210 



FRIMITIVE CRYSTALS OF SOME CARBONATES. 



spar!^ '^''"" The primitive angle of iron-spar is still more remote from 

 that of the carbonate of lime, which it exceeds by nearly two 

 degrees. I have examined various specimens of this substance, 

 some pure white, others brown, some transparent, others opake. 

 That which gives the most distinct image by reflection is of a 

 brownish hue, with the semitransparency of horn. It was 

 obtained from a tin mine, called Maudlin Mine, near Lost- 

 withiel, in Cornwall. By repeated measurement of small frag'- 

 ments of this specimen, the angle appears to be so nearly lo7°, 

 that I cannot form any judgment whether in perfect crystals it 

 will prove to be greater or less than that angle. 



In this instance the carbonate of iron is nearly pure, and so 

 perfectly free from carbonate of lime, as to render it highly 

 probable, that in other specimens, having the same angle, but 

 containing also carbonate of lime, or other ingredients inter- 

 mixed, the form is really dependent on the carbonate of iron 

 alone. 



Pearl spar. jt appears, however, not unlikely, that when substances. 



Mixtures pro- ,. , , . , . ... , . . , 



bably most Which agree so nearly m their primitive angle, are intermixed 



liable to curv- in certain proportions, they may each exert their power ; and 



ed surfaces. • . . r j r .„•.••, 



, may occasion that confused appearance or crystallization with 



curved surfaces, known by the name of pearl- spar. I cannot 

 say that I have made any accurate comparative analyses, which 

 may be adduced in support of the hypothesis, that mixture* 

 are more subject to curvature than pure chemical compounds ; 

 but it is very evident, from the numerous analyses that have 

 been made of iron-spar by other chemists, how extremely 

 variable they are in their composition, and consequently how 

 probable it is, that the greater part of them are to be regarded 

 as mixtures ; although it be also possible, that there may exist 

 a triple carbonate of lime and iron as a strict chemical com- 

 pound. 

 Perhaps some It seems not unlikely, that there may hereafter be found 

 carbonate, mo- gQj^e carbonate allied to the preceding, which may owe its 

 dified by man- ^ , ^ , • i ,. , 



ganese, exists, ioxva to the presence of manganese j but notwithstanding the 



liberality which happily prevails in general among those who 

 have it in theii* power to assist in such inquiries, I have not- 

 had the good fortune to meet with any such compound j and 

 I am unwilling, merely in the hope of making such an addi- 

 tion. 



