Optical Properties of Sulphate of Thallium. 249 



meters are 



a:b:c = l : 07319 : 0-5539; 



and the calculated and observed angles compared with the cor- 

 responding values for sulphate of potassium are : — 





Calc. 



Obs. 



KSO 4 . 



110. 



/ 



100= — 



53 48 



53° 16 



110. 



1 = 36 12 



36 21 



36 44 



210, 



, 1 = 34 20 



— 



33 49 



210, 



. 1 = 55 40 



— 



56 11 



110, 



,210= 19 28 



19 30 



19 27 



101 



,100= — 



61 1 



60 12 



101 



.101 = 57 58 



— 



59 36 



111 



.100 = 66 10 



66 11 



65 34 



111 



. 1 = 56 29 



56 29 



56 20 



111 



.110 = 46 50 



46 50 



46 15 



111 



. 1 1 = 33 31 



33 40 



33 40 



111 



. 1 1 1 = 67 2 



67 14 



67 19 



111 



. T 1 1 = 47 38 



47 48 



48 52 



The faces are very brilliant, and have an adamantine lustre, 

 probably due to the greater predominance (from its high che- 

 mical equivalent) of the element thallium in the compound : 

 thallium may thus be supposed to stand in the same relation to 

 potassium that lead does to the isomorphous barium. 



With regard to the optical properties of these crystals, I found, 

 on laying one of them on the large plane (1 0) in the polari- 

 zing apparatus, that, from the absence of interference-curves and 

 black hyperbolas, this plane is probably parallel to the plane of 

 the optic axes, and that therefore the axis a is parallel to the 

 axis of mean optical elasticity. Trying the compensating action 

 of a section of quartz, interference-curves were produced when 

 the quartz was turned on an axis parallel to the crystallographic 

 axis c; the interference-fringes thus produced were parallel with 

 the same direction. We may conclude from this experiment 

 that the smallest axis c coincides with the axis of least optical 

 elasticity, and that the mean crystallographic axis is parallel to 

 the greatest axis of elasticity. The optical orientation of these 

 crystals is therefore given by the symbol 



6ac. 



I succeeded afterwards in splitting out of one of the crystals 

 a section across the direction in which the crystals are prolon- 

 gated. This section showed directly, by the appearance of 

 coloured curves, that the plane of the optic axes is parallel with 



