202 CAST IRON. 



chloric acid, the solution evaporated to dry ness on the 

 water-bath, the mass digested with dilute hydrochloric 

 acid, and the silicic acid filtered off. 



III. PHOSPHORUS. From the solution filtered from 

 the silica, the phosphoric acid is separated as in 

 No. 22. If the iron contain arsenic, it is obtained as 

 arsenic acid, together with the phosphoric acid. 



Or a larger quantity of iron is dissolved in aqua- 

 regia, precipitated with ammonia, filtered, dried without 

 washing, mixed with about an equal portion of car- 

 bonate of soda, heated to redness for half an hour, the 

 mass completely dissolved in water, the solution concen- 

 trated and the phosphoric acid precipitated as in No. 9. 



The amount of phosphorus may be less accurately 

 determined by heating the fine iron-filings to redness 

 with 2 parts of nitre and 1 part of carbonate of soda, 

 extracting the mass with water, acidifying the solution 

 with hydrochloric acid, and adding excess of ammonia 

 and sulphate of magnesia. 



IV. ARSENIC. The presence of arsenic may be de- 

 tected by dissolving the iron in dilute sulphuric acid, 

 filtering off the black residue, and digesting it with 

 sulphide of ammonium. From the filtered solution 



Fig. 23. 



dilute sulphuric acid precipitates the pentasulphide of 

 arsenic. The precipitate is dissolved in aqua-regia t the 



