354 Dr Brewster on the Native Hydrate of Magnesia. 
fence takes place ; but this no doubt arises from adhering 
particles of carbonate of lime, or from a small quantity of car- 
bonic acid, which may have been absorbed by exposure to the 
atmosphere. 
The following analysis of this mineral has been made by Dr 
Fyfe, since the preceding account of it was drawn up 
Magnesia, - - 69.75 
Water, - - 30.25 
100.00 
a result which differs only a quarter of a per cent from that of 
Dr Bruce of New York. 
Optical Structure. — The Native Magnesia has One axis of 
double refraction perpendicular to the laminae, and exhibits the 
single system of coloured rings, traversed by a black cross. The 
character of its action is Positive^ like that of Quartz^ and the 
tints which it polarises are different from those of Newton’s scale, 
resembling somewhat those which surround the resultant axes of 
Selenite -f*. This mineral is not phosphorescent by heat. 
Distinctive Character. — The Hydrate of Magnesia is distin- 
guished from Talc^ by its having One axis of double refraction, 
while Talc has Two axes ; — by its lower specific gravity, and 
greater hardness ; — by its marking paper with a polished line, 
in place of a silvery one, as already noticed ; and by its solubi- 
lity in acids. It is distinguished from the Common Mica with 
two axes, by the elasticity of the latter, as well as by its 
two axes ; and it is equally distinguished from the less flexible 
Mica of Kariat of Greenland^ and from other Micas that have 
only one axis, by that axis being positive in the Magnesia and 
negative in the Mica., and also by the character of the tints with 
which the axis is encircled. 
It is distinguished from Selenite.^ by its having one axis of 
double refraction perpendicular to the laminas, whereas Selenite 
* Dr Fyfe analysed also a very small portion of the Hydrate of Magnesia 
from Hoboken, which I had used in my optical experiments, and found it to con- 
sist of 68.57 of Magnesia, and 31.43 of Water, with a trace of Lime. In some 
of Dr Hibbert’s specimens, Dr Fyfe obtained 0.5 of Lime and Iron, and 4.4 of Car- 
bonic Acid, w’hich, however, issued only from particular points of the fragment. 
-j- See VhiL Trans. 1818, p. 243. 
