origin) may be distilled or simply evaporated away like water or alcohol, and leave no residue. The fixed oils similarly treated are dissociated more or less completely.
Otherwise expressed, the boiling-point of the volatile oils is below their dissociation-point. The fixed oils are those which are dissociated at a temperature below their boiling-point.
My object in thus expressing this difference will be understood upon a little reflection. These volatile oils, when heated, being distilled without change are uncookable; while the fixed oils if similarly heated suffer various degrees of change as their temperature is raised, and may be completely decomposed by steady application of heat in a closed vessel without the aid of any other chemical agent than the heat itself. This "destructive distillation" converts them into solid carbon and hydrocarbon gases, similar to those we obtain by the destructive distillation of coal.
If we watch the changes occurring as the heat advances to this complete dissociation-point, we may observe a gradation of minor or partial dissociation proceeding gradually onward, resembling that which I have already described as occurring when sugar is similarly treated (see No. XIII of this series).
But in ordinary cooking we do not go so far as to carbonize the fat itself, though we do brown or partially carbonize the membrane which envelops the fat. What, then, is the nature of this minor dissociation, if such occurs?
Before giving my answer to this question, I must explain the chemical constitution of fat. It is a compound of a very weak base with very weak acids. The basic substance is glycerine, the acids (not sour at all, but so named because they combine with bases as the actually sour acids do) are stearic acid, palmitic acid, oleic acid, etc., and bear the general name of fatty acids. They are solid or liquid, according to temperature. When solid, they are pearly, crystalline substances; when fused, they are oily liquids.
To simplify, I will take one of these as a type, and that the one which is the chief constituent of animal fats, viz., stearic acid. I have a lump of it before me. Newly broken through, it might at a distance be mistaken for a piece of Carrara marble. It is granular like the marble, but not so hard, and, when rubbed with the hand, differs from the marble in betraying its origin by a small degree of unctuousness, but can scarcely be described as greasy.
I find by experiment that this may be mixed with glycerine without combination taking place; that when heated with glycerine just to its fusing-point, and the two are agitated together, the combination is by no means complete. Instead of obtaining a soft, smooth fat, I obtain a granular fat, small stearic crystals with glycerine among them. It is a mixture of stearic acid and glycerine, not a chemical compound; it is stearic acid and glycerine, but not a stearate of glycerine.