PLANT'S GROWTH AND DECAY 113 



light, and the hexane and heptane known as "petrol" in 

 England and "gasoline" in America, while higher members 

 of the series constitute the burning, lubricating, and fuel 

 oils which have played so important a part in the technical 

 world during the past fifty years. 



The Russian oils, on the other hand, contain hydrocar- 

 bons of a slightly different character, having as chief con- 

 stituents "naphthenes," a group which, although in many 

 properties similar to the saturated hydrocarbons, yet in 

 composition must be ranked with the unsaturated. So 

 laborious, however, is the separation of the hydrocarbons 

 present in these great natural distillates, that our knowl- 

 edge of their constituents is still far from perfect, and 

 recent researches upon the tars obtained at low tempera- 

 tures from coal show that they are characterized also by 

 the presence of the naphthene group. 



The valleys of the Alleghany, which gave so abundant 

 a supply of oil to Drake and the pioneers of the oil industry 

 in the early sixties, also yielded that great output of 

 natural gas which concentrated the manufacturing activity 

 of America to so large an extent in these districts. Al- 

 though such gas is found in small quantities in many parts 

 of the world, no output of the same magnitude has ever 

 been discovered. 



This gas, which is by far the most valuable of the gas- 

 eous fuels, is practically methane. 



Weight for weight natural gas is the most valuable of 

 all the fuels, having a calorific value of 12,008 calories 

 (21,615 British thermal units), and its history affords a 

 graphic object lesson of what within a hundred years will 

 be our condition with regard to coal supply, the actions, 

 however, having been concentrated into a period of not less 

 than fifty years. 



With the first discovery of natural gas waste of the gross- 

 est character took place, followed by a period in which, 

 the value of the gas having been realized, it was con- 

 sumed with the utmost prodigality, and no thought was 

 8 



