356 METALS AND THEIR COMBINATIONS. 



(Fig. 46) used for performing this test consists of a glass vessel (flask 

 or WoulPs 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 

 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.) 



FIG. 46. 



lllillllillllllllMIMII1!linil1lll1tl)l1limnilinnitiiMiiiiitiiiiimni 



uliliililiilllillllillliilllllllllM 



Marsh's apparatus for detection of arsenic. 



After having thus demonstrated the purity of the hydrogen, the 



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



chlonde (not as sulphide), is poured into the flask through the funnel- 



If arsenic is present in not too small quantities, the gas ignited 



the end of the glass tube shows a flame decidedly different from 



tiiut ot Durnino* hvdroo'pn TM fl K i 



appt^whichfs'ml^ " f P f ^^^it a wSdteud 



ovpr tli fl r SS se > a c ld test-tube held inverted 



lame will be covered upon its walls with a white deposit of 

 ctahedral crystals of n, senous oxide . a piece rf coId P porce . 



coated with a brown stain (arsenic 

 above in connection with 



