156 THE PRINCIPLES INVOLVED IN ROCK-WEATHEElNa 



not appear in the soils, as the nitrie organism attacks the latter 

 at once and converts it into nitric acid."^ (See further nnder 

 influence of plant and animal life, p. 180.) 



In considering the possible efficacy of these compounds, one 

 must not lose sight of the fact that the amount of nitrogen in 

 the soils is as a rule far too small to supply the demands of 

 growing plants, and it is probable that a very large proportion 

 of that which finds its way there is quickly taken up again by 

 these organisms. It is possible that other salts of ammonium 

 than the nitrate may be locally efficacious. M. Beyer, as quoted 

 by Van Den Broeck,^ has shown that the feldspars decompose 

 very rapidly under the influence of water containing ammonium 

 sulphate or even sodium chloride, either of which substance may 

 be found in vegetable soil. Daubree, who experimented by means 

 of revolving iron cylinders, found, however, that the presence of 

 sodium chloride retarded decomposition. (See p. 174.) 



(2) Carbonic Acid. — The amount of carbonic acid in the air 

 under natural conditions is not a widely variable quantity, ex- 

 cepting near volcanoes and in the immediate vicinity of gaseous 

 springs. In the vicinity of large cities and manufactories 

 consuming great quantities of coal, the amount is increased. 

 Although carbonic acid is the most abundant gas given off by 

 decomposing vegetable matter, it has apparently been definitely 

 ascertained that the amount in regions of abundant vegetation 

 is no greater than elsewhere. This has been accounted for on 

 the assumption that, as fast as liberated, it is taken up by grow- 

 ing organisms or carried by rains into the soil.^ 



^ Wiley, Principles and Practice of Agricultural Analysis. 



^Mem. sur les phenom&nes d 'Alteration des Dep6ts Superficial, p. 16. 



^Tiie researches of Boussingault and Lewey (Mem. de Cliemie Agrieole, 



etc.), as quoted by Johnson (How Crops Feed, p. 139), showed the following 



proportions existing between the COo of atmospheric air and that of various 



soils: — 



CO2 IN 10,000 Paets 



BY Weight 



Ordinary atmosphere 6 parts 



Air from sandy subsoil of forest 38 parts 



Air from loamy subsoil of forest 124 parts 



Air from surface soil of forest 130 parts 



Air from surface soil of vineyard 146 parts 



Air from pasture soil 270 parts 



Air from soil rich in humus 543 parts 



