SILICATES. 285 



phosphoric acid ; though, only, when the operator is 

 certain that none of the contents of the distilling flask, 

 which usually contain phosphoric acid, have spirted 

 into the condenser. The entire quantity of phospho- 

 rus is obtained by adding to that, thus determined, 

 what exists in the U tube. Its contents are treated 

 with nitric acid, the silver thrown down by hydrochlo- 

 ric acid, filtered through a washed filter, concentrated, 

 precipitated as phosphate of ammonia-magnesia, and 

 weighed as phosphate of magnesia. 



B. Detection of Phosphorous Acid. 



VI. In case free phosphorus itself has not been 

 detected by the above methods, it is needful to look 

 for the first product of its oxidation, viz., phosphorous 

 acid. To this end the residue of the distillation (II. a.), 

 or (V.), or also the residue of (IV.) is brought into the 

 apparatus, Fig. 44, and tested as described (IV. a.) as 

 to any green coloration of the evolved hydrogen. If 

 the phosphorous reaction appears, it is sufficient ; other- 

 wise organic matters may have hindered its production. 

 If, therefore, the flame is not colored, the clamp is 

 closed, and a U tube containing neutral solution of 

 nitrate of silver is affixed to the apparatus and the gas 

 is allowed to stream slowly through the silver solution 

 for many hours. In presence of phosphorous acid, 

 phosphide of silver is formed, which is filtered off and 

 examined as directed in (IV. a.). 



130. SILICATES.* 



A few silicates are directly attacked by acids, while 

 others cannot be decomposed by acids, except by the 

 addition of a base, as, for example, lime. 



* Methods of Sainte-Claire Deville, as given by Messrs. Grandeau 

 and Troost. 



