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weather ; but wind and weather, oxygen and water, have much less to do with 

 the process than carbonic acid, although it certainly requires the co-operation of 

 water. The carbonic acid acts with varying rapidity on silicates of sodium, 

 calcium, magnesium, and potassium, but is incapable of ousting the silicic acid 

 from its union with aluminium. The primitive rocks consist of a mixture of 

 different minerals whose capacity for weathering varies greatly. Looking at 

 granite, for instance, we find that quartz and mica are remarkably stable, whilst 

 the felspar (a double silicate of aluminium with potassium or sodium) weathers 

 more easily. Through the action of carbonic acid, carbonates of sodium or 

 potassium are produced from it, which are soluble in water, while on the other 

 hand, aluminium silicate (caolin, clay), which, while retaining water, is entirely 

 insoluble, is carried away by water in a state of suspension in very fine particles 

 and redeposited as a clay soil. When the originally compact rock is in this 

 way deprived of one of its ingredients, pits and cavities appear in it rendering 

 it liable to fresh attack from the carbonic acid, as well as to certain physical 

 effects of water which we need not refer to here. The net result of the whole 

 process is the decomposition of the granite into a mass of felspar, quartz, and 

 mica particles which, if they be still held together by the clay, are carried away 

 by water and redeposited to form an alluvial soil. Such a soil is better for the 

 plant than the original granite in two respects ; in the first place, it is not com- 

 pact, and hence plant roots can penetrate into it, and in the second place, it 

 contains soluble constituents, which may be continually formed until the last 

 particle of felspar has disappeared. The water, which such a soil holds, in 

 virtue of its capillarity, as well as that which actually percolates through it, 

 always contains substances in solution, although, it is true, only in limited 

 amount. Analyses of streams and rivers which have their sources in primitive 

 rocks teach us much on this subject (Knop, 1868, 124) : — 



When we compare the contents of these natural soil waters with our culture 

 solution, as regards the minerals they respectively contain, we find that the 

 former solutions contain nearly 100 times less solid in solution than the latter, 

 and further, that much of that solid is useless to the plant. It is also obvious 

 that oats or maize will be able to thrive only with great difficulty in such a 

 medium, seeing that phosphoric and nitric acids are present in such small 

 quantities that they do not appear in the analysis. Soil water ought to exhibit, 

 however, a composition similar to, and also show a concentration equal to, 

 that of spring and river water, and we are thus at a loss to understand how it 

 is possible for a plant to live in such a soil. Observation of natural conditions, 



