16 Dr. MacCulloch on certain products 



for the differences which characterize the several bitumens, from 

 naphtha placed at one extreme, to anthracite placed at the other. 

 The chasm in this series from asphaltum to fat coal, is in fact, rather 

 apparent than real, being more properly a mechanical or accidental, 

 than a chemical or essential one. I cannot here avoid taking notice 

 of the very loose experiments of Mr. Kirwan on the analysis of coal, 

 (which consisted in projecting portions of coal on melted nitre) as 

 his deductions are at war with this view of the subject, although not 

 more so than with all chemical reasoning. They were founded on 

 an assumption, that coal was carbon impregnated sometimes with 

 maltha and sometimes with asphaltum — a distinction quite unneces- 

 sary if the supposition were true. If we conceive coal to be com- 

 pounded in this way, it would be more obvious to consider it as 

 formed of carbon and petroleum, since by a regulated heat it can be 

 separated into those two substances. The theory of the experiment 

 is equally assumed and the conclusions equally groundless, when it 

 is inferred that of this compound, (coal) the carbon alone possesses 

 the power of decomposing the nitre, and that the proportions of these 

 supposed ingredients may thus be determined. The varying tem- 

 perature of the nitre, would necessarily produce considerable varia- 

 tions and uncertainty in its action, and in the consequent accuracy of 

 the results ; but it is plain, that the effect of this contrivance was to 

 separate by a sort of distillation the petroleum which fire elicits from 

 coal, and that the method could neither be so accurate as that of 

 ordinary distillation, from the greater irregularities to which it was 

 subject and the difficulty of conducting it, and that it proves nothing 

 with regard to the composition or nature of coal. In the examina- 

 tion of maltha, and asphaltum, the defect of this method is still more 

 apparent. If heat and flame be applied to these bitumens, with 

 access of air, they are either consumed without leaving any carbon, 



