OIL I49 



sity of the light increases and the illumination remains good 

 as long as the candle lasts. The heat of the burning wick 

 melts the fatty substance near it, and this liquid fat is quickly 

 sucked up into the burning wick. The heat of the flaming 

 wick is sufficient to change this liquid into a gas ; that is, to 

 vaporize the liquid, and furthermore to set fire to the gas 

 thus formed. 



Small particles of carbon are likewise set free, and these, on 

 coming in contact with the oxygen of the surrounding air, glow 

 with an intense heat and add to the luminosity of the candle 

 flame. In order that the gases may burn and the solid par- 

 ticle glow, a plentiful supply of oxygen is necessary. If the 

 quantity of air is insufficient, the carbon particles remain un- 

 burned and form soot. A lamp "smokes" when the air which 

 reaches the wick is insufficient to burn to incandescence the 

 rapidly formed carbon particles ; this explains the danger of 

 turning a lamp wick toq^ high and producing more carbon 

 particles than can be oxidized by the air admitted through 

 the lamp chimney. 



143. Oil. The most widely used illuminating oils are kero- 

 sene and gasoline, both of which are obtained from crude pe- 

 troleum, a dark, oily liquid occurring in the earth. The chief 

 oil-producing regions of the United States are California, 

 Oklahoma, Illinois, West Virginia, Ohio, Texas, Pennsylvania, 

 and Louisiana. 



The crude petroleum as it is taken from the earth is a mix- 

 ture of hydrocarbons ; that is, compounds of hydrogen and 

 carbon in varying combinations. The hydrocarbons must not 

 be confounded by the pupil with carbohydrates, Section 61, 

 compounds in which carbon is combined wjth hydrogen and 

 oxygen in definite proportions. 



Crude petroleum is separated into its various constituents 

 by distillation, and for this purpose iron retorts connected with 



