456 On the primilive Crystals of Carbonate of Lime, 



It is by no means my design to detract in any degree from 

 the merit of that justly celebrated crystallographer, to the 

 surprising accuracy of whose measurements I could, in va- 

 rious instances, bear testimony. I hope, on the contrary, 

 that in bringing forward two more observations similar to 

 the preceding, and intimately connected with it, I shall 

 offer what will not only appear interesting to crystallogra- 

 phers in general, but will be peculiarly gratifying to the 

 Abbe Haiiy. 



Jn his 1>aite de Mineralogie, and again more recently 

 in his Tableau Comparalif, the same primitive form is as. 

 signed to three substances very different in their compo- 

 sition, to carbonate of lime, to magnesian carbonate of 

 lime (or bitter- spar), and to carbonate of iron. 



It has been objected to Mons. Haiiy, that according to 

 his method identity of form should be accompanied by 

 identity of composition, unless the form were one of the 

 common regular solids. For though in that case any geo- 

 metrician would readily admit it to be very probable, that 

 many different substances might concur in assuming the 

 same form of cube, of octohedron, or of dodecahedron, 

 &c. there does not appear a corresponding probability that 

 any two dissimilar substances would assume the same form 

 of a particular rhomboid of 103'^ and a few minutes, to 

 which no such geometric regularity or peculiar simplicity 

 can be ascribed. 



Bui though so accurate a correspondence, as has been hi- 

 therto supposed to exist in the measures of the three car- 

 bonates above mentioned, might be justly considered as 

 highly improbable, no degree of" improbability whatever 

 attaches to the supposition, that their angles approach each 

 other bv some diflerence, so small as hitherto to have es- 

 caped detection. And this in fact I find to be the case. 



Since the angles observable in fractures of crystalline 

 substances are subject to vary a little at different surfaces, 

 and even in different parts of the same surface (as is evident 

 from the confused image seen by reflection from them), I 

 siiall not at present undertake to determine the angles of 

 these bodies to less than five minutes of a degree. This, 

 indeed, is the smallest division of the goniometer that I 

 usually employ, as I purposely decline giving so much time 

 to these inquiries as would be requisite for attempting to 

 arrive at sreater precision. 



The most accurate determination of the angle of car- 

 bonate of lime is probably that of Mons. IVlalus, who mea- 

 sured it by ujeans of a repeating circle, and found it to bs 



105° 5', 



