POTASSIUM, RUBIDIUM, C^SItTM, AND LITHIUM 558 



Gold and silver are soluble in potassium cyanide in the presence t)f 

 air, in which case the hydrogen, which would otherwise be evolved in 

 the reaction, combines with the oxygen of the air, forming water (Eissler, 

 MacLaurin, 1893), for example, 4Au + 4KCN + + H a O 2AuKCaN fl 

 + 2KHO, which is taken advantage of for extracting gold from its 

 ores (Chapter XXIV.). l ** to Platinum, mercury, and tin are not dis- 

 solved in a solution of potassium cyanide, even with access of air. 



Potassium nitrate, or common nitre or saltpetre, KKO^ 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 the 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 fornfation 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. 



Common nitre occurs in nature, but only in small quantities in 

 admixture with other nitrates, and especially with sodium, magnesium, 

 and calcium nitrates. Such a mixture of salts of nitric acid is formed 

 in nature in fertile earth, and in those localities where, as in the soil t 

 nitrogenous organic remains are decomposed in the presence of alkalis 

 or alkaline bases with free access of air. This method of the formation 

 of nitrates requires moisture, besides the free access of air, and takes 

 place principally during warm weather. 14 In warm countries, and in 



bu A -dilute solution of KCN is taken> not containing more than 1 per cent. KCN. 

 MacLaurin explains this by the fact that strong solutions dissolve gold less rapidly, owing 

 to their dissolving less air, whose oxygen is necessary for -the reaction. 



M Besides which Schloessing and Mtinta, by employing runilar methods to Pasteur, 

 fehowed that the formation of nitre in the decomposition of nitrogenous substances id 



