x Introduction 



at which the primary planes of crystals incline to each 

 other. And some of the mistakes which originate 

 from this supposition are so important, as to cast a 

 shade of disconfidence over his determinations relative 

 to the primary forms of crystals. 



His inaccuracy with respect to the angle of carbo- 

 nate of lime is a well known example of one of these 

 theoretic errors. 



His inaccurate measurements of many of the angles 

 of crystals, have probably been occasioned by the 

 comparatively imperfect instrument with which those 

 measurements were taken. That he should have con- 

 tinued to prefer this, to the more perfect goniometer 

 invented by Dr. Wollaston, may possibly have been 

 owing to the decay of sight incident to his period of 

 life, and to that dislike to change which so frequently 

 accompanies advanced age. 



But some of his inaccuracies are independent both 

 of his theory and his goniometer, and it would almost 

 appear that he had occasionally written from the dic- 

 tates of his fancy, without examining the minerals he 

 has described. 



The resemblance he imagines to exist between the 

 crystals of bournonite and those of sulphuret of anti- 

 mony is an instance of this nature; and on some of 

 his figures, as those of wolfram, and some of those 

 which he has still retained as stilbite, although they 

 belong to a distinct species of mineral to which I have 

 given the name of heulandite, he has placed imagi- 

 nary planes which have no existence on the crystals 

 themselves. 



tlis persisting in the identity of the angles of the 

 primary forms of carbonate of lime, bitter spar, and 

 carbonate of iron, if he has really been deceived by 

 his goniometer, evinces a carelessness in the use of 



