172 Elements of Soil. 
constitution is such that it is ready to combine with all other 
bodies; and, in the act of" combining, it gives rise to one general 
phenomenon, termed combustion: the only difference which be- 
longs to specific cases, is the rapidity of combination, the end or 
result being exactly the same. Thus oxygen combines with iron, 
and forms the black or red powder frequently called the rust of 
iron. If the combination goes on under the ordinary states of 
the air, it is an invisible action; but after a few days, the surface 
is red, and the oxide is formed, consisting only of oxygen and 
iron. If, however, we contrive some means by which a rapid 
combination takes place, it is then accompanied with all the ordi- 
nary phenomena of combustion, the emission of heat and light; 
but here it is an oxide which is formed, and nothing else, and the 
difference of the two cases is one of time only; for, undoubtedly, 
just as much light and heat are produced in one case as in the 
other; just as much ice might have been melted by the slow com- 
bustion, or as much light emitted, as by the rapid one. So in all 
other cases there is a combination of oxygen with some other sub- 
stance; as when wood burns, light and heat are attendant pheno- 
mena, the combination proceeding with such rapidity as to render 
itself both visible and palpable; but if the wood combines slowly 
with oxygen, as is the case when it rots, then time is required to 
make us sensible of the change, and yet the final result is but the 
reduction of the wood to the condition of an oxide, as in the pre- 
ceding case. The compounds which form in these and all other 
combinations, are called oxides, or acids, of the properties of 
w^hich we will not now speak, but refer the reader to books of 
elementary chemistry. 
Oxygen is the controlling element of both organic and inor- 
ganic matter. Few substances are known which are destitute of 
it; and even if the number were greater than it is, this would 
hardly affect the truth of the proposition. Its range of affinity is 
such, and so wide, that all the other elements are usually found in 
combination with it. Few functions in vegetable or animal life 
are performed without its agency. The leaves of the forest trees 
are spread out to exhale it, and the roots fill the soil to suck up 
fluids which contain it. The lungs of animals expand to absorb 
it, and vitalize the currents of blood; every organ, every tissue 
feels its stimulus. Every thing in nature is formed with reference 
to it. The tiny insect and the feeble worm are subjected to its 
action. Every living being breathes it; and though of the cold- 
blooded class, no animal can subsist without a certain quantity to 
which its nature is adjusted; diminish that quantity, and the ani- 
mal languishes and dies; increase it, and the animal dies from a 
too rapid combustion of its organs. Perfectly organized bodies 
cannot withstand the effects of oxygen, if made to inhale it in a 
