Notes on Chemistry, ^c. 85 



Art. VII. — Notes on Chemistry, &fc. ; by J. W. Bailey, Acting 

 Prof. Chem. he, U. S. Mil. Acad. West PoirU. 



1. On a new Test for Nitric Acid. — Chemical reagents maybe 

 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 whh 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 sim.ilar results with 

 the reagent added is present, and then if this result is 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 least 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. 



