POTASSIUM, RUBIDIUM, CAESIUM, AND LITHIUM 543 



the mass thus obtained be treated with water, the potassium cyanide 

 is partially decomposed by the water, but if it be treated with alcohol, 

 then the cyanide is dissolved, and on cooling separates in a crystalline 

 form. 12 A solution of potassium cyanide has a powerfully alkaline 

 reaction, a smell like that of bitter almonds, peculiar to prussic acid, 

 and acts as a most powerful poison. Although ^exceedingly stable 

 in a fused state, potassium cyanide easily changes when in solution. 

 Prussic acid is so very feebly energetic that even water decomposes 

 potassium cyanide. A solution of the salt, even without access of air, 

 easily turns brown and decomposes, and when heated evolves ammonia 

 and forms potassium formate ; this is easily comprehensible from the 

 representation of the cyanogen compounds which was developed in 

 Chap. IX., KCN + 2H 2 O=CHKO 2 + NH 3 . Furthermore, as carbonic 

 anhydride acts on potassium cyanide with evolution of prussic acid, and 

 as potassium cyanate, which is also unstable, is formed by the action of 

 air, it will be easily seen that solutions of potassium cyanide are very 

 unstable. Potassium cyanide, containing as it does carbon and 

 potassium, is a substance which can act in a very vigorously reducing 

 manner, especially when fused ; it is therefore used as a powerful 

 reducing agent at a red heat. A considerable quantity of potassium 

 cyanide is used in the arts, more particularly for the preparation of 

 metallic solutions which are decomposed by the action of a galvanic 

 current ; thus it is very frequently employed in electro-silvering and 



12 In this case it is evident that all the cyanogen which was in combination with the 

 iron is decomposed into nitrogen, which is evolved as gas, and into carbon, which com- 

 bines with the iron. In order to avoid this, potassium carbonate is added to the yellow 

 prussiate while it is being fused. A mixture of 8 parts of anhydrous yellow prussiate 

 and 8 parts of pure potassium carbonate is generally taken. On fusing, double decom- 

 position takes place, resulting in the formation of ferrous carbonate and potassium 

 cyanide. But by this method, as by the first, a pure salt is not obtained, (1) because a 

 portion of the potassium cyanide is oxidised at the expense of the iron carbonate and 

 forms potassium cyanate, FeCO 3 + KCN = CO 2 + Fe + KCNO ; (2) a portion of the iron 

 under the action of the water again passes into solution ; and (3) the potassium cyanide 

 very easily forms oxide, which acts on the sides of the vessel in which the mixture is 

 heated (to avoid this iron vessels should be used), &c. By adding one part of charcoal 

 powder to the mixture of 8 parts of anhydrous yellow prussiate and 8 parts of potassium 

 carbonate a mass is obtained which is free from cyanate, because the carbon absorbs the 

 oxygen, but then it is impossible to obtain a colourless potassium cyanide by simple 

 fusion, although this may be easily done by dissolving it in alcohol. Naturally, pure 

 potassium cyanide may be easily obtained if hydrocyanic acid be saturated with caustic 

 potash, and especially if caustic potash be dissolved in alcohol and hydrocyanic acid gas 

 be passed through this solution; crystals of potassium cyanide then separate directly from 

 the solution. Potassium cyanide is now prepared in large quantities from yellow prussiate 

 for gilding and silvering. "When fused in large quantities the action of the oxygen of the 

 air is limited, and the entire operation may be conducted with great care, and therefore, 

 on a large scale very pure salt is sometimes obtained. When slowly cooled, the fused 

 salt separates in cubical crystals like potassium chloride. 



