366 ELEMENTARY CHEMICAL MICROSCOPY 



From drops containing moderate amounts of mercury, the 

 metal may be separated by a fragment of magnesium, or it may 

 be deposited upon a bit of copper. If in the latter case the spot 

 of deposit be rubbed it becomes silvery white. If the coated 

 copper is placed in a subliming tube and heated the mercury 

 will be volatilized and will condense in characteristic globules. 



EXPERIMENTS. 



a. Test several mercurous and mercuric salts by heating them with Na2C0 3 . 

 Examine the sublimates. Rub them gently with a hair-like glass rod and note 

 that the globules unite. 



b. Obtain a deposit of Hg upon a tiny bit of Cu foil — i millimeter by 3 milli- 

 meters — by heating in a drop of a solution of an Hg salt acidified with HC1. 

 Dry and sublime. 



c . Introduce a fragment of iodine in one or more of the tubes, warm gently and 

 allow to stand about five minutes. Examine for crystals of Hgl2. 



B. Differentiating between Mercurous and Mercuric Salts. 

 Add Hydrochloric Acid. — With mercuric salts there is no 

 precipitation. Mercurous salts give an immediate amorphous 

 precipitate of a white chloride HgCl. Under unusual conditions 

 and exceedingly dilute solutions, mercurous chloride may some- 

 times be obtained in the form of slender needles. To charac- 

 terize the white precipitate, draw off the supernatant solution 

 and add to the residue a drop of dilute ammonium hydroxide. A 

 black compound of the formula NH 2 Hg 2 Cl is immediately formed. 

 Examined with a f- inch or an 8 millimeter objective the black 

 compound is seen to consist of a mass of tiny acicular crystals, 

 tiny squares, crosses and fusiform grains. 



EXPERIMENTS. 



a. Precipitate HgCl, examine with the microscope. 



b. Add NH4OH to the white precipitate and examine again. 



Add Potassium Bichromate and Nitric Acid. — To the drop to 

 be tested add nitric acid. Place nearby, a drop of solution of 

 bichromate. Warm the drops over the micro-flame and while 



