Oils, a term used to indicate a large class of compounds.  The more common animal and vegetable oils, which are the fatty or fixed oils, are all compounds of glycerine with fatty acids.  The term fats is usually given to the solid forms, and oils to the fluid.  The solid fats, however, become fluid when heated.  All these fats and oils are lighter than water and will not mix with it.  They penetrate paper or cloth, making it partly transparent, and leaving what is known as a grease-spot.  When pure and fresh they usually have little or no taste or smell; but, when exposed to the air, they become darker in color, have a disagreeable taste and smell, and are called rancid.  As examples of vegetable oils we have cottonseed-oil, linseed-oil, olive-oil, almond-oil and cocoa-butter.  Linseed is brought largely from Russia and India; Africa supplies palm-oil; India and the Pacific islands cocoanut-oil; while the best olive-oil is brought from Italy.  In animal oils the principal ones are butter, lard, tallow, neatsfoot-oil and sperm-oil.  Tallow is the fat of oxen and sheep, melted and purified.  Lard, obtained from the hog, is one of the great products of the United States, 60,000 tons yearly being sent to Great Britain alone.  Neatsfoot-oil is produced by boiling the feet of cattle.  Sperm-oil and other fish-oils are obtained from different varieties of fish and sea animals, as the whale, seal, cod, shark or herring.  The uses of the different oils are very numerous: as food and in the preparation of food, in soap-making, painting, machinery and in a thousand other ways they are of great importance.  There is a large class of substances known as the essential or volatile oils which resemble the fats somewhat in their properties, particularly in not mixing with water.  They make a grease-spot on paper which is not permanent.  These are quite varied in composition, are more or less volatile and have strong and characteristic odors.  Oils of turpentine, lemon and wintergreen are examples of a great number of these products which are largely used as solvents, for flavoring, in perfumery and in medicine.  For mineral oils see Petroleum.