182 § 103. ANALYSIS OF SOILS AND ROCKS. 



alone is very nearly pure, while it is that which is dissolved 

 by the hydrochloric acid that contains too much silica. 



Ignite the other half of the residue, to determine the 

 amount of mineral matters insoluble after treatment with 

 sulphuric acid. 



Pulverize the ignited mass very finely in an agate 

 mortar, separate the finer from the coarser portions by 

 levigation (§ 36) with distilled water, pulverize the coarse 

 part again, and repeat the levigation ; when in this way 

 the whole is reduced to the finest possible powder, evaji- 

 orate the water to dryness with the matters in suspension 

 in it, weigh out 3-4 grms. of the well-dried residue, and 

 treat it with hydrofluoric acid or amnionic fluoride 

 (§58,e). 



Examine the solution thus obtained according to Scheme 

 YII., § 94. The determination of ferric oxide will, how- 

 ever, be necessary, only when the precipitate by ammonia 

 is yellowish or reddish. 



If, as is usually the case, the solution is found to con- 

 tain only traces of lime and magnesia, the amount of 

 feldspathic minerals and of pure quartz sand in this in- 

 soluble part of the soil can be estimated from the amount 

 of alkalies found ; and, from the amount of aluminic sili- 

 cate, it may be judged how perfectly the clay was de- 

 composed by the j)revious treatment with sulphuric acid. 



h. According to A. Miiller, the relative proportion of 

 silicates and quartz sand in a soil can be determined with 

 accuracy by digestion with phosphoric acid at a certain 

 temperature; all the silicates are decomposed by this 

 treatment, and the silica is separated in a gelatinous form 

 while the quartz sand remains unchanged. 



For this purpose a syrupy acid is required containing 

 40-45°l g of anhydrous acid ; it may be obtained by con- 

 centrating the commercial acid. 



The insoluble residue to be treated with the acid must 

 be very finely pulverized, but it need not be levigated ; 



