H. L. Wells — Rubidium- Lead Halides. 37 



3NH 4 PbCI,.H,0 NH 4 Pb,Cl B 

 .... (XH 4 ) o PbBr 4 .H,0 ... NH 4 Pb 2 Br 5 



NH 4 PbI,.2H 2 

 NH 4 PbClI 2 .2H 3 .... 



An inspection of the table shows that the caesium salts differ 

 from the others in including the 4 : 1 type and in being with- 

 out any 2:1 salts. It is quite probable that we have not suc- 

 ceeded in preparing all the salts that are possible, but it seems 

 certain that the 1 : 1 rubidium, potassium and ammonium salts 

 cannot be made on account of the comparative insolubility of 

 the simple halides. The caesium salts also differ from the 

 others in being all anhydrous. The hydrous rubidium salts 

 have less water or lose it more readily than the potassium com- 

 pounds. KPbI,.2H 9 is stable in the air, but RbPbI,.2H a O 

 loses its water readily. There is evidently a gradation, in affin- 

 ity for water, from the caesium to the potassium compounds. 

 A gradation in water, from the chloride to the iodide, appar- 

 ently exists in the potassium and ammonium compounds of the 

 1 : 1 type. That such gradations in water exist among the 

 double halides, increasing with the atomic weight of the halo- 

 gens and decreasing with the atomic weight of the alkali metals, 

 has already been observed by Remsen.* 



The simplicity of the ratios in the four types of double hal- 

 ides of lead is noticeable. The 1 : 1 type, according to Werner's 

 remarkable theory,f may be considered as the ideal type of a 

 double halide of an alkali-metal and a bivalent metal, and as 

 the limit beyond which the ratios of alkali-metal to lead can- 

 not go. The type is represented, as Werner mentions, by 

 numerous double cyanides of bivalent metals, such as K 4 Fe 

 (CX) 6 and by other salts, such as K 4 CdCl 6 . 



The number of 2 : 1 lead salts that we have prepared is 

 rather small, but this is a very common type among the known 

 double halides of the other bivalent metals. 



The number of 1 : 1 lead salts is the largest of all. It is 

 remarkable that all the double iodides belong to this type. 

 This is also a well known type of bivalent metal double halides. 

 It is noticeable that the salt CsPbBr 3 is dimorphous, while three 

 mercuric salts of the same type,;): CsHgCl 3 , CsHgBr, and 

 CsHgClBr 2 are also dimorphous. 



The 1 : 2 salts are all anhydrous, and a chloride and bromide 

 were prepared with each of the alkali-metals and with am- 

 monium. They are formed under wide limits of conditions 

 and are therefore very easily prepared. It is noticeable that 

 the rubidium, potassium and ammonium chlorides of this type 



* Am. Chem. Jour., xiv. 88. -f Zeitschr. anorgan. Chem.. iii, 281. 



t This Journal, xliv, 222. . 



