to the purposes they served in the
physical economy. Perhaps the most convenient, though not an
unexceptionable division, is into the Saccharine, Oleaginous,
Albuminous, and Gelatinous groups. The first includes those substances
analogous in composition to sugar, being chemically composed of
hydrogen, carbon, and oxygen. Such are starch, gum, cellulose, and so
forth, which are almost identical in their ultimate composition, and
admit of ready conversion into sugar by a simple process of vital
chemistry. The oleaginous group comprises all oily matters, which are
even purer hydro-carbons than the first-mentioned class. The third, or
albuminous group, includes all substances closely allied to albumen,
and hence containing a large proportion of nitrogen in addition to the
other three elements. The last group consists also of nitrogenized
substances, which resemble gelatine in many of their characteristics.
The first two groups are called non-azotized, as they contain no
nitrogen; the last two, azotized, containing nitrogen. "All articles of
food that are to be employed in the production of heat must contain a
larger proportion of hydrogen than is sufficient to form water with the
oxygen that they contain, and none are appropriate for the maintenance
of any tissues (except the adipose) unless they contain nitrogen."
Hence the obvious restriction of the first two classes to the
heat-producing function, and of the last two (or azotized) to the
reparation of the tissues.

We have, then, the two natural divisions of calorifacient and plastic
foods: the one adapted to sustain the heat of the body, and enable us
to maintain a temperature independent of that of the medium we may be
in; the other to build up, repair, and preserve in their natural
proportions the various tissues, as the muscular, fibrous, osseous, or
nervous, which compose our frames. These two kinds of food we must have
in due proportion and quantity in order to live. Neither the animal nor
the vegetable kingdom furnishes the one to the exclusion of the other.
We derive our supplies of each from both. More than this, we consume
and appropriate certain incidental elements, which find their place and
use in the healthy system. Iron floats in our blood, sulphur lies
hidden in the hair and nails, phosphorus scintillates unseen in the
brain, lime compacts our bones, and fluorine sets the enamelled edges
of our teeth. At least one-third of all the known chemical elements