450 
ELEMENTARY CHEMICAL MICROSCOPY 
yield a precipitate with MgCl2. The fat may be removed from 
the wool by a mixture of alcohol and ether. 
Preparation of Potassium Mercuric Thiocyanate. — Prepare 
a saturated solution of mercuric nitrate in water containing 
I c.c. of concentrated nitric acid to every loo c.c. of water. 
Filter. Add an approximately 5 per cent solution of potassium 
thiocyanate as long as a precipitate is formed. Wash the pre¬ 
cipitate until no test is obtained with KI or with FeCls. To 
a 5 per cent solution of potassium thiocyanate add the mercuric 
thiocyanate until the mercury salt fails to dissolve. This reac¬ 
tion proceeds somewhat slowly and cannot be hastened. Heating 
is to be avoided. As soon as saturation is reached the solution 
is filtered, a few c.c. of the potassium thiocyanate solution is 
added and the whole evaporated on the water bath to crystalliza¬ 
tion. The crystals of potassium mercuric thiocyanate thus 
obtained should be recrystallized from water. The sodium and 
ammonium salts may be prepared in like manner but it has 
been found that both of the latter compounds are hydroscopic 
and do not keep well, while the potassium salt is stable and not 
subject to decomposition. 
