Dr Brewster on the Native Hydrate of Magnesia. S5S 
verge from k central line, and frequently occur in groups, with 
the laminas of one group inclined to those of another, like the 
masses of mica in granite. 
The colour of the laminae is white, and a slight tinge of 
green is sometimes observed, when we look upon their edges. 
They are perfectly transparent when separate ; but I have no- 
ticed • in specimens exposed to the weather, a dull and white 
opacity, which had been induced by the separation of the mine- 
ral into a greater number of minute lamina. This white part 
has the same relation to the transparent part as Albin has to 
Apophyllite^ (see Edm. Phil. Journal^ voi. i. p. 5.) and, as hap- 
pens with this mineral, the disintegration follows the crystalline 
structure of the body. One specimen of this kind exhibited a 
six-sided prism, the interior of which was undecomposed, while 
all the external part had a white opacity. 
The Native Hydrate of Magnesia scratches Talcy from which 
it may be easily distinguished, as the former marks white paper 
with a silvery powder, whereas the latter gives only a polished 
line, and leaves none of its own substance. Its hardness seems 
to be 1.5. 
In several specimens I have observed a distinct crystalline 
strupture in the form of the regular hexahedral prism. The 
pyramidal form being therefore excluded, it will belong to the 
system of Mohs. 
Its specific gravity is 2.336. It adheres very slightly to the 
tongue ; and it will constitute a new Genus of the 5th Order, 
or that of Mica, in the 2d Class of Mohs’ System, unless the or- 
der of Talc-Mica be modified to receive it. 
Locality. — Dr Hibbert found this substance, in 1817, at Swi- 
haness, in Unst, one of the Shetland Isles, traversing serpen- 
tine in all directions, being mixed with the magnesian carbonate 
of lime, and forming veins from half an inch to six or eight inches 
broad. 
Chemical Character.-— Llydccdie of Magnesia dissolves entirely 
in muriatic, nitric, and dilute sulphuric acids ; and I obtained 
from its solutions in muriatic and sulphuric acids, the deliques- 
cent salt of Muriate of Magnesia and regular crystals of Sul- 
pliate of Magnesia. On some occasions a very slight elferves- 
VOL. IV. ^^O. 8. APRIL 1821. 
55 
