12 Dr. Mac Culloch on certain products 



It seems evident however, that no very great change is wanting to 

 convert the one of these Into the other. 



The question so much agitated, of the conversion of vegetables 

 Into coal, would appear to receive some Illustration from the history 

 of the compound which I have been describing, and since (as I shall 

 by 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 resemblance between the mi- 

 neral ])Itumens and this vegetable bitumen. 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 bitu- 

 mens, 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 completely both in chemical and ordinary 

 characters, but which are approximated to each other by some 

 general resemblance. These are, amber, mellilite, and the subter- 

 raneous 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 sufficiently clear from what I have 



