150 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. Ifa 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 
