190 H. L. Wells — Halides of Potassium and Lead. 



added as long as a precipitate forms.* The precipitate is col- 

 lected on a filter and washed with hydrochloric acid. To re- 

 move traces of rubidium, the precipitate is thoroughly decom- 

 posed with successive, small quantities of hot water, then 

 hydrochloric acid and a little antimony trichloride are added 

 to the whole in order to repeat the precipitation. The last 

 precipitate is washed with hydrochloric acid. It usully shows 

 no rubidium when tested with the spectroscope. The caesium 

 antimony chloride is decomposed with hot water and hydrogen 

 sulphide is passed into the resulting solution. The filtrate 

 from the antimony sulphide gives, on evaporation, pure caesium 

 chloride. The filtrates from the antimony double salt are 

 freed from antimony, evaporated to dryness and the mixture 

 of caesium and rubidium chlorides, which should be very small 

 in amount, is preserved for use in subsequent purifications. 



Sheffield Scientific School, April, 1893. 



Art. XXYIII. — On some Peculiar Halides of Potassium 

 and Lead ; by H. L. Wells. 



In a preceding article I have described a series of double 

 chlorides of the type M 2 PbCl 6 , where M is NH 4 , K, Eb and 

 Cs. It has seemed desirable to extend the investigation by 

 attempting to prepare bromides and iodides corresponding to 

 these salts. A thorough search has been made, using the metals 

 of the potassium group and sodium, with the result that no 

 double bromides or iodides containing extra halogen could be 

 prepared except in the case of potassium. It is remarkable 

 that the potassium-lead bromide and iodide which have been 

 discovered do not correspond in composition to the chlorides. 

 The failure to prepare double salts of rubidium and caesium 

 corresponding to the new potassium compounds was unex- 

 pected, for, as a general rule, the insolubility and stability and 

 the consequent ease of preparation of such compounds become 

 greater from potassium towards caesium. The explanation of 

 the anomaly probably lies in the fact that extremely concen- 

 trated rubidium and caesium solutions containing a lead halide 

 and the corresponding halogen cannot be obtained, in the case 

 of the bromides and iodides, on account of the slight solubil- 

 ity of caesium triiodide, and of the double halides which are 

 formed with FbBr 2 and Pbl 2 . 



The compounds to be described probably have the composi- 

 tion represented by the following formulae : 



* Method of Godeffroy, Berichte, vii, 375. 



