§ 103. THE CHEMICz\L ANALYSIS. 181 



The amount of lime is usually small. Wolff directs 

 that the filtrate from the precipitate of calcic oxalate be 

 evaporated to dryness, the residue ignited gently in a 

 platinum dish, to expel ammoniacal salts, dissolved in di- 

 lute acid, and any silicic acid that may appear as an in- 

 soluble residue be filtered out; then add ammonia in 

 slight excess to the filtrate, filter out any flocculent pre- 

 cipitate of alumina that may also appear, and finally de- 

 termine sulphuric acid with baric chloride. 



This treatment of the soil with sulphuric acid serves 

 to determine the amount of clay in it, and Wolff has 

 found, by repeated trials, that the clay is completely 

 decomposed if the operation is carefully perfonned. He 

 gives importance to the determination, for it furnishes data 

 for controlling the results obtained by the silt analysis, 

 and' because it gives valuable information in regard to 

 the degree of insolubility of the other constituents of the 

 soil, and ^particularly the alkalies. 



The process is a good connecting link between the 

 treatment with hydrochloric acid on the one hand and 

 hydrofluoric acid on the other. 



Examination of the Residue Undecomposed by Sul- 

 phuric Acid. 



103. a. Dry this residue at 100°, burn the filter by it- 

 self, and weigh the ash and residue ; mix them well to- 

 gether, and, in half of the mixture, determine silica solu- 

 ble in alkaline carbonates. (§ 58, a, 2.) The silicic acid 

 thus found, together with the small quantity in the 

 hydrochloric and sulphuric acid solutions, gives, in con- 

 nection with the alumina found in the same solutions, 

 an approximate estimate of the pure anhydrous clay in 

 the soil. This amount of silicic acid is, in general, too 

 large in proportion to that of the alumina, for a part of 

 it was combined with ferric oxide, lime, etc. 



The clay that is decomposed by the sulphuric acid 



