544 PKINCIPLES OF CHEMISTRY 



gilding. An alkaline solution is prepared, 13 which is moderately stable 

 owing to the fact that potassium cyanide in the form of certain double 

 salts that is, combined with other cyanides is far more stable 

 than when alone (yellow prussiate, which contains potassium cyanide 

 in combination with ferrous cyanide, is an example of this). The pro- 

 perty of potassium cyanide of giving double salts with other cyanides is 

 very clearly shown by the fact that many metals dissolve in a solution 

 of potassium cyanide, with the evolution of hydrogen. For example, 

 iron, copper, and zinc act in this manner. In which case potassium 

 hydroxide is formed ; for example 



4KCN + 2H 2 O -i- Zn=K. 2 ZnC 4 N 4 + 2KHO + H 2 . 



Gold and silver are soluble in potassium cyanide in the presence of 

 air, in which case the hydrogen, which might be evolved in the reaction, 

 combines with the oxygen of the air, forming water. Platinum, 

 mercury, and tin are not dissolved in a solution of potassium cyanide,, 

 even with the access of air. 



Potassium nitrate, or common nitre or saltpetre, KNO 3 , is chiefly 

 used as a component part of gunpowder, in which it cannot be replaced 

 by the sodium salt, because the latter is deliquescent. It is necessary 

 that the nitre in gunpowder should be perfectly pure, as even small 

 traces of sodium, magnesium, and calcium salts, especially chlorides, 

 render the nitre and gunpowder capable of attracting moisture. Nitre 

 may easily be obtained pure, owing to its great disposition to form 

 crystals both large and small, which aids its separation from other 

 salts. The considerable differences between the solubility of nitre at 

 different temperatures aids this crystallisation. A solution of nitre 

 saturated at its boiling point (116) contains 335 parts of nitre to 100 

 parts of water, whilst at the ordinary temperature for instance, 20 

 the solution is only able to retain 32 parts of the salt. Therefore, in 

 the preparation and refining of nitre, its solution, saturated at the 

 boiling point, is cooled, and nearly all the nitre is obtained in the form 

 of crystals. If the solution be quietly and slowly cooled in large 

 quantities then large crystals are formed, but if it be rapidly cooled and 

 agitated then small crystals are obtained. In this manner, if not all 

 at all events the majority of the impurities present in small quantities 

 remain in the mother liquor. If an unsaturated solution of nitre be 

 rapidly cooled, so as to prevent the formation of large crystals (in whose 

 crevices the mother liquor, together with the impurities, would remain), 

 the very minute crystals of nitre known as saltpetre flour are obtained. 



13 The electro-deposition of metals generally proceeds more uniformly and gives a 

 purer deposit from alkaline, than from acid, solutions. 



