

 tion of this compound has already been described ; it melts, and even boils, below 



