192 THE WORLD'S LUMBER ROOM. 



of petroleum from New York and Philadelphia had con- 

 siderably exceeded 500,000,000 gallons a year. 



It is not improbable, however, that the petroleum of 

 North America may be in part of animal origin. American 

 oil has been exported to all parts of the world, and 

 until recently held almost exclusive possession of the 

 European markets ; but, now that the still vaster quantities 

 about Baku, on the Caspian, and throughout the whole 

 region north of the Caucasus are being explored and 

 systematically worked, the American oil seems likely to 

 be in a measure at least superseded. 



But whether we burn coal, rock-oil, gas, or any of the 

 various kinds of candles, our rooms are warmed and lighted 

 by the heat and light drawn from the sun by trees, ferns, 

 mosses, &c., ages ago. The carbon taken from the air is 

 sent back in the form of carbonic acid, and in one way or 

 another all that was drawn from the air is sent back to it to 

 feed new generations. 



There are a few other forms of vegetable refuse, how- 

 ever, which we must not quite pass over. 



At one time it was believed that the diamond, the 

 hardest of all known bodies, was simply a vegetable gum, for 

 it consists of pure carbon with a trace of ash, and its value 

 therefore arises, not from its composition, but from the form 

 in which it crystallises. Very minute but real diamonds 

 have been artificially obtained from a gas containing carbon 

 and hydrogen, the carbon being induced to crystallise by a 

 method which need not here be described These diamonds 

 are therefore of vegetable origin, but we are entirely ignorant 

 as to the way in which diamonds are formed in nature. 



