212 MEIALS AND THEIR COMBINATIONS. 



of potassium hydroxide solution and a few drops of the arsenic solu- 

 tion, which should not be acid. Provide paper cap as described in 

 previous test, and set the test-tube in a box containing sand heated 

 to about 90 C. (194 F.). A brown or black stain of metallic 

 silver will appear upon the paper. 



12. Marsh's test. While this test is not used now for qualitative 

 determinations as much as formerly, it is of great value because it 

 may serve for collecting the total amount of arsenic present in a 

 specimen, thus permitting quantitative estimation. The apparatus 

 (Fig. 15) used for performing this test consists of a glass vessel (flask 

 or Woulf's bottle) provided with a funnel-tube and delivery-tube 

 (bent at right angles), which is connected with a wider tube, filled 

 with pieces of calcium chloride or plugs of asbestos ; this drying-tube 

 is again connected with a piece of hard glass tube, about one foot 

 long, having a diameter of J inch, drawn out at intervals of about 

 3 inches, so as to reduce its diameter to J- inch. Hydrogen is gener- 

 ated in the flask by the action of sulphuric acid on zinc, and ex- 

 amined for its purity by heating the glass tube to redness at one of 



FIG. 15. 



Marsh's apparatus for detection of arsenic. 



its wide parts for at least 30 minutes; if no trace of a metallic mirror 

 is formed at the constriction beyond the heated point, the gas and the 

 substances used for its generation may be pronounced free from 

 arsenic. (Both zinc and sulphuric acid often contain arsenic.) 



After having thus demonstrated the purity of the hydrogen, the 

 suspected liquid, which must contain the arsenic either as oxide or 



