PRIMITIVE CRYSTALS OF SOME CARBONATES^ SOQ 



hedron, or of dodecahedron, &c. ; there does not appear a cor- 

 responding probability, that any two dissimilar substances 

 would assume the same form of a particular rhomboid of 105° 

 and a few minutes, to which no such geometric regularity, or 

 peculiar simplicity, can be ascribed. 



But though so accurate a correspondence, as has been The forms of 

 hitherto supposed to exist in the measures of the three carbo- bonatersfmf/* 

 nates above-mentioned, might be justly considered as highly jar, but noc 

 improbable, no degree of improbability whatever attaches to ' ^"^"^^ * 

 the supposition, that their angles approach each other by some 

 difference, so small as hitherto to have escaped detection. 

 And this, in fact, I find to be the case. 



Since the angles observable in Jractures of crystalline sub" Least measure 



stances are subiect to vary a little at different surfaces, and °^ ^" ^°^'® 

 -' •' usually em- 



even in different parts of the same surface (as is evident from ployed by the 



the confused image seen by refieciion from them) I shall 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 greater precision. 



The most accurate determination of the angle of carbonate Primitive an- 

 of lime is probably that of Mons, Malus, who measured it by '„^te^of [ime"" 

 means of a repeating circle, and found it to be 105° 5'. And 

 this, indeed, is the result to which I formerly came by a diffe- 

 rent method*. If it differ in any respect from this quantity, I 

 am inclined to think that it will more likely be found to be 

 deficient by a few minutes, than to exceed the measure here 

 assigned ; and accordingly to differ still more widely from those 

 angles which I am about to mention. 



In the magnesian carbonate of lime, or bitter-spar, the pri- of bitter-spar, 

 mitive form is well known to be a regular rhomboid, as well 

 as that of carbonate of lime, and so nearly resembling it, as 

 to have been hitherto supposed the same. I find, however, a 

 difference of 1° 10' iti the measures of these crystals ; for that 

 of the magnesian carbonate is full 106^°, as I have observed 

 with uniformity, in at least five different specimens of this sub- 

 stance^ obtained from situations very distant from each other. 



« Phil. Trans. 1802, p. 385 ;,or Journal, vol, IV, p. 150. 



Vol XXXIIL No. 153.— November, 1812. P Th« 



