350 ELEMENTARY CHEMICAL MICROSCOPY 



glass tubing or a fragment of solid may be pushed in by means 

 of a platinum wire. Close the tube by means of a drop of water 

 and the plug P. The reaction may be hastened by warming A 

 over the micro-flame. If arsenic is present the crystal of silver 

 nitrate turns yellow due to the formation of a compound believed 

 to have the composition AsAg 3 AgNOs, and rapidly changes 

 to black through the reduction to metallic silver. The lead 

 acetate remains unchanged unless hydrogen sulphide is evolved. 



In acid solution antimony will yield stibine which reacts upon 

 silver nitrate in a similar manner although the yellow compound 

 is practically never seen. Phosphine or hydrogen sulphide turn 

 the silver nitrate black at once, but the sulphur compounds should 

 have been held back by the lead acetate cotton. The crystal L 

 is introduced merely to make sure that any blackening of S can- 

 not be due to volatile sulphur compounds. 



To differentiate between arsenic and antimony we may sub- 

 stitute fragments of aluminum for the zinc and a solution of 

 potassium hydroxide for the acid. Under these conditions, no 

 stibine is evolved, only arsine passes off with the hydrogen. 

 Metallic antimony is precipitated in part and deposited in part 

 upon the aluminum. 



In place of a crystal fragment of silver nitrate we may employ 

 a fragment of mercuric bromide or a textile fiber soaked in mer- 

 curic bromide and dried; in the latter case a much finer capillary 

 tube can be used and the delicacy of the reaction is somewhat 

 increased. Arsine turns mercuric bromide red or brown. 



B. By Reduction to Metallic Arsenic and Subsequent Oxidation 

 to Arsenic Trioxide. 



The powdered material is mixed with a small quantity 

 of dry anhydrous potassium ferrocyanide and introduced into a 

 thin walled tube of hard glass drawn down to a point and fused. 

 The tube is tapped gently to cause all the material to collect in 

 the tip of the tube. Heat the material gently at first and finally 

 raise the temperature to a red heat. The arsenical compound is 

 reduced; arsenic is set free and condenses upon the walls of the 

 tube as a brownish mirror. Antimony will yield a black or metal- 

 lic mirror; mercury a sublimate of tiny silvery spheres. Certain 

 compounds of carbon or sulphur may yield deposits upon the 



