502 
On the Food of Plants. 
been brought from a distance and scattered about by the violent, 
although apparently transient, action of water : it Is usual to apply 
to these the general name of " alluvium." We have no difficulty 
in tracing to this disturbance of the surface the origin of the nu- 
merous mixed soils, whose fertility and value so far exceed those 
of the purer kinds. We owe also to the decomposition of some 
varieties of lava, mixed soils of extraordinary productiveness, aris- 
ing In a great measure from the quantity of alkaline salt they 
contain; the neighbourhood of Naples and manv other localities 
will furnish examples. It is not impossible that some of the 
effect produced is to be attributed to a remnant of the original 
heat in the interior, as such a mass of badly-conducting matter 
will cool with extreme slowness when the temperature to some 
depth approaches that of the air. 
Every soil in which plants have once grown contains more or 
less of a blackish or brown substance, called " humus," or vege- 
table mould. This substance certainly plays an important part in 
furnishing food to plants, although perhaps not exactly that which 
has usually been assigned to it. 
Liebig has taken great pains to show that this humus is a sub- 
stance of very uncertain composition ; it is In short nothing more 
than the woody fibre of previously existing plants in a state of slow 
combustion by the oxygen of the air, which continually finds its 
way through the porous soil, slowly and constantly giving off car- 
bonic acid and water, until at length nothing is left but a little 
black coaly matter, almost incapable of further change. 
This humus, a substance thus Incessantly changing In composi- 
tion as its decay advances, has been confounded with certain pro- 
ducts of the action of alkalis and acids on sugar and woody fibre, 
which resemble it somewhat in physical characters, but which 
seem to be really definite substances, consisting of carbon and the 
elements of water. It is pretty clear, however, that the bodies in 
question belong to a different class, and have no immediate con- 
nexion with the organic matter of the soil. 
The humus of the soil is not sensibly soluble In cold water; of 
this Liebig has adduced sufficient proof. Boiling water takes up 
a minute quantity and acquires a yellowish tint, but it Is very 
possible that in this case some chemical change has occurred by 
the action of the boiling liquid. When heated with dilute solution 
of alkali it is readily acted on^ furnishing a deep brown liquid 
from which acids precipitate a brown flocculent matter, soluble 
in about 2500 parts of water, to which the name " humic" or 
" ulmic acid " has been given. It must be distinctly understood 
that we have no reason to think that anything like humic acid 
ever exists in ordinary soil ; it is only formed by the action of 
alkalis on the decomposing vegetable matter. 
