Action of the Atmosphere upon neichj -deepened Soil. 461 



thirty centaries have beaten over them. The crumbling of granite 

 seems almost always owing to the decomposition of the felspar, 

 the silicate of potash dissolving out while the earthy silicate of 

 alumina remains behind in combination with two atoms of water 

 falling down into a white powder. 



Although the decay of felspar is generally referred to the 

 carbonic acid of the atmosphere and the solvent power of water 

 containing that agent, it appears not improbable that the am- 

 monia and nitric acid of the air may also have a share in pro- 

 ducing this effect. VVay has shown that the silicates have the 

 property of absorbing ammonia, and that it displaces several of 

 the other bases. It is true he found the natural silicates finely 

 powdered did not combine with the ammonia of a solution of 

 sal-ammoniac, but he observed that in these bodies artificially 

 produced, the silicates of potash, soda, and lime, were all de- 

 composed by ammonia : now, as the ammonia of the atmosphere is 

 considered to be in the form of a carbonate, it may act with 

 increased force by the carbonic acid seizing the base, such as 

 lime or potash, while the ammonia combines with the silica, 

 forming silicate of ammonia and carbonate of potash, soda, or 

 lime, as the case may be, thus giving rise to a double decom- 

 position : — 



Silica ^.^ ^ ' '" Carbonic acid. 



Potash — Ammonia. 



Both the new compounds being soluble would be gradually 

 washed away, leaving the silicate of alumina behind. Be this 

 however as it may, the felspar in the course of the changes 

 brought about gradually loses its transparency and lustre, the 

 surface becomes dull and earthy, and at length it falls down into 

 a powder. This powder forms the kaolin or porcelain clay ; its 

 composition is somewhat variable, but approximates to — 



Silica 47*2 



Alumina 39-1 



Water 13-7 



100-0 



A little iron and lime generally remains, and frequently some 

 potash or soda, according to what was the original constitution of 

 the mineral, and the degree of completeness with which the 

 alkalies have been washer! out. The following Table (p. 402) 

 contains analyses of some varieties of porcelain clay. 



A comparison of these with the analyses given of felspar will 

 at (mce show what has taken place. When the removal of t!ie 

 alkalies has been complete, or nearly so, a poor sterile clay will 

 be the consequence, but it is rarely that the alkaline silicate is 



VOL. XVII. 2 I 



