Action cif the Atmosphere vpon ncidy -deepened Soil. 431 



nesla is much more soluble than carbonate of lime. Another fact 

 of some interest, also noticed in their experiments, was the com- 

 parative readiness with which the magnesian and calcareo-magne- 

 sian silicates yield to the decomposing and dissolving action of 

 carbonated water and even simple water, thus explaining the rapid 

 decomposition of most rocks composed of hornblende, epidote, 

 &c., without calling in the agency of an alkali ; enabling us also 

 to trace the process by which plants are furnished with the lime 

 and magnesia they require from soils containing these silicates, 

 without having recourse to any mysterious decomposing power of 

 the roots of the growing vegetables. {Brit. Assoc. Report., 1849, 



Bisciiof has also remarked, that, although long-continued boiling 

 in water will separate the alkali from a mass of volcanic tuff., the 

 process is facilitated by the presence of carbonic acid, so that he 

 conceives the disintegration of fel spathic rocks to be brought 

 about by water impregnated with that ingredient. 



Professor Bunsen, of 31arbourg, experimenting upon the Pala- 

 gonite tuff of Iceland, of which the following is the composi- 

 tion — 



Silica 37-947 



Sesquioxide of iron .. .. 14"75i 



Alumina 11-619 



Lime 8-442 



Mai:;nesia 5-813 



Potash 0-659 



Soda 0-628 



AVater 16-621 



Eesiduc 4-lOS 



— found that under the action of water, at 212 F., silicic acid, 

 potash, and soda were dissolved ; but when the water was saturated 

 with carbonic acid and allowed to act upon the pulverized rock, 

 all the constituents, with the exception of alumina and oxide of iron, 

 were dissolved in the form of bicarbonates.* 



1000 grammes of the water solution; after 12 hours' digestion, 

 contained — 



Gramraes. 



Sih'ca 0-03716 



Soda 0-00^24 



Potash 0-00162 



0-04702 



* Silicates of potash and soda, silicate of lime, protosilicate of iron, and probably 

 protosilicate of manganese, are decomposed by carbonic acid at ordinary tempe- 

 ratures; but silicate of alumina and persilicatc of iron are not acted upon by 

 carbonic acid, as it docs not combine with alumina, nor witli peroxide of iron. 

 The alkaline carbonates also bring about many decompositions in the^ mineral 

 matter of the soil. 



2 g2 



