i 877*J i fl it s Geological Relations. 471 
are the processes to which many metalliferous deposits 
are due. 
Another result of the opposition of these two atmospheric 
gases is the defertilising of soils, and consequent failure of 
vegetation. An ordinarily fertile natural soil contains, 
amongst other things, silicates of alumina, lime, potash, 
and soda, with some peroxide of iron. The silicates of lime 
and soda will be decomposed by carbonic acid, and the bases 
removed as carbonates. The potash silicate is also decom- 
posed, and a part of the potash removed by aquatic plants 
under favourable circumstances, in marshy places, &c., — 
conditions under which the vegetation of the Coal era flou- 
rished, — and the ferric oxide is reduced to the ferrous state 
by the deoxidising influence of rotting vegetation. This 
having occurred, the roots of plants are for a time debarred 
from any access of oxygen, for any that permeates the soil 
will be immediately seized on by as much of the proto- 
compound of iron as has not been carried off in its soluble 
state, and this is again converted to the higher condition ; 
and these changes continue until they result in the total 
barrenness of the soil and its ultimate conversion into a 
hydrous silicate of alumina, almost entirely free from iron, 
such as we are acquainted with in the fire-clays of the coal- 
measures — those ancient soils on which the vegetation now 
forming our coal-seams once grew.* 
Ammonia and its Compounds. 
Ammonia exists in the air chiefly in the form of carbonate 
of ammonia, but the quantity, whilst always small, appears 
to vary greatly, and it is not positively ascertained whether 
the variation is to be ascribed to natural causes, or ought to 
be referred to the difficulty of accurate analysis when such 
small quantities have to be dealt with. It is quite possible, 
however, that the variability is natural. The minimum re- 
corded is o*i part of carbonate of ammonium in one million 
of air ; the maximum is 135 parts. f Rain-water, hail, 
snow, and dew contain appreciable quantities of ammoniacal 
* It is obvious that this only applies to natural soils, since the agriculturist 
by breaking up the ground affords a supply of oxygen much in excess of what 
is absorbed by the oxidisable matters present. 
f Watts, Chem. Didt., p. 439. P. Truchot finds that the amount of am- 
monia varies with the altitude. At Clermont-Ferrand, 395 metres above sea- 
level, the quantities were 0^93 m.grm. to 279 m.grms. in a cubic metre of air, 
— according as the day was clear or dull, — whilst at Picde Sancy, 1884 metres, 
it amounted to 5-27 and 5-55 m.grms. under the same conditions. Comptes 
Rendus, lxxvii., 1159 — 1161. 
