ABSORPTION OP 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, carbonic 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 carbonic 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 crust 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, &c. Of the carbonates of 

 lime and magnesia a great part is gradually dissolved and carried away upon the 

 invasion of water containing carbonic and nitric acids, and a proportion also of 

 the substances mixed with them, as above mentioned, is lixiviated. What remains 



