ORIGIN OF THE ELEMENTS OF SOIL. 49 



ant source, not only of the solubility of silica, a point 

 always of difficult explanation, in vegetable physi- 

 ology, but also of the production of sulphate of 

 lime. 



84. Similar remarks are applicable to the pres- 

 ence of the phosphates of lime, and iron, and alum- 

 ina in soil. Phosphate of lime is not a very uni- 

 versal ingredient in rocks. In certain localities it is 

 abundant, 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, have already been explained, 

 and accounts for the production of phosphates in 

 soil. Similar remarks are applicable to the source 

 of the chlorides or muriates ; for instance, common 

 salt in the potash of commerce. May not their 

 source be in chloride of silicon ? These are conjec- 

 tures, but conjectures only because, refined as mod- 

 ern chemical analysis is, it may not be so delicate, 

 as to detect the possible combinations, which nature 

 presents in silicates. What is the source of that 

 phosphoric odour, produced by the friction of frag- 

 ments of pure quartz on each other ? If not due 

 wholly to electrical excitement, may it not arise 

 from the presence of phosphoric elements ? The 



