SILVER, NITRATE OF 833 



SILVER, CYAMTDE OF. See CYAXIDES. 



SILVER FIR. Abies picca. This species yields the Burgundy pitch and Strasburg 

 turpentine. See ABIES. 



SILVER, FULMINATING. See FTJLMINATING SILVEE. 



SILVER GLANCE. Sulphide of silver. See SILVER. 



SILVER, HYPOSULPHITE OF. AgO.S 2 2 .HO (Ag 2 S 2 H 2 O 4 ). This salt 

 is formed in the process of fixing photographic pictures with hyposulphite of soda. 

 Solutions of the hyposulphite of soda, potash, or lime, which are bitter salts, dissolve 

 chloride of silver into liquids possessing a remarkable sweetness. See HYPOSULPHITE 

 OF SODA. 



SILVER, IODIDE OF. (Agl.) This compound of iodine and silver, which is 

 obtained when a solution of an iodide is added to nitrate of silver, is a pale yellow 

 powder. It is also found native, but not in large quantities. This silver salt is 

 remarkable, like some other metallic compounds, for changing its colour alternatively 

 with heat and cold. If a sheet of white paper be washed over with a solution of 

 nitrate of silver, and afterwards with a somewhat dilute solution of iodide of potas- 

 sium, it will immediately assume the pale yellow tint of the cold silver iodide. On 

 placing the paper before the fire, it will change colour from a pale primrose to a gaudy 

 brilliant yellow, like the sunflower ; and on being cooled, it will again resume the 

 primrose hue. These alternations may be repeated indefinitely, like those with the 

 salts of cobalt, provided too great a heat be not applied. The pressure of a finger 

 upon the hot yellow paper makes a white spot, by cooling it quickly. Iodide of silver, 

 when quite pure, is very slowly darkened when exposed to sunshine ; but if in com- 

 bination with an organic compound, or with an excess of nitrate of silver, it changes 

 colour with much rapidity. From this property it furnishes one of the most valuable 

 of our photographic agents. It is the active material in the calotype, the collodion, 

 the Daguerreotype, and other processes, See PHOTOGBAPHY. 



SILVER, NITRATE OF. AgO.NO 5 (AgNO 3 ). This salt was known to Geber, 

 and was chiefly used in medicine ; but since the discovery of photography, it has been 

 made on a very large scale. It is found in commerce in two different forms, viz. 

 crystallised, and in sticks, the former being more general ; in sticks it is called ' lunar 

 caustic,' and is used by the surgeon. It is prepared by digesting metallic silver with 

 moderately strong nitric acid ; the silver speedily dissolves, especially if heat be 

 applied. Some of the nitric acid is decomposed, yielding oxygen to the silver, and 

 liberating binoxide of nitrogen, which, in contact with the air, abstracts oxygen and 

 forms red vapours of hyponitric acid. 



The clear solution is evaporated, either to the crystallising point or to dryness ; if 

 for caustic, it is fused and cast into sticks. If ordinary standard silver be used, the 

 solution will contain some nitrate of copper ; in this case it must be evaporated to 

 dryness, and gradually heated till all the nitrate of copper is decomposed, which may be 

 known by taking a little of the salt, dissolving in water, and adding excess of ammonia ; 

 when, if copper be still present, the solutien will have a blue tint. When all the copper 

 is thus rendered insoluble, the fused mass is dissolved in distilled water, evaporated 

 and crystallised. When pure, nitrate of silver is white ; the crystals are transparent, 

 colourless, hexagonal tables, or right rhombic prisms, very soluble in water, requiring 

 only their own weight of cold water and half that quantity of boiling water for solu- 

 tion ; they are also readily soluble in hot alcohol, but the greater portion is again 

 deposited on cooling. Nitrate of silver possesses a strongly metallic and bitter taste. 

 It is not deliquescent, and when free from organic matter is not decomposed by light 

 (Scanlan). The dark colour of the outside of the ordinary sticks of the shops is caused 

 by the decomposition of the nitrate by the paper in which they are wrapped, as the 

 presence of organic matter reduces the silver to the metallic state. Nitrate of silver 

 is frequently adulterated to a considerable extent, principally with nitrate of potash, 

 but sometimes with other nitrates. The price at which it is sometimes sold is proof 

 enough that it is largely adulterated ; for instance, it may sometimes be bought for 

 3s. an ounce ; at that price it does not pay for the silver alone that should be in it : we 

 will prove this. Every ounce (437'5 grains) of pure nitrate of silver contains 278 

 grains of pure silver, and this itself, without taking notice of nitric acid and time of 

 preparation, is worth 3s. 2d . This clearly proves there must be considerable adultera- 

 tion ; but although the adulterating substances do not interfere generally with the 

 photographic processes, it is certain that no advantage can be gained by buying it at 

 so low a price. The way to detect the adulteration is to precipitate the silver by 

 hydrochloric acid, and evaporate the filtered liquid to dryness, when, if the salt is pure, 

 there will be no residue. 



As many, who use much nitrate of silver in photography, &c., throw away the 

 residues, and hence in course of time waste much silver, it will not be out of place 

 here to show how it maybe saved and converted again into nitrate of silver fit for use. 

 VOL- III. 3 H 



