142 On Crystallized Native Terrestrial Iron, &c. 
have been easy to detect the arsenic by its odor ; the common blow- 
pipe being insufficient, on account of the feeble proportion in which 
it exists, to render it obvious in either the North Carolina, or the 
Pennsylvania specimens. 
n the mass examined from Pennsylvania, which formed a part of 
a regular crystal, (the base being somewhat elongated, and the sum- 
mits obliterated,) I suggested that it might belong to the class of rhom- 
bic prisms ; an opinion which I am now happy in having an opportu- 
nity to rectify. The larger angle then obtained, overlooking the 
error of one degree in the measurement, it is obvious, belongs to the 
base of the octahedron ; while the smaller angle of 59° corresponds 
to the angle over the summit of this figure. 
2. Ferro-silicate of Manganese. 
1 am about to announce the existence of the present species im 
mineralogy, (recently established by Dr. Thomson,) in Cumberland, 
Rhode Island ; with some farther information concerning its crystal- 
lization, than is given in the original description : but as an account 
of this mineral is not yet to be found in any system of the science, 
and has not been noticed in this Journal, I shall first abstract from 
the original memoir, published in the “ Annals of the Lyceum of 
Natural History of New York,” (Vol. II. p. 28.) the substance of 
Dr. Thomson’s description, and then append to it the remarks pro 
posed, 
Color brown, with a shade of red ; externally, it is dull, having an earthy aspect 
and may be scratched by the nail ; but internally, it is foliated, splendent, of a grey 
color, with a shade of red ; hariineet nearly the same as felspar : specific gravity 
3.44, 
It has three cleavages, indicating a doubly oblique prism for its as form. 
on ee ee oe Se 
Pr: Oh - - . = 86. 30! 
oe ay © - . ea 86. 30’ 
The faces not being ac or flat, these measurements are only given as approxi- 
mations ; and on the examination of another crystal, Dr. T. suggests that the pri- 
mary form may be Sep ren prism 
A portion of the mineral was treated with muriatie acid, till it assumed a white 
color, Much chlorine was evolved, and nearly one fourth of the aegis was dis 
solved. The solution gave 
