Notes on Chemistry, &c. 87 
acid, on adding a concentrated solution of any nitrate, (except those 
of silver and mercury,) the red color of the scale or bi-iodide will 
speedily disappear, and will be followed by the dark hue of iodine. 
Even when the sulphuric acid forms an insoluble precipitate, the 
action may be seen, by stirring up the precipitate with a glass rod, 
when the dark spots will be easily observed. 
This method of testing may sometimes be used, but is liable to 
the objection that a chromate, chlorate, and probably some other 
salts, would give the same result. It is greatly inferior to the method 
by distillation, as given above. 
2. Locality of Hyalite at West Point.—Last autumn I noticed 
at a quarry of gneiss on the shore of the Hudson, about a quarter 
of a mile below West Point, a beautiful incrustation on some of the 
masses of gneiss which had been removed from situ by blasting. On 
examination by the microscope, and by analysis, I am satisfied that 
it is Hyalite, although gneiss is I believe a very unusual situation for 
this mineral.* The mineral forms a crust of about the thickness of 
one or two wafers, upon what appears to have been a natural seam 
of the rock, and it does not appear to have been produced by any 
decomposition of the surrounding mass. Its color is white; lustre 
varying from silvery or pearly to glassy. When examined by a 
magnifier it is very beautiful, presenting a brilliant collection of small 
mammillated masses, in some places opaque, and at others transpa- 
rent as glass. By analysis it gave about 84 per cent. of silica, 6 per 
cent. of alumina and oxide of iron, and 10 per cent. of water. 
Specimens are sent with this; those in the paper marked (A) 
show the mineral in its purest state. 
On the same surface with the above, and apparently passing into 
the same, was a silicious crust, which by the magnifier showed no 
signs of the mammillated structure, nor any lustre, but small eleva- 
ted straight lines, about one fourth of an inch in length, were ob- 
served, crossing each other in various directions, and indicating an 
imperfect crystallization. A specimen (B) showing this character 
is also sent. 
* It is found in the gneiss at Haddam, Conn., in exactly similar form and cir- 
cumstances.—Eb. 
+ Not received, Jan. 26, 1837—Ep. 
