374 



Mr, Brooke on the 



[Nov. 



Article IX. 



On the Crifstalline Forms of Artificial Salts, 

 By H. J. Brooke, Esq. FRS. 



(.Continued from p. 288.) 



Having dissolved and recrystallised several of the salts 

 described in these communications, I have observed differences 

 in the figures of what may be termed different crops of crystals 

 obtained from the same solution. Having dissolved some chro- 

 mate of soda, the crystals first deposited, or Jirst crop, as we may 

 term them, were all lengthened in the direction of the great 

 diagonal of their terminal planes, so as to be almost acicular. 

 These crystals having been taken out of the solution, a second 

 crop was soon deposited, many of which nearly agreed in form 

 with the engraved figure already given, but most of them were 

 much flattened or reduced in height, so as to become what has 

 been termed tabular, and apparently bearing no relation to the 

 slender crystals first produced. 



The same difference of character is found to obtain in many 

 other salts. When these varieties of figure occur, the goniometer 

 will afford sufficient evidence that their differences are only 

 apparent, and that they are really analogous forms whose cha- 

 racter has been varied by a disproportionate extension of some 

 of the planes of the crystals in particular directions. 



Acetate of Lead* 



I have received some brilliant crystals of this substance from 

 Mr. R. Phillips, several of which have given measurements on 

 the corresponding natural planes agreeing within 3' or 4', and 

 affording an example of unusual regularity of form. 



The crystals may be cleaved parallel to 

 the lateral and terminal planes, of a right 

 oblique angled prisfu, which may be regarded 

 as its primary form. The only modification 

 I have observed is exhibited in the annexed 



fiffure. 



dond' 128° 0' 



doiiU 116 



donT 98 30 



M on T 109 32 



Oxalate of Ammonia. 



I have not observed any distinct cleavage of the crystals of 

 this salt, but their forms are referable to a right rhombic prism as 

 the primary. They are subject, however, to an irregularity of I 



