122 M. C. Lea—Detection of Hydrocyanic Acid. 
This test is very delicate. Used in the manner to be pres- 
ently described, a solution of potassic cyanide, containing only 
ssa Of a grain of anhydrous hydrocyanic acid, gives a per- 
fectly distinct reaction. It is therefore not exceeded in deli- 
cacy by any known test for that substance. 
The solution of iron and uranium must not be acidulated, 
ed. 
Cobaltous nitrate may be substituted for the uranium salt, 
and gives an almost equally delicate reaction, but the color of 
the cobalt salt is an objection. 
2. Prussian Blue Test. 
f ss'ya Of a grain of anhydrous prussic acid, the Prussian blue 
to ammonia ferric alum or ferric chloride. The best mode of 
proceeding is as follows: 
eak solution of iron is to be made, containing a ferrous 
salt, to which a little ferric ammonia citrate is to be added. Of 
this solution, acidified with hydrochloric acid, two or three 
drops only are to be placed in a white porcelain capsule. A 
drop of the liquid to be tested is then allowed to slip down the 
side of the capsule, and this meeting the iron solution will give 
rise to the production of a blue cloudiness. 
If a grain of potassic cyanide be dissolved in four ounces of 
pure water, rendered slightly alkaline with caustic alkali, and a 
single drop of the solution be allowed to flow down the capsule 
in the manner just described, a distinct blue coloration — 
result. This drop will have contained about z,'55 of a grain of 
cyanide, or about ;;';5 of a grain of anhydrous prussic acid, 
a degree of delicacy very far surpassing that found by Mr. 
Taylor; surpassing, indeed, that claimed by him for the sulpho- 
cyanhydric test. 
