Carbonate of Lime, Bitter- Spar, and Iron-Spar . 161 
The most accurate determination of the angle of carbonate 
of lime is probably that of Mons. Malus, who measured it by 
means of a repeating circle, and found it to be 105° f. 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,! 
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- 
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 i° 10' 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. 
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 semi-transparency of horn. It was 
obtained from a tin mine, called Maudlin Mine, near Lostwithie! 
in Cornwall. By repeated measurement of small fragments 
of this specimen, the angle appears to be so nearly 107°, 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 
* Phil. Trans. 1802, p. 385. 
MDCCCXII. Y 
