378 ELEMENTARY CHEMICAL MICROSCOPY 
To the washed precipitate add one or two drops of strong 
ammonium hydroxide. After a second or two of contact, draw 
off the ammoniacal solution from any undissolved precipitate. 
Do not heat the preparation. Allow the preparation to stand. 
Almost immediately the drop becomes turbid around the edges, 
because of the separation of minute crystals of silver chloride; 
these crystals increase slowly in size, but are always very small, 
requiring a moderately high power for distinguishing their form. 
From ammoniacal solutions silver chloride seems to separate 
almost invariably in the form of cubes and hexagonal and rec¬ 
tangular plates. Only rarely are octahedral crystals obtained. 
Of the two recrystallization methods, that with ammonium 
hydroxide will be found to be the better, as well as also the more 
convenient, because of the greater solubility of the precipitate 
in this reagent, and because the employment of ammonium 
hydroxide eliminates many interfering substances. 
Lead chloride is precipitated in the form of white acicular 
crystals, irregular crystallites and X-like dendrites, soluble in 
hot water and therefore easily removed. 
Mercurous salts yield a granular precipitate, but sometimes 
minute needles. Recrystallized from concentrated hydrochloric 
acid tetragonal crystals may be obtained but no cubes and part 
of the salt is converted into soluble mercuric chloride. Mercu¬ 
rous salts therefore interfere with the satisfactory detection of 
traces of silver by masking the tiny cubes of silver chloride. 
Thallous salts yield cubes and stars. 
Treated with ammonium hydroxide, silver chloride dissolves 
with the formation of the compound AgCl-2X113 (Isambert). 
If mercurous chloride is present the precipitate turns black under 
the action of the reagent, an insoluble compound being formed 
which Barfoed has shown to be a mixture of metallic mercury 
and the compound Hg-NH2-Cl. If, therefore, silver chloride 
is present only in traces in a precipitate consisting chiefly of 
mercurous chloride, ammonium hydroxide may dissolve practi¬ 
cally no silver chloride, since the finely divided metallic mercury 
may reduce the greater part of the silver salt to metallic silver. 
(Silver follows mercury in the electrochemical series.) Under 
