Measurement and Cleavage of certain Primitive Crystals. 255 



and transparent, and bearing at first sight as much the aspect of 

 quartz as of corundum. From this, I succeeded in obtaining, among 

 others, four very minute portions with brilliant and perfectly re- 

 flecting planes. These were procured by the assistance of the 

 pincers; but it must be confessed that, as the directions of the 

 natural joints were not at all visible, it was more by chance than 

 regular design that they were obtained at all. Owing to the 

 extreme hardness of the substance, I found it requisite, after placing 

 the specimen in the pincers, to envelope it and them in a piece of 

 cloth, to prevent the escape of the fragments ; as the force which 

 it is requisite to use would otherwise have caused them to fly in 

 various directions. The same mode was for similar reasons pur- 

 sued in regard to the oriental ruby and the sapphire : the latter 

 may be cleaved with the utmost beauty and regularity in one 

 direction ; in the others it is difficult. 



One of the fragments of corundum yields the measurements of 

 86° 4' and 93° 56' ; the other three, each 86° 4'. Two minute 

 portions of the sapphire give each 86° 4' ; another 93° 56'. One 

 fragment of the oriental ruby yields 86° 4'. In the whole seven 

 corresponding measurements of 86° 4' and two of 93° 56, which 

 therefore I consider to be the true value of the angles of the primi- 

 tive rhomboide. 



Sulphate of Strontian. 



Fig. 2. 



The form of the primitive crystal of the sulphate of strontian is 

 considered to be a right prism with rhombic bases. Its angles are, 

 according to Haiiy, 75°. 12'. and 104°. 48'. On submitting several 

 crystals with perfectly reflecting planes to the goniometer, I found 



2 k 2 



