282 EXAMINATION FOR PHOSPHORUS. 



and Fresenius, in the following manner. It is brought 

 into a flask with doubly-perforated stopper, water is 

 added, if necessary, and dilute sulphuric acid to aid 

 reaction. Washed carbonic acid gas* is now slowly 

 conducted through the mixture by means of a glass 

 tube passing through the cork and reaching nearly 

 down to the bottom of the flask. From a short tube 

 above the current of gas is led through one or two 

 V-formed tubes which contain neutral solution of ni- 

 trate of silver. When the flask is filled with carbonic 

 acid it is warmed in a water-bath. The experiment is 

 kept up for several hours. If free phosphorus be 

 present, a portion of it volatilizes unoxidized in the 

 stream of carbonic acid, and on passing into the silver- 

 solution produces there an insoluble black precipitate 

 of phosphide of silver, together with phosphoric acid. 

 Since a black insoluble precipitate may be caused by 

 various volatile reducing agents or by hydrosulphuric 

 acid, its appearance is not proof of the presence of 

 phosphorus, though its non-formation demonstrates 

 conclusively that free phosphorus is absent. 



a. A PRECIPITATE formed in the silver solution in 

 the above experiment is collected on a filter (which 

 has been previously washed with dilute nitric acid and 

 water), and is well washed with water. The phosphide 

 of silver, which may be contained in this precipitate, 

 is detected by the method of Blondlot, improved by 

 Dussard. a, Fig. 44, is an apparatus for evolving hy- 

 drogen ; b is filled with fragments of pumice-stone 

 drenched with concentrated potassa-lye ; c is a common 

 spring clamp ; d a clamp that can be nicely adjusted 

 by means of a screw or wedge ; e is a platinum jet 

 which is kept cool by means of moistened cotton. 

 This platinum jet is essential, since the flame would 

 be colored yellow if burned directly from a glass tube. 



* The apparatus, Fig. 41, may be conveniently employed. 



