SANGER AND BLACK. — QUANTITATIVE DETERMINATION OF ARSENIC. 307 



test solution, say for an hour. This preliminary saturation may be 

 also conveniently secured by leaving zinc and acid in the apparatus 

 over night. 



By observation of the above precautions we have obtained fairly 

 regular and uniform bands of color from equal amounts of arsenic, — 



TABLE I. 



using apparatus of definite size, reagents of definite concentration, care- 

 fully sensitized paper, and by passing the arsenical gas over the paper 

 in a condition of moisture which is as carefully regulated as possible. 

 Without these precautions, which involve no great care, the method 

 will not give satisfactory quantitative results. 



