lastic_. The plastic materials are
composed of carbon, hydrogen, oxygen, nitrogen, and a little sulphur
and phosphorus. Albumen, fibrine, and casein are plastic elements of
nutrition; they form the lean flesh, or muscles, the membranes, and
cartilages, the gelatine of the bones, the skin, the hair, and, in
short, every part of the body which contains nitrogen. The _non-plastic_
elements of nutrition include fat, oil, starch, sugar, gum, and certain
constituents of fruits, such as pectine.

All non-plastic substances--and of each kind there are numerous
varieties--are capable of conversion, in the animal mechanism, into fat
and oil. The non-plastic food substances do not contain nitrogen, hence
they are commonly termed non-nitrogenous elements. The oily and fatty
matters contain a large proportion of carbon, their next most abundant
component is hydrogen, and they contain but little oxygen. Unlike the
plastic elements, they are--except the fats of the brain and nervous
tissue--altogether destitute of sulphur and phosphorus. The starchy,
saccharine, and gummy substances are composed of the same elements as
the fatty bodies, but they contain a higher proportion of oxygen.
According to Liebig, fat is used in the animal economy as a source of
internal heat. We all know that it is a most combustible body, and that
during its inflammation the most intense heat is developed. It is less
evident, but not less true, that heat is evolved during its slow
oxidation, or decay.

The more rapidly a body burns, the greater is the amount of heat evolved
by it in a _given time_; but the total amount of heat developed by a
specific weight of the body is the same, whether the combustion takes
place rapidly or slowly. An experiment performed with phosphorus
illustrates the case perfectly. If we burned two pieces of equal weight,
the one in oxygen, the other in atmospheric air, we should find that the
former would emit a light five times as brilliant as that evolved by the
latter, for the simple reason that its combustion would be five times as
rapid. The white, vapor-like matter into which phosphorus is converted
by its combustion, is termed _phosphoric acid_. It is composed of
phosphorus and oxygen. In forming an ounce of this compound, by the
direct oxidation, or combustion of phosphorus, the amount of force,
either as heat, or as heat and light, evolved is precisely the same,
whether the time expended in the process be a minute or a month