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term commonly used to indicate a variety of greasy, fluid substances that are, in general, viscous liquids at ordinary temperatures, less dense than water, insoluble in water but soluble in alcohol and ether, and flammable. These substances, however, differ so much among themselves in chemical composition that, in chemistry, their classification in one group is not practical and is employed only in a general way in accordance with popular usage. Petroleumpetroleum,
oily, flammable liquid that occurs naturally in deposits, usually beneath the surface of the earth; it is also called crude oil. It consists principally of a mixture of hydrocarbons, with traces of various nitrogenous and sulfurous compounds.
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 and substances obtained from it, which are mixtures of hydrocarbonshydrocarbon
, any organic compound composed solely of the elements hydrogen and carbon. The hydrocarbons differ both in the total number of carbon and hydrogen atoms in their molecules and in the proportion of hydrogen to carbon.
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, are classed together, because of their origin, as mineral oils. They are widely used as fuels, illuminants, and lubricants. Distinguished from these in that they are obtained from animals and plants and are mixtures of carbon-hydrogen-oxygen compounds are the fatty oils or fixed oils. There is fundamentally no difference between fatty oils and fats (see fats and oilsfats and oils,
group of organic substances that form an important part of the diet and also are useful in many industries. The fats are usually solid, the oils generally liquid at ordinary room temperatures.
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). Such oils are used extensively as lubricants and in the making of soapsoap,
a cleansing agent. It cleanses by lowering the surface tension of water, by emulsifying grease, and by absorbing dirt into the foam.

Ancient peoples are believed to have employed wood ashes and water for washing and to have relieved the resulting irritation with
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. Depending upon their ability to oxidize when exposed to the atmosphere and form a thin, skinlike layer over substances upon which they are spread, the fixed, or fatty, oils are classed as drying or nondrying oils. The drying oils, e.g., linseed, hempseed, and poppy seed oil, are used in making paints and varnishes. On the other hand, such vegetable oils as olive, rapeseed, and castor oil and such animal oils as lard oil and neat's-foot oil do not possess this property and fall into the nondrying group. Another large and varied group of oils is recognized, the essential oilsessential oils,
volatile oils that occur in plants and in general give to the plants their characteristic odors, flavors, or other such properties. Essential oils are found in various parts of the plant body (in the seeds, flowers, bark, or leaves) and are also concentrated in
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 or volatile oils, which occur in plants but differ from the fixed, or fatty, oils in that they are volatile. In general, they give to the plant in which they are found its characteristic odor, flavor, or other properties peculiar to it.
References in classic literature ?
I caught a drop of oil that fell from your knee-joint," confessed Ojo.
I'm afraid I shall have to scold the fellow, for I can't be dropping oil wherever I go.
And that oil oozed up and ruined the sunset; and as for the moonbeams, they positively reeked of paraffine.
At the end of that trip we met together at midnight in a lonely field, under a blasted oak, and took an awful oath (we had been swearing for a whole week about the thing in an ordinary, middle-class way, but this was a swell affair) - an awful oath never to take paraffine oil with us in a boat again-except, of course, in case of sickness.
Next the Wizard poured a pool of oil from the can upon the glass floor, where it covered quite a broad surface.
We cannot forbear giving the following remarkable page from the history of the times, to show how the need for reinvestment of the Standard Oil surplus crushed out small capitalists and hastened the breakdown of the capitalist system.
He had reached the limit of profitable investment of profits in the oil industry.
Of course I make more money with the Standard Oil Company and I'm only telling you," he added.
After they had washed them and got them quite clean, they laid them out by the sea side, where the waves had raised a high beach of shingle, and set about washing themselves and anointing themselves with olive oil.
But Ulysses said, "Young women, please to stand a little on one side that I may wash the brine from my shoulders and anoint myself with oil, for it is long enough since my skin has had a drop of oil upon it.
He rushed downstairs and sent somebody--they said the furnace man or somebody in the basement--out to a drug store for some oil and things to bind it up with.
You must go to New Bedford to see a brilliant wedding; for, they say, they have reservoirs of oil in every house, and every night recklessly burn their lengths in spermaceti candles.