14 Dr. Mac Culloch on certain product s^ 



performed by the action of heat, or of other causes volatiUzlng the 

 hydrogen, or by the contact of oxygen converting it into water, 

 cannot now be determined by any facts that we are acquainted with. 

 Experiments on the induration of the essential oils may throw some 

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

 is a difference in the substances as they are produced artificially 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 alcohol. Yet, this circumstance may 

 arise from the insensible gradation of difference which I have above 

 remarked in the similar compounds, and thus in the series of grada- 

 tion, specimens absolutely corresponding, 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 undefinable petrolea, so 

 this analogy may be extended to the next general variety of the 

 bitumens, coal. 



The several varieties of coal are supposed to consist of charcoal 

 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 pit 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 state. It will be 

 more consonant to the analogies of the other bituminous substances, 

 to consider coal in its several varieties as a bitumen, varying in its 

 composition, from the fattest specimens of Newcastle to the driest 

 of Kilkenny, and owing its compactness, as well as the other modi- 

 fications which it exhibits, to the peculiar circumstances under which 

 it has been formed, the chatiges it may subsequently have under- 



