^li^a. 
TO 
THE CHEMISTRY OF SOILS AND MANURES. 
causes." A few agencies in producing these changes partake of the nature of both ; they act partly 
chemically, partly mechanically. 
I. Chemical causes of the degradation and disintegration of rocks. 
1. One of the principal chemical agencies in effecting a gradual disintegration of solid rocks is the 
atmospheric oxygen. For many mineral elements oxygen possesses a most powerful affinity, and con- 
sequently a strong tendency of forming new compounds. These compounds as oxides,' being always 
more voluminous, looser and less compact, are the primary cause of the bursting of many rocks, par- 
ticularly of those containing much iron. In the course of the formation of these oxides, the compact 
texture of the rock is broken up, and the whole mass of the rock gradually crumbles down. In basalt 
rocks this process of disintegration can be well observed. Those sides of such rocks which are most 
exposed to the atmospheric influences will invariably be found on the surface softer and less compact 
than in the interior, whilst at the same time the brown coating of oxide of iron, which in the course 
of time grows deeper and deeper towards the interior, illustrates the action of the atmospheric oxygen, 
to which wc have just alluded. 
2. A second and no less powerful chemical agency in the formation of soils is the carbonic acid of 
the atmosphere, which is carried down by the rain. The affinity of carbonic acid for different mineral 
compounds varies greatly ; the action which this acid exercises on limestone, dolomitic rocks, sand- 
stones, clay, basalt, &c, which minerals and rocks differ greatly in composition, is not therefore the 
same in every case. Limestones, for instance, are easily attacked by rain-water, as the carbonic acid 
which the water contains dissolves the carbonate of lime, whilst pure sandstones and quartz-rocks arc 
scarcely acted upon by rain-water. Generally speaking, the less compact and less pure a limestone 
is, the more easily it is attacked by the carbonic acid of the atmosphere. Pure marble, a hard, crys- 
talline variety of carbonate of lime, is little affected by this agency ; whereas a limestone containing 
much clay crumbles to powder with comparative ease, furnishing what is called a marly soil. Most 
limestones are impure, that is to say, arc mixtures of carbonate of lime with clay and other mineral 
compounds; the more clay a limestone contains, the more readily it absorbs and retains rain, the 
carbonic acid of the latter, dissolving the lime, leaves crevices and fissures in the rock ; the rock thus 
loses its coherence, and gradually crumbles down to form a marly soil, which always contains a pre- 
ponderance of clay, and often but small quantities of carbonate of lime. The energetic dissolving 
action of rain-water on carbonate of lime, which is the principal cause of the formation of all clay 
marls, is surprising. Some clay marls contain but three per cent, of carbonate of lime, whereas the 
limestone rocks from which they are derived exhibit no less than eighty per cent, of carbonate of 
lime. In the formation of stalactites, which arc generally found in those caverns which abound in 
limestone rocks, another familiar example of the dissolving action of rain-water is presented. 
Dolomitic rocks arc affected by rain-water in the same manner as limestone rocks, — not, however, 
so energetically. 
On felspar, granite, and other minerals, consisting, like many zoolites, of double silicates of alumina 
and an alkaline silicate, carbonic acid and water exercise a highly-important action. Under their 
influence these minerals arc decomposed into alkaline silicates, which in their turn give rise to silica 
and carbonate of potash or soda, and into silicate of alumina, the chief mass of clays. In other words, 
clays are produced, whilst at the same time the highly-fertilising alkaline salts which exist in 
felspar, granite, trap, and other rocks in an insoluble state, are changed into the easily soluble 
carbonates, and thus rendered available to the immediate use of plants. 
3. In the formation of soils from solid rocks the superior orders of plants and animals take an 
active share. The seeds of lichens and mosses Boating in the air attach themselves to the roughened 
and partially decomposed surface of rocks, and finding here sufficient food, germinate and throw out 
roots, which penetrate the little crevices in the rocks like wedges. These widening and multiplying 
the crevices, hasten the final disintegration of the rock. Mosses and lichens likewise retain the 
atmospheric water, and keep the surface of the rock moist for a longer time, giving in tins manner 
rain-water a better chance of exercising its dissolving powers on the constituents of the rocks. 
Insects and other animals of the lower orders collect and feed on the lichens and mosses, and both 
insects and plants in due time die, decay, and leave all the mineral matter which they have originally 
derived from the rock behind, mixed with vegetable and animal remains, or hnnms. Dining the 
decay of the inferior plants and animals, carbonic acid and ammonia are given off, which assist the 
atmospheric water in its dissolving action on the constituents of the rock. A thin layer of a icon 
\ fertile soil is thus formed, on which plants of a higher order may spring up; in the course of time 
A these die, and enrich and increase the soil in the same manner as the mosses. Tims one small i 
•&) assists another, and all together produce the mightiest i 
