Bitter-Spar, and Iron- Spar. 457 



!05^ 5'. And this, inciectl, is the result to which I for- 

 merly came by a different method *. If it differ in any re- 

 spect from this quantity, I am inclined to think that it will 

 more likelv 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 

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

 well as ih:it of carbonate of lime, and so nearly rcseuibling 

 it, as to have been hitherto supposed the same. I find, 

 however, a difference of i'^ lo' in 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 substance obtained from situations very 

 distant from each other. 



Tlie 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 

 tran-parent, others opake. That which gives the most 

 distinct imatre by reflection is of a brownish hue, with the 

 scmi-lransparencv of horn. It wasobtained from a tin mine^ 

 called Maudlin Mine, near Lostwilhiel in Cornwall. By 

 repeated measurement of small fraoments of this specimen, 

 the an2;le appears to be so nearly 107^, that I cannot form 

 any judgement 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 iime, as to render it 

 highly probable that in other specimens having,the same 

 ancle, hut containing also carbonate of lime or other ingre- 

 dients interaiixed, the form is really dependent on the car- 

 bonate of iron alone. 



It appears however not nnlikelv, that when substances 

 which agree so nearly in their primiti'-'e angle are inter- 

 mixed in certain proportions, they may each exert their 

 power; and may occasion that confused appearance of cry- 

 stallization with curved surfaces, known by the name of 

 pearl-spar. I cannot sav that I have made any accurate 

 comparative analyses which niay be adduced in support of 

 the hypothesis, that mixtures are more subject to curvature 

 than pure chemical compounds : but it is very evident f^rom 

 the numerous analyses that have been made of iroil-sparby 



• Plill. Trans. 180'A p. '.83. 



Vol. 39. No. 170. June J SI 9. Gg other 



