Prof. Miller on the Forms of some Compounds of Thallium, 153 



It will be seen from the foregoing Table that Kopp's values are 

 lower than the others ; and bearing in mind that the coefficient of 

 expansion of mercury, when deduced by means of these, falls below 

 that obtained by Regnault, but when deduced from Despretz's or 

 my own agrees closely with Regnault' s, we are led to conclude that 

 Kopp's values must be somewhat incorrect. 



" On the Forms of some Compounds of Thallium." By W. H. 

 Miller, M.A., For. Sec. R.S., Professor of Mineralogy in the Uni- 

 versity of Cambridge. 



Nitrate of Thallium, 

 Prismatic, I 0, 1 1=38° 8'-l ; 1 00, 1 1 = 62° 56''3. 



1 0, 1 1 

 LOO, 1 10 

 1 0, 2 1 

 1 0, 1 1 1 

 1 0, 2 1 1 

 I 1 0, 1 1 1 

 01 1, Oil 



1 1, 21 1 



1 1 0, 1 1 



2 1 0, 2 1 1 

 2 1 0, 2 1 



1 1, 1 1 1 



1 1 I, Tl 1 

 1 1 1, 1 I 1 

 1 1 1,11 1 

 21 1, 21 1 

 21 1, 2ll 

 21 1, 2l I 



90 

 62 56-3 

 44 23 

 68 6-5 

 51 13 

 34 57'5 



103 44 

 38 47 

 54 7'4 

 28 46 



91 14 

 21 53-5 

 43 47 

 93 44-8 



110 5 

 77 34 

 75 38 



122 28 



Fig. I 



Observed combinations :— 1 0, 1 1 1 ; 10 0, 111, 211; 10 0, 

 Oil, 111, 211; 10 0, 110, 2 10, 111, 2 11; 10 0, 11, 

 1 10, 2 10, 1 1 1, 2 1 1. 



No cleavage observable. 



From the observed minimum deviation of the brightest part of 

 the solar spectrum formed by refraction through the faces 10 0, 

 1 1 0, it appears that the index of refraction of a ray in the plane 

 01, and polarized in that plane, is about 1*817. The refrangibi- 

 lity of the other ray is greater, its minimum deviation through the 

 same faces being 93° nearly. 



