Whenever oils are mentioned, without a prefix, the fixed or fatty oils are always understood.  The volatile or essential oils are a distinct class.  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.

The Osseids comprise ossein, gelatin, cartilage, &c., from bone, skin, and connective issue.  They approach the proteids in composition, but unlike them they cannot form flesh or fulfil the same purpose in nutrition.  Some food chemists wish to call the osseids, albuminoids; what were formerly termed albuminoids to be always spoken of as proteids only.

Jellies are of little use as food; not only is this because of the low nutritive value of gelatin, but also on account of the small quantity which is mixed with a large proportion of water.

The Vegetable Kingdom is the prime source of all organic food; water, and to a slight extent salts, form the only food that animals can derive directly from the inorganic kingdom.  When man consumes animal food—­a sheep for example—­he is only consuming a portion of the food which that sheep obtained from grass, clover, turnips, &c.  All the proteids of the flesh once existed as proteids in the vegetables; some in exactly the same chemical form.