Occasionally, the fixed oils are called hydrocarbons, but hydrocarbon
oils are quite different and consist of carbon and hydrogen alone. Of
these, petroleum is incapable of digestion, whilst others are poisonous.

Vegetable Acids are composed of the same three elements and undergo
combustion into the same compounds as the carbohydrates. They rouse the
appetite, stimulate digestion, and finally form carbonates in combination
with the alkalies, thus increasing the alkalinity of the blood. The chief
vegetable acids are: malic acid, in the apple, pear, cherry, &c.; citric
acid, in the lemon, lime, orange, gooseberry, cranberry, strawberry,
raspberry, &c.; tartaric acid, in the grape, pineapple, &c.

Some place these under Class III. or food adjuncts. Oxalic acid (except
when in the insoluble state of calcium oxalate), and several other acids
are poisonous.

Proteids or Albuminoids are frequently termed flesh-formers. They are
composed of nitrogen, carbon, hydrogen, oxygen, and a small quantity of
sulphur, and are extremely complex bodies. Their chief function is to form
flesh in the body; but without previously forming it, they may be
transformed into fat or merely give rise to heat. They form the essential
part of every living cell.

Proteids are excreted from the body as water, carbon dioxide, urea, uric
acid, sulphates, &c.

The principal proteids of animal origin have their corresponding proteids
in the vegetable kingdom. Some kinds, whether of animal or vegetable
origin, are more easily digested than others. They have the same
physiological value from whichever kingdom they are derived.