CYANIDE OF POTASSIITM. K Cy. The principal employment of this salt in photography is in forming a small proportion of cyanide of silver with the iodide, and in fixing. The former of these uses is not to be commended. As met with in commerce, it often contains half its weight of carbonate of potassa, which is detected by the effervescence prochiced when an acid is added to its solution : from the usual mode of its preparation, it often also contains cyanate of potass, but not in very large quantity, and this is not injurious. It is deliquescent, and especially so when contaminated with carbonate, but it is not soluble in cold alcohol. By dissolving it in boiling alcohol the impurities are removed, and on cooling, pure cyanide of potassium is deposited. When dissolved in water it may be crys tallized in cubes. It is as poisonous as prussic acid (hydrocyanic acid) itself : the best antidote, and one very convenient to the pho tographer, is protosulphate of iron. It is used from 2 to 20 grains to the ounce of water in fixing photographs, according to the purity of the article. Its properties for this purpose differ from those of hyposulphite of soda, and it is more energetic. Like " hypo " it forms double salts with the chloride and other compounds of silver, which contain one equivalent of cyanide of silver and one of cyanide of potassium : they are not decomposed by water as the double salts produced by other fixing agents are, so that no apprehension need be entertained of precipitating cyanide of silver in the washing : and it dissolves much more silver on this account than weak hypo will do. It also acts more quickly ; being weak, it is soon saturated, and is more quickly and completely removed by washing, so that very little comparative washing is required. It is never found to crystallize in
the film, and but, very few instances have occurred in which the washing has been so careless as to leave enough in the film to react on the picture. Cyanide of potassium has also an affinity for oxygen, and has therefore considerable reducing powers, so that the oxides of copper and other metals thrown into it in the fused state, are presently brought to the state of pure metal, and the cyanide becomes cyanate. This reducing power is also seen in fixing collo dion pictures, which have a more metallic lustre when fixed with it than when the " hypo" salt is used. All these are important advan tages, but it is not adapted to fixing pictures on albumen, paper, and negative collodion, from the power it has of dissolving the organic basis, and reduced silver in combination with it, as well as the un altered chlorides and other salts. When it is used to fix pictures on the very active negative collodion, which has been mentioned in the article on collodion, it becomes quite red with the organic matter and silver which it removes from the plate, and the picture becomes faint and yellow. The same thing, precisely, occurs when it is used to fix paper impressions, and therefore it is not adapted to these organic pictures. When a collodion negative has density, and to spare, or when the image is blue and metallic, negatives may be fixed with this salt without danger. The solution of cyanide of potassium deteriorates very slowly when exposed to air, by the gradual produc tion of carbonate of potassa.