Heat 



89 



apart to break away and form a gas. In order to do this, they 

 must overcome the air pressure. In high altitudes, the boiling 

 temperature of liquids is lower than at sea level as the air pres- 

 sure is less, and so the molecules do not have to move so fast 

 to break away from the liquid and go off as a gas. 



Fuels. There are, perhaps, no more important industries than 

 those which provide fuels for households and industrial plants. 



Brown Brothers. 



FROM SUCH FORESTS COAL WAS FORMED 



Fuels are usually classified as solid, liquid, and gaseous. Wood, 

 cod, peat, charcoal and coke are solid fuels. Gasoline, naphtha, 

 kerosene, alcohol, and fuel oil are liquid fuels. Coal gas, water 

 gas, gasoline gas and acetylene gas are gaseous fuels. All of these 

 are composed largely of two elements, carbon and hydrogen, upon 

 which depends their usefulness as heat producers. 



It is thought that wood was the first fuel used by man. Wood 

 is composed chiefly of water, starch, gum oil, cellulose, and resin, 



