450 ELEMENTARY CHEMICAL MICROSCOPY 



yield a precipitate with MgC^. 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 ioo 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 FeC^. 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. 



