560 PEINCIPLES OF CHEMISTKY 



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 

 their 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 soluble. Therefore, if a solution of tartaric acid 

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

 potassium salts, 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 

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

 the yellow light emitted by the sodium salt depends on a group of light 

 rays having a definite index of refraction which corresponds with the 

 yelloV portion of the solar spectrum, having the index of refraction 

 of the Fraunhofer line (strictly speaking, group of lines) D, whilst the 

 salts of potassium give a light from which these rays are entirely absent, 

 but which contain rays of a red and violet colour. Therefore, if a 

 potassium salt occur in a "flame, on decomposing the light (after passing 

 it through a narrow slit) by means of a prism, there will be seen red 

 and violet bands of light situated at a considerable distance from each 

 other ; whilst if a sodium salt be present a yellow line will also appear. 

 If both metals simultaneously occur in a flame and emit light, the 

 spectrum lines corresponding to the potassium and the sodium will 

 appear simultaneously. 



