Candles may be defined as rods of fatty or waxy materials surrounding a central wick, and designed for purposes of illumination. The simplest form of candle was the "rushlight," made by simply dipping the pith of rushes into ordinary bacon or other fat melted in an iron pot. The process of manufacture is now considerably more complicated, and varies for the different kinds of candles. The chief substances employed for the manufacture are tallow, stearin, paraffin, ozokerit, or wax. For tallow candles, fat is melted and either cast in moulds around the wick, or, as in the primitive method, formed by dipping the wick into the melted material. Fat consists of glycerin in combination with various fatty acids, as stearic, palmitic, etc., and it has been found that better candles are obtained if instead of the fat the acid itself is used. This is done by suitable chemical operations, and stearin and composite candles are so obtained. The paraffin is a mixture of hydrocarbons, and is obtained by distillation of bituminous shale, petroleum, and mineral oils. Ozokerit is found native in Bohemia and Galicia. Wax candles cannot, like the above, be manufactured by casting in moulds, as the wax shrinks on cooling. They are generally made either by squeezing through a cylindrical mould, or by pouring the melted wax on the wick, and then working into a cylindrical form on smooth wood or marble. Beeswax or Chinese wax bleached by exposure or by the action of chromic acid is generally used. The wick is usually made of cotton yarn. In the burning of a candle the upper portion of the wax or tallow melts and runs up the wick [Capillarity], and is there by the heat decomposed into combustible gases which burn round the wick.