MTCROCHEMICAL REACTIONS OF ARSENIC 305 



Sb, Pb, Sn, Pt, Ir, Os, Pd, Ru. All salts of this series are iso- 

 morphous (Groth). 



EXPERIMENTS. 



Test a concentrated and a dilute solution containing Sn. 



ARSENIC. 



Crystal Forms and Optical Properties of Common Salts of 

 Arsenic. 



A. ISOTROPIC. — Trioxide (I, also, but rarely mono- 



clinic). 



B. ANISOTROPIC. 



Hexagonal. — Triiodide; silver arsenate (second- 

 ary, normal is I ?). 



Tetragonal. — Secondary potassium arsenate. 



Orthorhombic. — Calcium - ammonium arsenate; 

 magnesium-ammonium arsenate. 



Monoclinic. — Primary ammonium arsenate; pri- 

 mary sodium arsenate. 



Triclinic. 



DETECTION. 



A. Through the Formation of Arsine and its Reaction with a 

 Crystal of Silver Nitrate. 



Use the distilling tube, Fig. 156, page 296, as a generator, 

 as indicated in Figs. 161 and 162. 



Fit the side tube with a plug of soft wood P. Introduce two or 

 three fragments of arsenic-free zinc Z, and through a pipette 

 dilute hydrochloric acid A (the acid will not flow into the lower 

 part of the tube until the plug P is loosened). Insert a loose 

 plug of absorbent cotton C which has been soaked in lead acetate 

 and dried. The plug P is. next withdrawn. The acid is allowed 

 to flow upon the pure zinc; a tiny drop of water s is introduced 

 into the side tube and the plug reinserted. This drop makes a 

 tight seal and prevents loss of gas. The tube is now tipped 

 downward and a tube drawn down to a capillary and containing 



