ABSORPTION OF FOOD-SALTS BY LAND-PLANTS. 83 
Streams fall into lakes, rivers into the sea, and hence the water ascends into the 
atmosphere in the form of vapour, and returns once more to earth as snow, rain, 
and dew. Through porous earth it percolates until it has filled all the interspaces. 
If its further descent be impeded by impervious strata, it spreads literally as sub- 
terranean water, or else comes up at some special spot as a spring. Earth which is 
richly endowed with decaying vegetable remains is able to absorb vapour in addition 
from the atmosphere. When this occurs, carbonie and nitric acids are always 
absorbed along with the aqueous vapour. These are contained, as has been mentioned 
before, in atmospheric deposits, and another source of these acids is afforded by the 
decay of dead parts of plants. Water precipitated from the atmosphere, and con- 
taining carbonic and nitric acids, is able by their means to decompose the compounds 
in all the rocks which come in its way as it percolates through the ground, especially 
when its action is long continued. The siliceous compounds or so-called silicates— 
felspars, mica, hornblende, and augite in particular—and quartz, the anhydride of 
silicic acid, which form the preponderant mass of the rocks of the solid crust of our 
earth, either contain a great quantity of silica, alumina, and alkalies, or if they are 
relatively poor in silica they may be rich in iron. The former are found chiefly 
in granite, gneiss, mica-schist, and argillaceous slate; the latter preponderate in 
serpentine, syenite, melaphyr, dolerite, trachyte and basalt. First the felspars are 
decomposed by the acid water. Their alkalies combine with the carbonic and nitric 
acids forming soluble salts, and the alumina and silica remain behind as clay. Iron 
is also converted into soluble salts. The most difficult substances to decompose are 
the mica and quartz, and it is on that account that they so often appear in the 
form of glittering scales and angular nodules mixed with the clay produced from 
the decomposition of felspar. But, ultimately, even they are unable to withstand 
the continuous action of the acidulated water. The result of these chemical 
changes is an earth, which, according to the nature of the parent rock, contains 
a preponderating amount of clay, of quartzose sand or of mica, which is coloured 
in various ways by iron compounds. Of substances useful to plants these 
earths yield generally on analysis the following: potash, soda, lime, magnesia, 
alumina, ferrous and ferric oxides, manganese, chlorine, sulphuric acid, phosphoric 
acid, silica, and carbonie acid, sometimes one sometimes another in greater 
proportion relatively, and traces of many substances often so slight as hardly 
to be detected. 
It is true that limestone and dolomite, which, next to the above-mentioned 
rocks, enter most largely into the composition of the solid erust of the earth, 
consist chiefly of carbonate of lime and magnesium carbonate respectively; but 
wherever they occur in extensive strata and piles, they always contain in addition 
an admixture of alumina, silicic acid, ferrous oxide, manganese, traces of alkalies 
in combination with phosphoric and sulphuric acids, &e. Of the carbonates of 
lime and magnesia a great part is gradually dissolved and carried away upon the 
invasion of water containing carbonie and nitric acids, and a proportion also of 
the substances mixed with them, as above mentioned, is lixiviated. What remains 
