and
effects of animal exuviae, and vegetable matters or manures--properly
so called.

In order to understand the nature of these, and the peculiarity of
their influence upon our fields, it is highly important to keep in
mind the source whence they are derived.

It is generally known, that if we deprive an animal of food, the
weight of its body diminishes during every moment of its existence.
If this abstinence is continued for some time, the diminution
becomes apparent to the eye; all the fat of the body disappears, the
muscles decrease in firmness and bulk, and, if the animal is allowed
to die starved, scarcely anything but skin, tendon, and bones,
remain. This emaciation which occurs in a body otherwise healthy,
demonstrates to us, that during the life of an animal every part of
its living substance is undergoing a perpetual change; all its
component parts, assuming the form of lifeless compounds, are thrown
off by the skin, lungs, and urinary system, altered more or less by
the secretory organs. This change in the living body is intimately
connected with the process of respiration; it is, in truth,
occasioned by the oxygen of the atmosphere in breathing, which
combines with all the various matters within the body. At every
inspiration a quantity of oxygen passes into the blood in the lungs,
and unites with its elements; but although the weight of the oxygen
thus daily entering into the body amounts to 32 or more ounces, yet
the weight of the body is not thereby increased. Exactly as much
oxygen as is imbibed in inspiration passes off in expiration, in the
form of carbonic acid and water; so that with every breath the
amount of carbon and hydrogen in the body is diminished. But the
emaciation--the loss of weight by starvation--does not simply depend
upon the separation of the carbon and hydrogen; but all the other
substances which are in combination with these elements in the
living tissues pass off in the secretions. The nitrogen undergoes a
change, and is thrown out of the system by the kidneys. Their
secretion, the urine, contains not only a compound rich in nitrogen,
namely urea, but the sulphur of the tissues in the form of a
sulphate, all the soluble salts of the blood and animal fluids,
common salt, the phosphates, soda and potash. The carbon and
hydrogen of the blood, of the muscular fibre, and of all the animal
tissues which can undergo change, return into the atmosphere. The
nitrogen, and all the solubl