, sheep, and pigs, which is
consumed in maintaining their vital functions, has not been accurately
ascertained; probably, as in the case of man, it is strictly
proportionate to the animal's weight. We can determine the amount
of plastic food consumed by an animal during a given period: we can
ascertain the increase (if any) in the weight of its body; and finally,
we can weigh and analyse its egesta. With these data it is comparatively
easy to ascertain the quantity of food which produced the increase in
the animal's weight; but they do not enable us to determine the amount
expended in keeping it alive, because the egesta might be largely made
up of unappropriated food--organised matter which had done no work in
the animal body. When we come to know the precise quantity of nitrogen,
in a purely, or nearly pure, mineral form[14] excreted by an animal,
then we shall be in a position to estimate the proportion of its food
expended in sustaining the essential vital processes which continuously
go on in its body. But although we are in ignorance as to the precise
quantity of flesh-formers expended in keeping the animal alive, we know
pretty accurately the amount which is consumed in producing a given
weight of its flesh, or rather in causing a certain increase in its
weight. This knowledge is the result of numerous investigations, of
which by far the most valuable are those of Lawes and Gilbert. These
experimenters found that fattening pigs stored up about 7-1/2 per
cent. of the plastic materials of their food, whilst sheep accumulated
somewhat less than 5 per cent. That is, 92-1/2 out of every 100 lbs.
weight of the nitrogenous food of the pig, and 95 out of every 100 lbs.
of that of the sheep, are eliminated in the excretions of those animals.

It appears from the results of Lawes and Gilbert's experiments, that
pigs store up in their _increase_ about 20 per cent., sheep 12 per
cent., and oxen 8 per cent. of their (dry) food. The relative increase
of the fatty, nitrogenous, and mineral constituents whilst fattening,
are shown in this table.

---------------------+-------------------------------------------------
|Estimated per cent. in Increase whilst Fattening.
CASES. +--------+-----------+-----------+----------------
|Mineral |Nitrogenous| |
| matter |matter | Fat (dry).| Total dry
|(ash). |(dry).