( 868 ) 



The first angle is 11' larger, tlie latter 6' smaller than that of the 

 it6-salt. From the last fractions however, — which, of course, are 

 very rich in potassium-salt, — both monociinic crystals and tetragonal 

 double pyramids are deposited. They are monaxial and faintly positive. 



3. Potassium-, and Caesium-salts. 



Up to the present the most peculiar phenomenon, however, was 

 found with mixtures of the potassium-, and the caesium-^d\\.. The 

 latter is exceedingly soluble ; from the mixed solution are deposited 

 fii'St mixed crystals, richer in potassium. Although possessing a 

 somewhat different habit (Figs. 7 and 8), these crystals, which, 



Fig. 7. Fig. 8. 



Mixed crystals of Potassium- and Caesium-saW.. They exhibit the form of 



the Rubidiiim-sa\{. 



though small, were irreproachably l)uilt, appeared to exhibit the 

 monociinic form of the Rb-sah \ The remarks made in the description 

 of the thallous-tribromoacetate also apply hei'e : the monociinic form, 

 which was the more stable one in the Rb-ssiit, appears to also occur 

 by preference in the mixed crystals as an exceedingly stable struc- 

 tuie. If we adhere to the principal difference between Rb- and Cs- 

 forms, there must, probably, be three hiatus in this mixing series. 



That indeed the form of the monociinic Rb-ssi\t and not that of 

 the monociinic CVsalt was present is proved by the following mea- 

 surements : 

 c:q = (001) : (Oil) = 72°32' m : m = (110) : (ifO) = 111°40' 



w :^ = (110):(0T1) = 37°13' 



c : m = (001) : (110) = 88°21' q ■ q = (OJ 1) : (Oil) = 34°o2' 



m:(7 = (TlO):(011)==38°21' 



The angles in the different mixed crystals of this series diverge 

 about -|- 4' and — 5' from the above values. 



Up to the present, no tetragonal mixed crystals have been observed ; 

 also no monociinic ones in the second form of the C'v-salt. 



