120 Mr, Brooke on the [Ato. 



which follow. The crystals, however, so speedily lose their 

 brilliant surfaces when exposed to the air, or even when inclosed 

 in a bottle, that the measured angles of the secondary faces are 

 less to be relied upon than those afforded by the cleavage planes, 



MonM' 101° 30' 



Mon/i 129 15 



M on ff 163 23 



gong^ 134 45 



a ona'i .... .^ .. . •.. Ill 60 



aonb...i 161 30 



honcx i 139 45 



/*onca 121 15 



c. onc« 161 30 



Ci onc\ 117 30 



Sulphate of Iron, — Sulphate of Cobalt,* 

 The crystaUine form assigned by the Abbe Iliuy to sulphate 

 of iron is a rhomboid ; but it was, I believe, first observed by 

 Dr. Wollaston, that its true form was an oblique rhombic prism. 

 I do not find any published account of the 

 ordinary figure of the crystals, or of the 

 measurements of the planes ; and as its 

 form approaches very nearly to that of sul- 

 phate of cobalt, I am induced to give the 

 measurements of both substances m refer- 

 ence to the annexed figure. 



In sulphate of cobalt another plane some- 

 times appears as ei, which measures about 

 124° with P. And in both these sulphates 

 there are also other planes a and e, which occur on some of the 

 crystals. 



Sulphate of iron. 



P on M, or M' 99° 20' ... . 



MonM' 82 20 .... 



P onei 153 00 



Pone* 123 55 . . . . 



Vonai 159 00 .... 



Pon«» 136 10 .... 



Pone' 119 15 



Chr ornate of Potash, 



The primary form has been determined from some very perfect 

 and brilliant crystals which I have received from M. Teschema- 

 cher, and the measurements given below have very nearly coin- 

 cided on several of these. 



There is a distinct cleavage parallel to the plane A, but appa- 

 rently in no other direction. The primary form inferred from 

 that of the crystals, as shown in fig. 1, is a right rhombic prism ^ 



* Foi this salt I am indebted to Mr« Cooper. 



