POTASSIUM, BUBIDIUM, C-3SSIUM, AND LITHIUM 551 



tained by means of lime in exactly the same manner in which sodium 

 hydroxide is prepared from sodium carbonate. 11 Therefore, in order to 

 complete our knowledge of the alkali metals, we will only. describe two 

 salts of potassium which are of practical importance, and whose 

 analogues have not been described in the preceding chapter, potassium 

 cyanide and potassium nitrate. 



Potassium cyanide, which presents in its chemical relations a certain 

 analogy with the halogen salts of potassium, is not only formed accord- 

 ing to the equation, KHO -f HOIST ==: H 2 -f KCN, but also when- 

 ever a nitrogenous carbon compound for instance, animal matter is 

 heated in the presence of metallic potassium, or of a compound of 

 potassium, and even when a mixture of potash and carbon is heated in 

 a stream of nitrogen. Potassium cyanide is obtained from yellow 

 prussiate, which has been already mentioned in Chapter IX., and 

 whose preparation on a large scale will be described in Chapter XXII. 

 Tf the yellow prussiate be ground to a powder and dried, so that it loses 

 its water of crystallisation, it the"n melts at a red heat, and decomposes 

 into carbide of iron, nitrogen, and potassium cyanide, FeK 4 C 6 N 6 

 = 4KCN 4- FeC 2 f N 2 . After the decomposition it is found that the 

 yellow salt has been converted into a white mass of potassium cyanide. 

 The carbide of iron formed collects at the bottom of the vessel. If 

 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 



11 Caastic potash is not Only formed by the action of lime on dilute eolations of 

 potassium carbonate (as sodium hydroxide is prepared from sodium carbonate), but 

 by igniting potassium nitrate with finely-divided copper (see Note 15), and also by mixing 

 solutions of potassium sulphate (or even of alum, KA1S 2 O 8 ) and barium hydroxide, 

 BaH 2 O 3 . It is. sometimes purified by dissolving it in alcohol (the impurities, for example, 

 potassium sulphate and carbonate, are not dissolved) and then evaporating the alcohol. 



The specific gravity of potassium hydroxide is 2*04, but that of its solutions (see 

 Chapter XII., Note 18) at 15 8=9,992+90-4j? + 0'28p (here^ 8 is +, and for sodium 

 hydroxide it .is - ). Strong solutions, when cooled, yield afcry stallo-hydrate, KHO,4H ? O, 

 which dissolves in water, producing cold (like 2NaHO,7H20), whilst potassium hydroxide 

 in solution develops a considerable amount of heat. 



12 When the yellow prussiate is heated to redness, all the cyanogen which was in 

 combination with the iron is decomposed into nitrogen, which is evolved as gas, and 

 carbon, which combines with the iron. In order to avoid this, potassium carbonate i 

 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. Double 

 decomposition then 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, 

 because a portion of the potassium cyanide is oxidised at the expense of the iron 

 carbonate end forms potassium cyanate, FeCO 3 + KCN=CO a + Fe-f KCNOj and tho t 

 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). By adding 



