The essential oils of different plants differ much in their specific gravity, and also in the manner by which they are affected by cold, some being heavier and others lighter than water, and some being more difficultly, and others more easily, congealed. Though the differences with respect to _weight_ and _consistency_ in these oils is probably owing to the state of the acid that is combined with them, these two properties are wholly independent of each other; some essential oils being very thin and yet heavy, and others thick and yet light. Essential oils are used in perfumes, and also in medicine, acting powerfully the nervous system. Essential oils are very apt to be adulterated. If it be with sweet oil, it may be discovered by evaporation on white paper, or by a solution in spirit of wine, which will not act upon the sweet oil. If spirit of wine be mixed with it, it will be discovered by a milky appearance upon putting water to it, which uniting with the spirit, will leave the oil much divided. If oil of turpentine, which is the cheapest of essential oils, be mixed with any of the more valuable kinds, it will be discovered by evaporation; a strong smell of turpentine being left on the paper, or cloth, upon which the evaporation was made. Animal oil, like the vegetable, is of two kinds; the first _butter_, or _fat_, which is easily congealed, owing to the quantity of acid that is intimately combined with it. It resembles the sweet oil of vegetables in having no smell or taste. The other kind of animal oil is extracted by distillation from the flesh, the tendons, the bones, and horns, &c. of animals. It differs essentially from the other kind of animal oil, by containing an alkali instead of an acid. By repeated distillation it becomes highly attenuated and volatile; and in this state it is called the _oil of Dippel_, the discoverer of it. All oil exposed to much heat is in part decomposed, and acquires a disagreeable smell; and in this state it is said to be _empyreumatic_: but this property is lost by repeated distillations. Besides the vegetable and animal oils above described, there is a fossil oil called _bitumen_, the several kinds of which differ much in colour and consistence; the most liquid is called _petroleum_, from being found in the cavities of rocks, and the more solid kinds are _amber_, _jet_, _asphaltum_, and _pit-coal_. When distilled, the principal component parts of all these substances are an oil and an acid. But all fossil oil is probably of vegetable or animal origin, from masses of vegetables or animals long buried in the earth. Their differences from resins and other oily matters are probably owing to _time_; the combinations of mineral acids and oils so nearly resembling bitumens, the principal difference being their insolubility in spirit of wine.