48 ORIGIN OF THE ELEMENTS OF SOIL. 



bility of silica, is yet to be detected, by exact chem- 

 ical analysis. It is to be looked for in the sulphu- 

 rets and phosphurets of silicon, which probably exist 

 in rocks. The action of sulphuret of iron, as explain- 

 ed, would demand its universal diffusion, to account 

 for the presence of sulphate of lime. Sulphate of iron, 

 must either now exist, or have ages ago existed, as 

 widely diffused as the silicates. But though common 

 in rocks, its presence as a sulphuret, will not account 

 for the quantity of sulphate of lime found in soil. 

 Vast quantities of this salt are annually borne off in 

 crops ; while at the same time, a large portion of 

 that hardest, and as is generally supposed, utterly 

 insoluble earth, silex is withdrawn by every plant 

 which grows. How is this rendered soluble ? 



83. This question may be answered, if it be ad- 

 mitted, that a large portion of the silica of rocks, 

 exists as a sulphuret of silicon. The action of air, 

 and moisture upon this, will be understood by refer- 

 ing to section 68, where it is stated, that sulphuret 

 of silicon, is decomposed by water. The sulphur, 

 in this case, is evolved as sulphuretted hydrogen gas, 

 the silica deposited, and in this state, is abundantly 

 soluble in water. The sulphuretted hydrogen, would 

 act on the lime of the silicates and gradually, sul- 

 phate of lime would be formed. Here is an abund- 



