( 365 ) 



No distinct ciccivage was found. The coujpoimd iss ccrlcünly isunior- 

 plioiis witii the K-, and the (iV//^)-salt; the parameters are 7iearlij 

 identical with those of the ammoninni-saU. The crystals are optically 

 monaxial without circular polarisation. 



In contrast, however, with the K-, and (A^//J-salts, they })ossess 

 a negative double refraction. 



The clear crystals, if kept for a long time, sometimes become, 

 locally, opaque and white like porcelain. While retaining their out- 

 ward shape, they gradually become so altogether. 



Topic parameters : ^.yp .oi = 6,2547 : 6,2547 : 4,7987. 



§ 4. VI. In order to confirm experimentally the presumption of 

 a possible existence of eventual monoclinic, less stable modifications 

 of the thallium-salt, the Thallous-trtbromoacetate was prepared and 

 investigated. 



And indeed, the monoclinic form proved to be here the more stable 

 one, and the analogy with the form of the rubidiiun-sah of the 

 previous series is at once a striking one. 



Acid thallium-tribromoacetate : TlCOJJBr^. HCO^CBr^. The 

 analysis gave: 25,6 "/o Tl ; calculated 25,59 7o- 



Sp. gr. at 18°, 3,923; molecular volume = 203,16. 



Small, colourless or very pale, very lustrous crystals, which mostly 

 have the form of thick crystal-plates of quadratic shape. They are 

 shown in Fig. 6. 



Acid Thallous-Tribromoacetate. 

 Fig. 6. 



25* 



