1819.] and on the Blue Carhonate of Copper. . 455 



The example I have selected of carbonate of lime evinces that 

 this theoretic simplicity does not always exist in nature ; and 

 when we observe that it leads to an error amounting to more 

 than double the difference between two known species of 

 minerals, the magnesian and ferriferous carbonate of lime : 

 this difference being only 15', and not knowing that there 

 are not other distinct species approaching still nearer in their mea- 

 surements, a theory which is satisfied with an error of 37' does 

 not appear to have attained to that degree of perfection which 

 warrants its being regarded as " sufficiently near the truth." 



But the intelligence which has conceived this theory can 

 easily remedy its imperfections ; and we may hope that the 

 second edition of his Mineralogy, which the scientific author is 

 preparing for the press, will rather adopt and reason upon the 

 dimensions which nature has given to crystals than clothe them 

 with an imaginary character which must be erroneous in propor- 

 tion as it deviates from nature. 



The immediate occasion of these remarks is the appearance of 

 a memoir by M. Cordier on the blue carbonate of copper, in 

 the first part of the Annales des Mines for 1819, which is 

 stated to contain the latest observations of the Abbe Haiiy on 

 that substance. 



A list is given in this memoir of the values of 31 angles, 

 primary and secondary ; and as far as I have compared them 

 with the measurements I have taken on the natural planes of 

 some brilliant crystals, and on some planes obtained by cleavage, 

 not one of them is correct. 



The form assumed in this memoir as the pri- 

 mary one is an octahedron with a rhomboid, or 

 oblique angled parallelogram, for the common 

 base of the two pyramids ; and for the sake of 

 rendering the position of some of the measured 

 planes apparent to the reader, I have given this 

 figure, although I do not concur in the neces- 

 sity of adopting this as the primary form, having 

 observed a cleavage parallel to the edge C, and 

 to a line joining the summits E and E' of the octahedron. 



The following are the values of some of the angles as given by 

 M. Cordier contrasted with those afforded by the reflective 

 goniometer : 



Cordier. 



Reflective Goniometer. 



Pon P' 116° 44' 121° 00' on the natural planes or cleav- 

 ages of three crystals. 

 121 03 on a cleavage by Mr. Phillips. 

 P M 127 32 .... 125 25 

 V W 82 14 .... 81 35 

 P A 121 38 .... 119 30 A is a plane parallel to tht 



M M' 97 46 



98 30 



edge (■• 



