88 



On the Form and Optical Constants of Nitre. 



become as minute as those of the water, and equally semi- 

 transparent. A candle held before fig. 3 is faintly reflected 

 from the second surface, as there is no actual contact of atoms 

 in nature; all substances, even the hardest metals, are porous. 

 In my next communication I shall give a theory of the 

 Iriscope, compared with Nobili's rings. 



I remain, Gentlemen, 



Your obedient servant, 

 London, June 8, 1840. J. READE, M.D. 



VI. On the Form and Optical Constants of Nitre. By 

 Professor MILLER*. 



THE following values of the angles between normals to the 

 several faces of nitrate of potash are calculated from means 

 of the best measurements of a large number of crystals at a 

 temperature of about 19 centigrade. The close agreement 

 of the results afforded by different crystals, renders it proba- 

 ble that the errors of the concluded angles do not in any case 

 exceed half a minute. 



Fig. 1. 



Fig. 2. 



PF 

 s s 1 

 hy 



iy 



oy 



70 3'-5 

 109 

 65 4-1 

 45 50 

 54 1'5 



In twin crystals having the twin axis perpendicular to one 

 of the faces m (fig. 2). 



mm, 57 40' h h t 61 10'. 



The parameters are respectively proportional to the num- 

 bers 2-4285; 1'4352; 1'7023. 



The cleavage most easily obtained is parallel to the faces p. 

 When the temperature of the crystal is increased 100 C. 



Communicated bv the Author. 



