38 



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. 

 Professor Miller*. 



By 



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 

 exeeed half a minute. 



Fie. 2. 



Fig. 1 









s' ^\W' 



^\ -^C\~ -^ 



<?'U 



d 



o 



X 



I 1 



S 



iy y 



\5 



f / 



\ ,v ^^ 



V^vS" 



\^\.x- 







/», N N 



^^TTV 



lo 



90° 0' 



PP 1 



70° 3'*5 



o h 



90 



s s' 



109 



hi 



90 



hy 



65 41 



mini 



61 10 



iy 



45 50 



x a/ 



38 38 



oy 



54 1-5 



In twin crystals having the twin axis perpendicular to one 

 of the faces m (fig. 2). 



mm l 57° 40' h h, 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. 



