26 Dr. Mac Culloch on certain products 



As It was no part of my design to examine the vegetable elements, 

 I did not pursue any experiments with this substance distilled in 

 earthen vessels so as to ascertain whether in this case also it would 

 contain iron, but I did enough to satisfy myself that the pitch was 

 essentially the same in which ever way produced. 



It is already known that a substance resembling plumbago is 

 formed in water, it having been discovered by Fabroni in the coun- 

 try round Naples. It is equally known to be formed in the iron 

 foundries, and the advocates for the igneous origin of coal have also 

 contended for that of plumbago, and have supposed it to have been 

 produced by the contact of melted greenstone with beds of coal. But 

 even if we admit this cause of its formation, something else seems 

 necessary for the production of the substance, and some other mode 

 of applying the heat required before it can be produced. Nor 

 indeed does the explanation sufficiently correspond with the general 

 geological position of plumbago. 



In numerous trials to combine Iron with charcoal so as to form 

 this substance, I have uniformly failed of success, except where as in 

 the case above related, the charcoal or carbon has been in a state of 

 previous combination, or was actually held in solution. In many 

 trials on this principle, the results have been tolerably successful. If 

 therefore we are to adopt an Igneous theory of the formation of 

 plumbago, it will be as easy to suppose that the action of subterra- 

 neous fire on mixtures of bitumen and iron has produced the com- 

 pound of charcoal and iron, on the principles I have described, and 

 this supposition will be more consonant to the chemical facts. But 

 we are too little acquainted with the geological relations of plum- 

 bago to lay much stress at present upon this or any other hypothesis. 

 It is evident that plumbago may be a produce of art, and could it be 

 produced in as solid and compact a state as Nature aflfords it, the 



