echnically
called fallows--the repose of the fields--we recognise by science
certain chemical actions, which are continually exercised by the
elements of the atmosphere upon the whole surface of our globe. By
the action of its oxygen and its carbonic acid, aided by water,
rain, changes of temperature, &c., certain elementary constituents
of rocks, or of their ruins, which form the soil capable of
cultivation, are rendered soluble in water, and consequently become
separable from all their insoluble parts.

These chemical actions, poetically denominates the "tooth of time,"
destroy all the works of man, and gradually reduce the hardest rocks
to the condition of dust. By their influence the necessary elements
of the soil become fitted for assimilation by plants; and it is
precisely the end which is obtained by the mechanical operations of
farming. They accelerate the decomposition of the soil, in order to
provide a new generation of plants with the necessary elements in a
condition favourable to their assimilation. It is obvious that the
rapidity of the decomposition of a solid body must increase with the
extension of its surface; the more points of contact we offer in a
given time to the external chemical agent, the more rapid will be
its action.

The chemist, in order to prepare a mineral for analysis, to
decompose it, or to increase the solubility of its elements,
proceeds in the same way as the farmer deals with his fields--he
spares no labour in order to reduce it to the finest powder; he
separates the impalpable from the coarser parts by washing, and
repeats his mechanical bruising and trituration, being assured his
whole process will fail if he is inattentive to this essential and
preliminary part of it.

The influence which the increase of surface exercises upon the
disintegration of rocks, and upon the chemical action of air and
moisture, is strikingly illustrated upon a large scale in the
operations pursued in the gold-mines of Yaquil, in Chili. These are
described in a very interesting manner by Darwin. The rock
containing the gold ore is pounded by mills into the finest powder;
this is subjected to washing, which separates the lighter particles
from the metallic; the gold sinks to the bottom, while a stream of
water carries away the lighter earthy parts into ponds, where it
subsides to the bottom as mud. When this deposit has gradually
filled up the pond, this mud is taken out and piled in heaps, and
left