58 



Royal Society : — 



In the foregoing comparative Chart of the physical properties of 

 the two salts, the axis has heen assumed to coincide with a line 

 drawn through the short diagonal of the primary rhombic crystal, 

 which will coincide with the long diameter of the a-prism, and the 

 plane of the breadth of the /3-prism, and is therefore the T" of the 

 tiiree rectangular crystallographic axes. 



It has been compiled from the observations of Professors Stokes 

 and Haidinger and the author. It appears to form a complete 

 optical description of the two salts, as far as they are at present known. 



Whilst in both salts the indicative body-colours, or those due to 

 the more absorbed pencils (3), are only to be seen in the thinnest 

 crystals, it is evident that the reflected rays may be seen indifferently 

 in crystals of all thicknesses ; and the author is inclined to believe 

 that "the cinchonidine salt possesses even greater tourmaline absorbent 

 powers upon ordinary light, inasmuch as much thinner plates are 

 required in order to obtain the indicative body-colours, — perfect 

 absorption, and therefore total obstruction, being more early arrived 

 at than in the case of the qiunine salt. 



The author's more recent analyses of the cinchonidine salts have 

 produced the following results : — 



Sulphate of lodo-Cinchonidine. 



100-000 

 which lead to the following composition :- 



100-000 



One other remarkable difference exists between the quinine and 

 cinchonidine salt, which is, that the optical crystals of the last salt, 



