Chemical and Physical Papers. 83 



perfectly stable, while the cyanides readily yield hydrocyanic acid, 

 which may be detected in the distillate through the formation of 

 Prussian blue or by the precipitation of silver cyanide in an acid 

 solution of silver nitrate. The simplest method of all, though, is to 

 place a little of the solution to be tested in a small beaker, add 

 ammonium hydroxid and ammonium chlorid; as before, cover 

 with a small watch-glass carrying on the under side a small piece 

 of paper wet with a one-fourth normal solution of copper ammonia 

 sulfate. The beaker is then slightly warmed and set aside for some 

 time. If cyanides are present the paper will lose its blue color, 

 owing to the change of the copper ammonia ion into the colorless 

 cuperous cyanogen ion. By this means as little as one-tenth of a 

 milligram of potassium cyanide may be detected. In order to get 

 the full delicacy of the test the paper must be small and the action 

 must go on for several hours. In the absence of cyanides, but in 

 the presence of the ferrocyanides, etc., the paper will retain its blue 

 color for days. 



