Oil n.  Any one of a great variety of unctuous combustible substances, more viscous than and not miscible with water; as, olive oil, whale oil, rock oil, etc.  They are of animal, vegetable, or mineral origin and of varied composition, and they are variously used for food, for solvents, for anointing, lubrication, illumination, etc.  By extension, any substance of an oily consistency; as, oil of vitriol.
 Note: ☞ The mineral oils are varieties of petroleum.  See Petroleum.  The vegetable oils are of two classes, essential oils (see under Essential), and natural oils which in general resemble the animal oils and fats.  Most of the natural oils and the animal oils and fats consist of ethereal salts of glycerin, with a large number of organic acids, principally stearic, oleic, and palmitic, forming respectively stearin, olein, and palmitin.  Stearin and palmitin prevail in the solid oils and fats, and olein in the liquid oils.  Mutton tallow, beef tallow, and lard are rich in stearin, human fat and palm oil in palmitin, and sperm and cod-liver oils in olein.  In making soaps, the acids leave the glycerin and unite with the soda or potash.
 Animal oil, Bone oil, Dipple's oil, etc. Old Chem., a complex oil obtained by the distillation of animal substances, as bones. See Bone oil, under Bone.
 Drying oils, Essential oils. Chem. See under Drying, and Essential.
 Ethereal oil of wine, Heavy oil of wine. Chem. See under Ethereal.
 Fixed oil. Chem. See under Fixed.
 Oil bag Zool., a bag, cyst, or gland in animals, containing oil.
 Oil beetle Zool., any beetle of the genus Meloe and allied genera.  When disturbed they emit from the joints of the legs a yellowish oily liquor.  Some species possess vesicating properties, and are used instead of cantharides.
 Oil box, or Oil cellar Mach., a fixed box or reservoir, for lubricating a bearing; esp., the box for oil beneath the journal of a railway-car axle.
 Oil cake. See under Cake.
 Oil cock, a stopcock connected with an oil cup. See Oil cup.
 Oil color. (a) A paint made by grinding a coloring substance in oil.  (b) Such paints, taken in a general sense. -- (b) a painting made from such a paint.
 Oil cup, a cup, or small receptacle, connected with a bearing as a lubricator, and usually provided with a wick, wire, or adjustable valve for regulating the delivery of oil.
 Oil engine, a gas engine worked with the explosive vapor of petroleum.
 Oil gas, inflammable gas procured from oil, and used for lighting streets, houses, etc.
 Oil gland. (a) Zool. A gland which secretes oil; especially in birds, the large gland at the base of the tail.  (b) Bot. A gland, in some plants, producing oil.
 Oil green, a pale yellowish green, like oil.
 Oil of brick, empyreumatic oil obtained by subjecting a brick soaked in oil to distillation at a high temperature, -- used by lapidaries as a vehicle for the emery by which stones and gems are sawn or cut. --Brande & C.
 Oil of talc, a nostrum made of calcined talc, and famous in the 17th century as a cosmetic. [Obs.] --B. Jonson.
 Oil of vitriol Chem., strong sulphuric acid; -- so called from its oily consistency and from its forming the vitriols or sulphates.
 Oil of wine, Œnanthic ether. See under Œnanthic.
 Oil painting. (a) The art of painting in oil colors. (b) Any kind of painting of which the pigments are originally ground in oil.
 Oil palm Bot., a palm tree whose fruit furnishes oil, esp. Elaeis Guineensis. See Elaeis.
 Oil sardine Zool., an East Indian herring (Clupea scombrina), valued for its oil.
 Oil shark Zool. (a) The liver shark. (b) The tope.
 Oil still, a still for hydrocarbons, esp. for petroleum.
 Oil test, a test for determining the temperature at which petroleum oils give off vapor which is liable to explode.
 Oil tree. Bot. (a) A plant of the genus Ricinus (Ricinus communis), from the seeds of which castor oil is obtained. (b) An Indian tree, the mahwa. See Mahwa. (c) The oil palm.
 To burn the midnight oil, to study or work late at night.
 Volatle oils. See Essential oils, under Essential.


 Vol·a·tile a.
 1. Passing through the air on wings, or by the buoyant force of the atmosphere; flying; having the power to fly.  [Obs.]
 2. Capable of wasting away, or of easily passing into the aeriform state; subject to evaporation.
 Note: ☞ Substances which affect the smell with pungent or fragrant odors, as musk, hartshorn, and essential oils, are called volatile substances, because they waste away on exposure to the atmosphere. Alcohol and ether are called volatile liquids for a similar reason, and because they easily pass into the state of vapor on the application of heat. On the contrary, gold is a fixed substance, because it does not suffer waste, even when exposed to the heat of a furnace; and oils are called fixed when they do not evaporate on simple exposure to the atmosphere.
 3. Fig.: Light-hearted; easily affected by circumstances; airy; lively; hence, changeable; fickle; as, a volatile temper.
    You are as giddy and volatile as ever.   --Swift.
 Volatile alkali. Old Chem. See under Alkali.
 Volatile liniment, a liniment composed of sweet oil and ammonia, so called from the readiness with which the latter evaporates.
 Volatile oils. Chem. See Essential oils, under Essential.


 Es·sen·tial a.
 1. Belonging to the essence, or that which makes an object, or class of objects, what it is.
    Majestic as the voice sometimes became, there was forever in it an essential character of plaintiveness.   --Hawthorne.
 2. Hence, really existing; existent.
 Is it true, that thou art but a name,
 And no essential thing?   --Webster (1623).
 3. Important in the highest degree; indispensable to the attainment of an object; indispensably necessary.
 Judgment's more essential to a general
 Than courage.   --Denham.
    How to live? -- that is the essential question for us.   --H. Spencer.
 4. Containing the essence or characteristic portion of a substance, as of a plant; highly rectified; pure; hence, unmixed; as, an essential oil. “Mine own essential horror.”
 5. Mus. Necessary; indispensable; -- said of those tones which constitute a chord, in distinction from ornamental or passing tones.
 6. Med. Idiopathic; independent of other diseases.
 Essential character Biol., the prominent characteristics which serve to distinguish one genus, species, etc., from another.
 Essential disease, Essential fever Med., one that is not dependent on another.
 Essential oils Chem., a class of volatile oils, extracted from plants, fruits, or flowers, having each its characteristic odor, and hot burning taste. They are used in essences, perfumery, etc., and include many varieties of compounds; as lemon oil is a terpene, oil of bitter almonds an aldehyde, oil of wintergreen an ethereal salt, etc.; -- called also volatile oils in distinction from the fixed or nonvolatile.