450 Mr. Brooke on the [June, 



With a view to render the descriptions of these as simple and 

 as practical as possible, it is not my intention to consider them 

 mathematically, and in relation to the theory of decrements. 

 The information the chemist requires to be posssessed of con- 

 cerning the crystals which may be formed during his operations, 

 is the character of their simplest or primary forms;* their cleav- 

 ages where they can be given; their modified or secondary 

 forms ; the angles at which their planes severally incline to each 

 other; with occasional notices on their predominating charac- 

 ters, and on any peculiar habitudes which may be observed to 

 belong to particular crystals. 



Descriptions of several of the artificial salts, founded on these 

 characters, will form the substance of this and some following- 

 communications. These will be accompanied by figures which 

 are not drawn with geometrical truth, and are intended merely 

 as diagrams to which the measurements of the crystals may be 

 more conveniently referred, and which will, at the same time, 

 convey a general idea of the form of the substance described. 



Crystals deposited from the Oil ofCuhebs. 



Of the chemical nature of the substance of these crystals, 

 which I received from Mr. Teschemacher, I know nothing. 



The predominating form of the crystals is 

 that of an octahedron with a rhombic base, 

 as shown in fig. 1, measuring as follows : 



P on P' about 115 



P over plane v 



PonP" 



n on o 

 n on v 

 n on a 



• ••••■ 



Arseniate of Potash. 



The primary form of this substance may 

 be regarded as a right square prism. 



In the crystals I have measured, the 

 terminal edges of the prism are replaced, as 

 seen in fig. 2, measuring as follows : 



Fig. 2. 



M on c 



W on c 

 „/ 



'} 



»••••• loo 1 <_) 



cone 122 2 



The predominating form of most of the 



• The nomenclature of forms, and the letters placed on the crystals, are those which 

 are used in the introductory volume already alluded to. 



