Description and Analysis of the Sillimanite. 115 
_ Having thus given a brief history of this mineral, I shall 
now proceed to state the results of its examination, and 
shall then offer my reasons for considering it a new species. 
Description. 
The color of this mineral is dark gray, passing into clove 
brown. 
It occurs crystallized in rhomboidal prisms, whose angles 
are about 306° 30’ and 73° 70’; the inclination of the base 
to the axis of the prism being 113°. It has but one cleav- 
age which Is parallel to the longer diagonal of the prism. 
The sides and angles of the crystals are frequently round- 
ed. 
Its hardness is greater than that of quartz; even the to- 
paz may be scratched by some of the specimens. It is trans- 
lucent on the edges, and in small fragments ; is brittle and 
may easily be reduced to powder. 
Its fracture in the direction of the longer diagonal is lam- 
ellar, and displays a brilliant lustre; the cross fracture is 
uneven and splintery. 
It does not become electric either by heat or friction, 
nor does it give any indicationsof magnetism even when 
tested by the method proposed by M. Haiiy. 
Its specific gravity is 3.410. 
Before the blow pipe it is infusible per se, and also when 
heated with borax. 
The nitric, muriatic and sulphuric acids do not act on its 
powder, even when digested upon it with the assistance of 
heat. 
This mineral occurs in a vein of a quartz, penetrating 
gneiss in the town of Saybrook, Connecticut, where, I am 
informed, it is found in considerable quantities. 
ANALYSIS. 
A. Three grammes of the mineral reduced to an impal- 
pable powder were exposed, during half an hour, to a red 
heat, in a platina crucible. The colour of the powder was 
not altered by ignition, after which it weighed 2.985 gram- 
