the Corundum Stone , and its Varieties , &c. 309 
fracture shows that they are very evidently laminated. In the 
substance of this spar are contained a great number of small 
prismatic crystals of mica, of a beautiful ' yellow colour, like 
that of the topaz ; they have also the lustre, and the transpa- 
rency, of that precious stone, for which they might the more 
easily be mistaken, as several of them, which show the sides of 
their prisms on the exterior part of the stone, appear to have 
their surface slightly rounded.* Very thin laminae may without 
difficulty be detached from the terminal faces of the crystals ; 
these laminae are perfectly elastic. 1 
There are .also, in this calcareous spar, small pieces of a 
metallic substance, which deserves to be particularly described. 
The colour of this substance is gray, slightly inclining to red, 
so as very much to resemble that of arsenical cobalt, or of nickel. 
Iffie substance is very brittle ; the slightest blow breaks it ; and 
it may, by a moderate degree of pressure, be reduced into a 
black powder. Its fracture is conchoid, with a very fine and 
compact grain ; and it has a very brilliant lustre. The magnet 
* All the authors who have treated of mica, say that it is transparent only when in 
very thin laminae. This is a mistake. When the crystals of this substance are in as 
perfect a state as they possibly can be, that is to say, when their crystalline lamina: are 
in complete contact with each other throughout the whole extent of their surface, (a 
circumstance very uncommon, but which is known by the sides of their prisms being 
perfectly smooth,) they are usually transparent. I have seen crystals of mica, of a pretty 
considerable thickness, which were perfectly transparent, in whatever direction they 
were viewed ; although sometimes such crystals, when their terminal faces have a very 
shining silvery lustre, (which shows that they reflect all the light that falls upon 
them,) have not the smallest transparency, when viewed in a direction perpendicular 
to them axis ; many of them, however, appear transparent, when viewed through the 
edges 01 the lamina:, that is to say, in a direction parallel to that of their axis. The 
above is not the only mistake that has been made with respect to this substance ; 
a correct description of which, I hope, some time hence, to be able to lay before the 
Royal Society. 
MDCCCII. s s 
