562 CHEMISTRY IN AGRICULTURE AND PHYSIOLOGY. 
on a large scale in free nature, and in small artificial expe¬ 
riments. 
The bowels of the earth rumble and heave; Vesuvius 
opens her fiery mouth, and vomits forth a sea of vapours and 
molten lava; the fumes of her sulphurous breath slowly 
descends like a mist, which is absorbed by the sand and ashes 
around her cooling feet. Time rolls on, the lava is bleached 
and becomes porous fossile, and honeycombed, till at length 
it crumbles into powder, the type of a fertile soil. 
This soil, being derived from the disinegration of lava, 
cannot possibly, owing to its origin, contain the smallest 
trace of vegetable matter; yet every one knows that when 
lava or volcanic ashes have been exposed for a time to the 
influence of air and moisture, all kinds of plants grow on 
them with the utmost luxuriance. 
It is perfectly universal experience, that when the vegeta¬ 
tion is left to itself upon a particular soil, and its products 
are not removed from the ground, organic substances are 
formed, in consequence of the death of plants accumulating 
from year to year, which can of course only be the case 
through each generation of plants producing a greater quan¬ 
tity of organic substances than it consumes. It is not re¬ 
quisite to demonstrate more minutely how these circumstances 
show the total error of the view, supported, indeed, less by 
vegetable physiologists than by <c popular ” writers on agri¬ 
cultural chemistry, that plants subsist solely on the moul¬ 
dering remains of former plants or animals. 
The inorganic compounds which are taken up by plants as 
food, and which furnish them with the four principal elemen¬ 
tary bodies which they require for their formation, are water, 
carbonic acid, ammonia. As the absorption of watery fluids 
has already been discussed, we now turn to the consideration 
of carbonic acid. This, it is well known, exists universally 
diffused in atmospheric air and in water; experiments prove 
that plants do not absorb the carbonic acid dissolved in water 
with the latter by means of its roots, but that their leaves 
possess in a high degree the faculty of absorbing carbonic 
acid, and of liberating oxygen. 
We owe the more accurate knowledge of this process to 
the admirable experiments of Saussure, Grischovv, and Boul- 
singault. When a leaf-shoot, with its lower end dipping 
in water containing carbonic acid, is enclosed in a glass 
globe, its leaves exhale more oxygen than when its lower end 
is dipped in common water. A leafy shoot still connected 
with the tree, enclosed in a glass globe, increases the oxygen 
