DETECTION OF UNOXIDIZED PHOSPHORUS. 283 



At the outset it is needful to test the sulphuric acid 

 and zinc to demonstrate that they yield hydrogen free 

 from phosphuretted hydrogen. For this purpose allow 

 the gas to evolve until air is displaced from the appa- 



Fig. 44. 



ratus, then close c until the acid has been forced into/ 

 then close d, open c, and lastly open d, cautiously in- 

 flaming the gas at the jet and properly regulating its 

 issue. If the flame, when examined in a rather dark 

 place, is colorless, exhibits no trace of a green cone in 

 its interior and no emerald-green tinge when a porce- 

 lain dish is depressed into it, the hydrogen is pure. 

 After verifying this result by a second trial, the preci- 

 pitate to be examined is rinsed into/, care being taken 

 that it passes completely into a, and the flame is again 

 observed as before. In case but a minimum of phos- 

 phide of silver be present the green inner cone and 



