in his Paper on a saline Substance from Mount Vesuvius. 131 
It would be endless to describe further the phenomena of 
druses, all leading to the same conclusion : the various manners 
in which the prisms cross each other, divide and unite again. 
But there is a particular circumstance, which is exemplified in a 
process that we see going on, that of the crystallization of salts 
in liquids. Each salt has its particular character and shape ; 
and if a diquid hold in solution different salts, they crystallize 
separately in known circumstances, especially in different tem- 
peratures. Now there is a similar phenomenon in the crystalli= 
zation of rock crystal, showing that in some of the cavities in 
which it has been formed, the liquid contained in solution other 
mineral substances, one in particular, whence have proceeded 
sinall erystals of geen shorl, from which many prisms of rock 
erystal ave made, unfit for any other use than that of variety in 
mineral collections. 
These green shorls were at first supposed to be mosses that 
had grown in these cavities; a supposition contradicted by its 
being impossible that land vegetables should grow in the sea ; 
but by observing these shorls with a magnifying glass, the mis- 
take is soon discovered, by the angular form of their thread. 
We have a prism of rock crystal made almost opake by these 
shorls, in which one of them having been destroyed, has left an 
angular hole from one side to the other, through which a horse- 
hair can pass. ; 
I now come to the assertion of the author, that none of the 
erystals has ever been seen inclosing drops of water. This might 
be, without affording any argument against the aqueous origin, 
and for instance, of rock crystal ; for there is no reason @ priort, 
that water should be inclosed in any of its prisms. However, the 
fact itself contradicts his assertion. It is natural to infer that he 
has not seen the case himself, since he never alludes to rock cry- 
stal, though so important to the natural history of the primitive 
strata. But I have seen many prisms of that crystal containing 
water, and there is one in our collection, the particulars of which 
I shall describe. 
- This prism is about two inches in diameter ; it is traversed by 
many small erystals of green shorl, between which a small quan- 
tity of water remained engaged during the increase of its size. 
That water is manifested by a bubble of air, which remained in- 
closed with the water retained also by the shorls. By inclining 
the prism, that bubble moves gradually from nearly one end to~ 
the other of the prism; but as, according to the shorls that it 
meets in its way, its direction is determined, it does not follow the 
same course in every trial, except when the inclination of the 
prism happens to be the same and on the same side, and it shows 
thus the space which is occupied by the inclosed water, 
12 Now, 
‘ 
