divided into two classes, _i.
e._, that portion which contains nitrogen--such as gluten, albumen,
etc., and that which does not contain nitrogen--such as starch, sugar,
oil, etc.

The inorganic part of food may also be divided into _soluble_ matter and
_insoluble_ matter.

DIGESTION AND ITS PRODUCTS.

[Of what does that part of dung consist which resembles soot?

What else does the dung contain?

In what manner does the digested part of food escape from the body?]

Let us now suppose that we have a full-grown ox, which is not increasing
in any of his parts, but only consumes food to keep up his respiration,
and to supply the natural wastes of his body. To this ox we will feed a
ton of hay which contains organic matter, with and without nitrogen, and
soluble and insoluble inorganic substances. Now let us try to follow it
through its changes in the animal, and observe its destination. Liebig
compares the consumption of food by animals to the imperfect burning of
wood in a stove, where a portion of the fuel is resolved into gases and
ashes (that is, it is completely burned), and another portion, which is
not thoroughly burned, passes off as _soot_. In the animal action in
question, the food undergoes changes which are similar to this burning
of wood. A part of the food is _digested_ and taken up by the blood,
while another portion remains undigested, and passes the bowels as solid
dung--corresponding to soot. This part of the dung then, we see is
merely so much of the food as passes through the system without being
materially changed. Its nature is easily understood. It contains organic
and inorganic matter in nearly the same condition as they existed in the
hay. They have been rendered finer and softer, but their chemical
character is not materially altered. The dung also contains small
quantities of nitrogenous matter, which _leaked out_, as it were, from
the stomach and intestines. The digested food, however, undergoes
further changes which affect its character, and it escapes from the body
in three ways--_i. e._, through the lungs, through the bladder, and
through the bowels. It will be recollected from the first section of
this book, p. 22, that the carbon in the blood of animals, unites with
the oxygen of the air drawn into the lungs, and is thrown off in the
breath as carbonic acid. The hydrogen and oxygen unite to form a part of
the water which constitutes the moisture of the breath.

[Explain the escape