396 



ELEMENTARY CHEMICAL MICROSCOPY 



loosely a tiny crystal S of silver nitrate and one L of lead acetate 

 is attached by means of a short piece of rubber tube R. From 

 time to time the crystal S is examined to see if it changes color. 

 If after some minutes S remains clear and colorless remove P, 

 insert the material to be tested by means of a bit of drawn-out 



FIG. 162. 



Apparatus for the Detection of Arsenic. 



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 AgNO 3 , 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 



