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 
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 AsAgs • 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 
