POTASS i r.M. KU i;i i HUM, CAESIUM, AND LITHIUM 551 



oxide ; potassium peroxide contains KO 2 , but it is probable that in the 

 combustion of potassium an oxide KO is also formed. Potassium, 

 like sodium, is soluble in. mercury.- 2 In a word, the relation between 

 sodium and potassium is as close at that between chlorine and bromine, 

 or, better still, between fluorine and chlorine, as the atomic weight of 

 sodium, 23, is as much greater than that of fluorine, 19, as that of 

 potassium, .'39, is greater than that of chlorine, 35 '5. 



The resemblance between potassium and sodium is so great that 

 (/!>'!/ compounds can only be easily distinguished in the form of certain 

 of their salts. For instance, the acid potassium tartrate, C 4 H 5 KO 6 

 (cream of tartar), is distinguished by its sparing solubility in water and 

 in alcohol, and in a solution of tartaric acid, whilst the corresponding 

 sodium salt is easily dissolved. Therefore, if a solution of tartaric acid 

 be added in considerable excess to the solutions of the majority of 

 potassium salts, then a precipitate of the sparingly- soluble acid salt is 

 formed, which does not occur with salts of sodium. The chlorides KC1 

 and NaCl in solutions easily give double salts K 2 PtCl 6 and Na 2 PtCl 6 , 

 with platinic chloride, PtCl 4 , and the solubility of these salts is very 

 different, especially in a mixture of alcohol and ether. The sodium salt 

 is easily soluble, whilst the potassium salt is insoluble or almost so, and 

 therefore the reaction with platinic chloride is that most often used 

 for the separation of potassium from sodium, as is more fully described 

 in works on analytical chemistry. 



It is possible to discover the least traces of these metals in admix- 

 ture together, by means of their property of imparting different colours 

 to aflame. The presence of a salt of sodium in a flame is recognised 

 by a brilliant yellow coloration, and a pure potassium salt colours a 

 colourless flame violet. However, in the presence of a sodium salt 

 the pale violet coloration given by a potassium salt is quite undistin- 

 guishable, and it is at first sight impossible in this case to discover the 

 potassium salt in the presence of that of sodium. But by decomposing 

 the light given by a flame coloured by these metals, or a mixture of 

 ithem, by means of a prism, they are both easily distinguishable, because 



l The definite crystalline amalgam of potassium contains twice as much mercury, 

 KH<;. 2 (as the potassium peroxide contains twice as much oxygen, Chap. XII. Note 89). 



Potassium forms alloys with sodium in all proportions. The alloys containing 1 and 

 3 equivalents of potassium to one equivalent of sodium are liquids, like mercury at the 

 ordinary temperature. Joannis, by determining the amount of heat developed by these 

 .alloys in decomposing water, found the evolution for Na^K, NaK, NaK 2 , and NaK 3 to be 

 44-5, 44-1, 48'8 and 44'4 thousand heat units respectively (for Na 42'6 and for K 45'4). 

 The formation of the alloy NaK 2 is therefore accompanied by the development of heat, 

 whilst the other alloys may be regarded as solutions of potassium or sodium in this alloy. 

 In any case a fall of the temperature of fusion is evident in this instance as in the alloys 

 of nitre (Note 14). 



