88 THE PRODUCTION AND USE OF HEAT 



HYDROCARBONS are compounds of hydrogen and carbon. 

 Petroleum is a very complex mixture of hydrocarbons. Some 

 of the hydrocarbons of petroleum are gases at ordinary tem- 

 perature, some are liquids, and some are solids (Chap. I, 

 Sec. IV). The liquids predominate. By distillation, the 

 gaseous, liquid, and solid hydrocarbons are separated. Gaso- 

 line and kerosene are common liquid hydrocarbons, while the 

 paraffin of which the candle is made is a solid hydrocarbon. 

 There are not large amounts of uncombined hydrogen in 

 nature, but the compounds of hydrogen and carbon known 

 as hydrocarbons are abundant, and in these the hydrogen and 

 carbon are combustible. Acetylene is a gaseous hydrocarbon. 

 It is one of the few manufactured hydrocarbons. 



99. Summary of Section II. The Chemistry of Combus- 

 tion. (1) Fuels may be classified as solid, liquid, or gaseous. 

 (2) Carbon and hydrogen are fuel elements. (3) Carbon and 

 hydrogen are also chemical elements. Chemical elements 

 are substances that, so far, have defied all attempts on the 

 part of man to change them into simpler substances. There 

 are about 80 chemical elements. (4) "Energy is work and 

 everything that can arise from work and be converted into 

 work." Heat, light, electricity, kinetic energy, potential 

 energy, and chemical energy are different forms of energy 

 which may be converted, the one into another. During 

 such changes there is no gain nor loss of energy. Chemical 

 energy is the cause of the heat and the light produced when sub- 

 stances burn. (5) A chemical change is that kind of change 

 in matter in which one or more new substances are formed 

 and in which some kind of energy is produced or absorbed. 

 (6) Chemical compounds result from the union of two or 

 more chemical elements with the accompanying energy 

 change. (7) The proportion of the elements in a compound 

 is fixed. (8) Oxygen is the element commonly used to 

 liberate the chemical energy of fuels as heat or light or both. 

 (9) Oxygen constitutes more than Y$ of the air. (10) Oxygen 

 may be prepared from a number of compounds, potassium 



