212 071 cer lain Products olta'ined 



a daiker oil than that of petroleuni, because ito composition 

 cannot be dissolved but in a higher temperature. 



For tlie same reason also petroleum is easily rectified into 

 naphtha. Asphaltum, in its ordinary state, contains the two in- 

 gredients in a ratio in which the carbon bears a large proportion 

 to the hydrogen, and that ratio is reduced to the minimum, or 

 becou'iCS ev.-mescent, when by the continuance of, distillation 

 charcoal alone remains behind. A large portion of the oxygen, 

 and also of the azote, is disengaged during this process, but not 

 the whole, since the darker compounds still give it over on re- 

 peating the process. The naphtha is probably entirely exempt 

 from oxygen. With this view, we cannot accede to the notion 

 that the absorption of oxygen is capable of converting naphtha 

 or petroleum into asphaltum; or that the harder bitumens ori- 

 ginate from the oxygenation of tlie more liquid. It is more con- 

 sonant to the nature of these substances to suppose that the 

 change consists in the alteration of the relative proportions of 

 the hydrogen and carbon ; but whether this is performed by the 

 action of heat, or of other causes volatilizing the livdrogen^ or 

 by the contact of oxygen converting it into water, cannot now 

 be determined by any facts that we are acquainted with. Ex- 

 periments on the induration of the essential oils niav throw some 

 light on this question. It will here perhaps be remarked, that 

 there is a difference in the substances as they are produced artifi- 

 cially by the distillation of coal, and as they are found in nature. 

 Thus, for example, the artificial petroleum of coal differs from 

 that of nature, in being much more soluble in a.lcohol. Yet this 

 circumstance may arise from the insensible gradation of dif- 

 ference which I have tbove remarked in the similar compounds; 

 and thus, in the series of gradation, sj)ecimens absolutely corre- 

 sponding, whether artificial or natural, may exhibit the same 

 chemical characters. 



Thus, as I have shown that there is a sort of gradation from 

 naphtha to asphaltum, through a series of uudefinable petrolea, 

 30 this analogy may be extended to the next general variety o; 

 the biiuuiens, coal. 



The several varieties of coal are supposed to consist of char- 

 coal and asphaltum, or of charcoal and bitumen, combined in as 

 many different proportions. Charcoal is undoubtedly found 

 mixed with coal, but it does not appear correct to consider pi)f ' 

 coal as either a mixture or combination of any bitumen with 

 charcoal. The action of naphtha on its varieties, often none and 

 always sparing, shows that bitumen does not exist in it in a mixed 

 slate. It will be more consonant to the analogies of the other 

 bituminous substances, to consider coal in its several varieties as 



a bitumcii. 



