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 dis-appear, 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 Hable 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 

 raammillated 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.f 



* It is found in the gneiss at Haddam, Conn., in exactly similar form and cir- 

 cumstances. — Ed. 

 + Not received, Jan. 26, 1837.— Ed. 



