MICROCHEMICAL REACTIONS OF ALUMINUM 387 
acetic acid. Cobalt will immediately be precipitated. If lead 
and nickel or copper are present the yellow or black or both triple 
nitrites will eventually separate. If none appears, a little lead 
acetate is added; tiny black squares and cubes indicate copper. 
Powerful oxidizing agents must be absent. 
EXPERIMENTS. 
a. Test for Cu in CUSO4; in Cu(N03)2. 
b. Try the reaction in acid solution; in ammoniacal solution. 
c. Try in like manner a mixture of Cu and Ni, Cu and Co. 
C. Other Useful Reactions for Copper, which may arise in 
Testing for Other Elements. 
Cesium chloride forms two very characteristic double 
chlorides with copper CsCl • CuCb in golden yellow rectangular 
plates, squares and short stout prisms and a less frequently met 
with orange colored salt of unknown formula. These char¬ 
acteristic double salts frequently appear when testing for tin, 
antimony, or bismuth with cesium chloride or on rare* occasions 
when testing for aluminum. Ferric chloride also forms a yellow 
double chloride with cesium chloride. The color and the appear¬ 
ance of the cesium iron chloride is quite different from the copper 
salt and the combination does not take place so readily. 
Potassium ferrocyanide in acetic acid solutions yields an 
amorphous red-brown precipitate. Added to ammoniacal solu¬ 
tions there appear after a time white dendrites of copper ferro¬ 
cyanide ammonia 2 (NH3) • Cu2Fe(CN)6.^ The addition of acetic 
acid causes these dendrites to become red. 
ALUMINUM. 
Crystal Forms and Optical Properties of Common Salts of 
Aluminum. 
A. ISOTROPIC. —alums (I). 
B. ANISOTROPIC. 
Hexagonal. — Sulphate; chloride (6 H2O). 
Tetragonal. 
^ Behrens, Anleitung, p. 75. 
