33 a Mr. Gregor’s Experiments on a Mineral Substance 
impressed form of which the pieces thus separated, retain. The 
surface of these, which was in immediate contact with the 
quartz, exhibits the several minute crystals of which the mass. 
consists, matted together in various directions. 
These crystalline assemblages are, in general, white ; a 
nearer inspection of the individual crystals proves that they 
are transparent. Sometimes they are stained of a yellowish 
hue by ochry water. 
The size of these crystals varies considerably in different 
specimens. Sometimes they assume the appearance of a white 
powder raised up in small heaps, upon the surface of the stone, 
to which they adhere. In other specimens they resemble a 
tender down. And the larger sort varies, in relative size, in the 
proportion, perhaps, in which a human hair, horse-hair, and a 
hog’s bristle, severally differ from each other in magnitude. 
They seldom exceed a quarter of an inch in length. The 
figure of these crystals is not easily ascertainable, on account 
of their minuteness. By the help of a very powerful micros- 
cope, they appear to consist of four-sided prisms ; where these 
are broken off, the section exhibits a rhomboidal, approaching 
indeed to an elliptical figure, from the circumstance of the 
angles of the prism being worn away ; but that the prism itself 
is rhomboidal, cannot be inferred from hence, unless we 
could be certified, that the section were at right angles with 
the axis of it. 
Imbedded amongst these crystals two species of crystalline 
laminae are frequently discoverable : the one consisting of par- 
allelopipedon plates with truncated angles, applied to each 
other, of a green colour of various tints, from the emerald to 
the apple-green : the other species, consisting of an assemblage 
