40 



Mr, Brooke on the 



[July, 



30' 



00 



4 



45 



is exhibited in front of tlie figure, but inverted in its position 

 so as to be terminated at its lower extremity by the planes 

 g and c. 



PonM, orM' 105° 



M on M' 72 



Pone' 119 



P on g, or g' 131 



The planes M and M' are generally curved, and the cleavage 

 planes parallel to these partake also of the same character. 



Sulphate of Magnesia, 



The primary form of this substance has 

 been given by the Abb6 Haiiy as a right 

 prism with a square base. But from the 

 measurement of several crystals, and from 

 the character of the secondary forms of 

 fiome of those, the primary may be regarded 

 as a right prism with a rhombic base, whose 

 andes are 90° 30' and 89° 30'. 



1 have found only one cleavage, which is 

 parallel to the short diagonal of the prism, 

 and consequently to the plane h of the accom- 

 panying figures. 



Fig. 1 represents a crystal of a form 

 "which frequently occurs, and of which the 

 following are the measurements : 



Mon M' 90° 30' 



M on ^ 134 45 



Mone 129 00 



a on a' 120 nearly 



Fig. 2 represents a form under whicli the 

 crystals also frequently appear. In this 

 form, only two of the four planes e are seen 

 on each summit, and alternating in position 

 as shown in the figure. 



On some of the crystals, however, which resemble this figure, 

 the two other planes e may be perceived, but they are very 

 minute. 



Tartrate of Potash and Antimony — Emetic Tartar, 



The'general character of the crystals of this compound is that 

 of an octahedron with a rhombic base. I cannot discover more 

 than one distinct cleavage, which is parallel to the plane a of the 

 accompanying figure. 



The following are the nearest to coinciding measurements 

 taken on several crystals ; 



Fig. 2. 



