15° ELEMENTS OF APPLIED MICROSCOPY. 



the metallic elements are first tested by the addition of a 

 fragment of metallic zinc. If metallic crystals form, either 

 silver, copper, bismuth, lead, tin, cadmium, or thallium 

 is present. If a blackish coating appears upon the zinc, 

 gold, platinum, iridium, palladium, or mercury is indi- 

 cated. Should neither phenomenon follow, the other 

 groups of elements may be tested for by the addition of 

 nitric acid, magnesium ribbon, ammonium chloride, and 

 magnesium and ammonium-phosphate solution; we are 

 here, however, concerned only with the first group of 

 metals, as typical of the general method of analysis. 



If the solution contains one of the metals precipitated 

 by zinc, drops of it must be tested with several reagents, 

 each of which points out certain of the elements. Thus 

 a grain of potassium chloride is added, and with silver, 

 lead, and thallium a characteristic reaction is obtained. 

 Silver salts produce an immediate white curdy precipi- 

 tate of silver chloride, lead salts, crystals of lead chloride, 

 and thallium compounds, very minute characteristic 

 crystals of thallium chloride. In each case confirmatory 

 tests must be applied. With silver, for example, the 

 amorphous precipitate should be first separated by pour- 

 ing off the liquid and then dissolved in a drop of ammo- 

 nium hydrate. The colorless solution thus obtained on 

 warming and then cooling yields large cubical crystals of 

 silver chloride. If silver be present, a drop of the original 

 solution, warmed and made faintly acid with nitric acid, 

 produces on the addition of potassium bichromate large 

 dark-red hexagonal plates and prisms of silver bichromate, 

 soluble both in nitric acid and ammonia. A grain of 



