in the Distillation of Wood, &c. 21 1 



distinguished from them. It seems evident, however, that no 

 very great change is wanting to convert the one of these into the 

 other. 



The qnestion so much agitated, of the conversion of vegetahles 

 into coal, would appear to receive some illustration from the his- 

 tory of the compound whicli I have been dc.>5cribing; and shice 

 (as I shall bv and by show) it has actually been confounded with 

 bitumen, and has been adduced as an instance of the artificial 

 production of coal by the action of fire, I shall make no apology 

 for pursuing this subject. Indeed the general chemical resem- 

 blance between the mineral bitumens and this vegetable bitu- 

 men, if it may be so called, is so striking, that we may at first 

 sight be easily led to suppose that the same agent has produced 

 both, and excuse the mistakes which seem to have occurred on 

 this subject. But a cursory view of the several substances which 

 have been classed under the head of bitumens, may enable us to 

 form a clearer notion of the limited extent of this analogy, at the 

 same time that it will perhaps assist us in correcting some errors 

 which have crept into our arrangements of them. 



It is necessary to separate from the bitumens three or four 

 mineral substances, which differ completelv both in chemical and 

 ordinary characters, but which are approximated to each other 

 by some general resemblance. These are, amber, mellilite, and 

 the subterraneous resins of Cologne, Bovey, and Highgate. The 

 two first are more nearly associated by the property they have of 

 yielding a peculiar acid ; and of the three last, it may perhaps 

 be fairly doubted, whether they are more entitled to be ranked 

 among the mineral substances strictly so called, than the other 

 vegetable matters which are found in alluvial soils. 



The nature and relations of naphtha, petroleum, maltha, and 

 asphaltum, will, I trust, appear suificienlly clear from what I have 

 above related ; Ijut I cannot forbear remarking on some false hy- 

 potheses which have been held resj)ecting these substances, and 

 their relation to other bodies. It is evident, from considering 

 the products of their decomposition, that the basis of napht.'ui 

 and of all the intermediate stages of bitumen, down to asphaltum, 

 are carbon and hydrogen, modified by certain small proportions 

 of oxygen and azote. It is in the relative proportions chiefly of 

 these two ingredients that naphtha differs from petroleum, pe- 

 troleum from maltha, and maltha from asphaltum. If we distil 

 either of these more soliil substances with a very gentle heat, we 

 obtain naphtha, in which the proportion of the hydrogen to the 

 carbon is increased to a maximum ratio. If the heat is greater, 

 we obtain a sulistance of a darker colour, in which that ratio i > 

 Icbs 5 and, for this reason, the distillation of asphaltum affords 

 2 a darker 



