FUEL FOR GAS TRACTORS 117 



The weight of a given volume of these successive groups 

 increases in a ratio roughly corresponding to their chemical 

 composition. To obtain liquids which should each contain 

 but a single chemical compound, hence be homegeneous in 

 composition, the temperature of distillation for each fraction 

 of the oil would have to be based on the boiling point of one 

 compound. Even then it would be possible to obtain a pure 

 chemical product only after several further refining processes, 

 which would make the cost prohibitive. The very best com- 

 mercial oils then, from the standpoint of uniformity, are the 

 mixtures of several substances very nearly alike in composition 

 and evaporating at nearly the same temperatures. 



All petroleum distillates are composed of varying propor- 

 tions of carbon and hydrogen and on that account are known as 

 hydrocarbons. In the process of combustion they unite readily 

 with oxygen, usually forming water and carbon dioxide, or 

 carbonic acid gas. The lowest unit in which the two elements 

 are chemically combined is known as the molecule, and the 

 lightest molecule found in the Pennsylvania oils is that of 

 marsh gas, or methane. It contains one atom of carbon and 

 four of hydrogen, represented by the formula CH 4 . The bulk 

 of the more complex compounds occur in a practically regular 

 series known as the paraffin series. Each contains one more 

 atom of carbon and two of hydrogen than the one next below. 

 Thus we have C 2 H 6 ,C 5 H 12 , etc., as formulas for higher com- 

 pounds. The latter is the lowest which is normally liquid, 

 being one of the lightest constituents of gasoline. The carbon 

 atom is heavier than the hydrogen atom in the ratio of 12 to 1. 

 The marsh gas molecule, therefore, is three fourths carbon. 



Every reader knows that hot air will sustain a greater 

 weight of vapor than cold air. A given volume of gas contains 

 exactly the same number of molecules, no matter what kind it 

 is; hence a cylinderful of kerosene vapor will weigh more than 

 an equal volume of vapor from gasoline. From this fact arises 

 the necessity for a higher temperature within the cylinder to 



