( 369 ) 



4. Potassium-, and Aimnouium-salts. 



From mixed solutions of K- and (NllJ-tricliloroat'etale, on slow- 

 evaporation, mixed crystals are dei)0sited which exhibit solely the 

 tetragonal-bipyramidal form, and often possess a pronounced la) cr- 

 slructure. They are optically positive, — even the crystals deposited 

 later ; the interference image is often a little distorted, which points 

 to a not quite homogeneous structure. 



5. Rubidium-, and Tlialliuin-saH. 



From mixed solutions were obtained the mixed crystals of the 

 form of the 7^6-salt and chiefly in the habit of Fig. 7. The angle 

 (ilO):(lïO) was 72°42\'; (Ollj : (OJf) = lll°4i'. Up to the present 

 no individuals of the jfV-type have been noticed. 



6. Caesium-, and Thallium-salts. 



Even with a very small YY-salt content, mixed crystals of the 

 form of the Rb-?,i\\\. are deposited from the solutions of the two 

 trichloroacetates. At a very large concentration of the 6V-salt, 

 however, there were also found — besides thes aid monoclinic forms, — 

 monoclinic, rectangular thick plates, which were cleavable towards 

 |001| and exhibited in convergent light a beautiful axial-image, with 

 a very small axial-angle, a strong dispersion . q^ i', and with the 

 first bisectrix nearly ± upon |001j. The character of the first diagonal 

 was negative. The following measurements were taken : (001) : 

 (111) = 53°5i'; (111) : (IlO) = 37°1'; (110 : (001) = 89°9'. The 

 mixed crystals, therefore, possess the first form of the pure caesium- 

 salt ; those with the second form ha\"e nexer been found as yet. 



7. Caesium-, and Rubidium-salts. 



As in the case of the caesium-thallium-salt-mixtures, mixed crystals 

 of the i^6-type appear first ; these possess chiefly the habit shown 

 in Fig. 8. 



The following measurements were taken : (110) :(1Ï0) = 72° 247./; 



(Oil : (Oil) = 111° 207, 001) : (111) = 61° 4' ; (111) : (110) = 27° 21'; 

 (001) : (110) =r 88° 25'. 



At a very great concentration of the caesium salt, however, one 

 finds very thin, square, little plates, which appeared to be mixed 

 crj^slals of the first caesium-type, which are deposited alongside 

 with the other mixed crystals. They are biaxial, with a very small 

 axial angle and negative character of the bisectrix. The dispersion 

 is weak: q<^^ ^ï^<^ with rhombic symmetry. Mixed crystals of the 

 second C's^-type have neither here been met with as yet. 



Inoiy. Clieiii. Lab. of the University. 



Groningen, September 1911. 



