' On Cry st allograph^', tpt 



In order that T-may be better understood, I sball take aa 

 example from lildspar. Mi*. Kirvvan, to whom we are in- 

 debted for a treatise on mineralogy, in which that cele- 

 brated author lias brought together, in the development of 

 the science, the external characters of minerals and the 

 results of his own researches as well as of those of othci^ 

 chtmiists, as to the composhion of these bodies, cites 13 

 analyses of the substance in question, to which we may 

 add a 14lh made by Vauquelin. No\V^ not only do the 

 products vary among each other in the proportions of 

 the same principle?, but there are ingredients which are 

 found in certain products, and not in others. Thud 

 Mr. Kirwan has procured from a reddish feldspar eleven 

 per cent, of barytes and eight of magnesia ; whilst the re- 

 sult obtained by Wieglicb in another feldspar of a red co- 

 lour, furnished neither of these earths, but only silex and 

 alumine, with a small quantity of oxide of iron and fluoric 

 acid. Vauquelin found about one seveiMh of potash in 

 the feldspar called adular^ and in the green feldspar of 

 Siberia, and yet no other analysis has presented this alkali. 

 Besides, this expert chemist has discovered neither magnesia 

 nor barytes in the same mineral. 



Mr. Kirwan concludes, from the diflferent analyses quoted 

 by him, that every compound of silex and alumine (the 

 silex being predominant) to which is added a slight pro- 

 portion of lime and magnesia, or of lime, of magnesia and 

 barytes, (but sufficient to render tlie whole fusible at a de- 

 gree of heat not exceeding 140'',) would form a feldspar ;'and 

 we ought not to hesitate m giving it this name, if at the same 

 time it presents a lamellous texture. But he adds that iron 

 seems in this case to be an accidental principle. 



I do not observe that this rule laid down by Mr. Kirwan 

 leaves any thing to be desired, so far as simplicity and pre- 

 cision are concerned; but in spite of the efforts of the 

 author to render it general, at the hazard of loading it with 

 conditions, it is no longer applicable to the result of the 

 analysis made by M. Vauquelin of the feldspar known by 

 the name of adulary ; and finally, if we should undertake 

 to give similar rules for all minerals, the result would be a 

 complication from which it would be difficult to extricate 

 ourselves, and it would even very probably, happen, that 

 a rule which should have for its object such a particular 

 species might be applied nearly equally well to an entirely 

 different species. 



I shall not examine if all the analyses alluded to by Mr. 

 Kifwan deserve an equal confidence. But we may at least 



conclude 



