Notes on Chemistry, &§c. 85 
Art. VII.—Notes on Chemistry, &c.; by J. W. Bartey, Acting 
Prof. Chem. &c., U.S. Mil. Acad. West Point. 
1. Ona new Test for Nitric Acid.—Chemical reagents may be 
divided into two classes; first, those which produce with the sub- 
stance they are employed to detect, an action which they will pro- 
duce with no other known body ; an example is starch, as a test for 
free iodine: secondly, those which cause a certain action with a 
small number of bodies, which they will not exhibit with any others; 
as, for example, sulphuretted hydrogen, which causes a black pre- 
cipitate with a few metals. 
The first class are, of course, the most valuable reagents, as they 
require no subsequent operation to determine whether certain sub- 
stances are present or not; while with those of the second class, we 
only determine that one of a certain number of bodies must be pres- 
ent, but must then resort to other means to ascertain which particular 
one it may be. 
There are many cases, however, when we may know that only 
one of those bodies which are capable of giving similar results with 
the reagent added is present, and then if this result 2s produced, the 
evidence is as satisfactory as can be desired. 
The test which I would propose, must be placed among those of 
the second class, and is therefore inferior in value to morphia as a re- 
agent for nitric acid; but I think it at /east as valuable as the method 
by means of gold leaf and hydrochloric acid, or by the bleaching of 
indigo. 
The substance I now suggest, as a new reagent for nitric acid, is 
the cyano-hydrargyrate of iodide of potassium, discovered by M. 
Caillot. It is formed by mixing together bicyanuret of mercury and 
iodide of potassium, (one equivalent of each,) dissolved in small 
quantities of warm water. It soon crystallizes in a very beautiful 
manner. This is the same salt which has recently been recom- 
mended as a means of detecting the presence of hydrochloric acid 
in hydrocyanic acid. (See Lond. and Ed. Phil. Mag. Nov. 1835.) 
Its use as a test for nitric acid depends upon the fact, that if one 
of the scale-like crystals be introduced into most acids, it immediately 
becomes of a beautiful red, being changed into the bi-iodide of mer- 
cury ; while in concentrated nitric acid, (spec. grav. 1.4 to 1.5,) the 
scale instantly becomes almost black, from the liberation of iodine. 
