ACTION OF ELEMENTS OF SOIL. 61 



presence of sulphate of lime. Sulphuret 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 ; whi-Je at the same time, a large portion 

 of that hardest, and, as is generally supposed, utterly insol- 

 uble earth, silex, is withdrawn by every plant which grows. 

 How is this rendered soluble ? 



83. This question may be answered, if it be admitted 

 that a portion of the silica of rocks exists as a sulphuret of 

 silicon. The action of air and moisture upon this will be 

 understood by reference to section 68, where it is stated 

 that sulphuret of silicon is decomposed by water. ,The sul- 

 phur, in this case, is evolved as sulphuretted hydrogen- gas, 

 the pilica deposited, and in this state is abundantly soluble in 

 water. The sulphuretted hydrogen would act on the lime of 

 the silicates, and gradually sulphate of lime would be 

 formed. Here is an abundant source, not only of the solu- 

 bility of silica, a point always of difficult explanation in 

 vegetable physiology, but also of the production of sulphate 

 of lime. 



84. Similar remarks are applicable to the presence of the 

 phosphates of lime, and iron, and alumina in soil. Phos- 

 phate of lime is not a very abundant ingredient in rocks, 

 except in certain localities ; yet its occurrence is too rare to 

 account for the vast amount of phosphate of lime in soil. 

 The phosphorus possibly exists in combination with silicon, 

 as phosphuret of silicon. The effect of air and moisture on 

 this has already been explained, and accounts for the pro- 

 duction of phosphates in soil. Similar remarks are'applica- 

 ble to the source of the chlorides or muriates ; for instance, 

 common salt in the potash of commerce. May not their 



