180 § 102. ANALYSIS OF SOILS AND EOCKS. 



the addition of a few drops of nitric acid towards the 

 close of the evaporation, to destroy organic matter and 

 oxidize ferrous salts, and eliminate silica. (§ 58, a, 1.) 



Examine the filtrate from the silica, which is to be 

 made up to 1000 c.c. and well mixed, according to 

 Scheme II., § 94. 



Or, in order to have a larger quantity of solution for 

 the determination of phosphoric and sulj^huric acids, the 

 analysis may be performed by Scheme I., in which a and 

 h may be united, and the sulphuric acid determined as 

 usual, while half the filtrate from the precipitate by 

 ammonia for phosphoric acid will answer for the determi- 

 nation of the alkalies. 



Examination of the Residue Insoluble in Hot Hydro- 

 chloric Acid. 



102. Dry it, and remove it from the filter as completely 

 as possible, burn the latter, and weigh ash and residue, 

 and separate the carefully prej)ared mixture of the two 

 into three accurately weighed portions of 10 grms. (a), 

 10-15 grms. (5), and 15-20 grms. (c). 



a. Ignite this portion, to determine the amount of min- 

 eral matter insoluble in the hot acid. 



b. In this portion determine the silica soluble in car- 

 bonated alkali. (§ 58, a^ 2.) 



c. Pour five times its weight of concentrated sulphuric 

 acid over this portion, heat until the excess of acid is 

 removed, and the residue forms a light, dry powder ; the 

 evaporation of the acid should be performed slowly and 

 with constant stirring, and should require from six to 

 eight hours. Moisten the residue freely with concentrat- 

 ed hydrochloric acid, remove this acid by long heating in 

 the water-bath, boil the residue repeatedly with water to 

 which a little hydrochloric acid has been added, filter, and 

 wash the insoluble residue carefully. 



Examine the solutions and washings, after concentra- 

 tion, according to Scheme VII., § 94. 



