4 Dr. Mac Culloch on certain products 



posing the fluids thus generated to a second heat, that so large a 

 portion of the petroleum is distilled unchanged. By causing it to 

 pass a second time in contact with heated iron, while in the state of 

 vapour, it may be resolved completely into inflammable gas and 

 charcoal, and the produce of gas be thus considerably increased. 

 This circumstance explains also the contradictory accounts given by 

 different persons of the relative products of distillation, as applied to 

 the various compound inflammables. To instance the case of cam- 

 phor, which according to the mode of managing the process, may 

 be caused to yield essential oil or inflammable gas or a mixture of 

 both in various proportions. 1 need scarcely point out the advan- 

 tages so obviously to be derived from this consideration in the oeco- 

 nomical process of procuring light from pit-coal, an operation at 

 present conducted with less skill than it demands. 



I distilled a portion of this tar in such a way as to obtain inflam- 

 mable air only, and took the gas in five portions. The first burnt 

 very faintly, the second rather better, the third and fourth portions 

 with a good white flame, and the fifth burnt feeble and blue. No 

 portion of it was equal in brilliancy of inflammation to the gas from 

 pit-coal. On examination, it was found to contain much carbonic 

 oxide, by which its nature, as far as it differs from the gas of coal, 

 is readily understood. The cause of this difference will be apparent 

 when the other circumstances in the constitution of this substance 

 have been detailed. I thought it superfluous to examine accurately 

 the nature of these gasses, but they probably consist of different 

 mixtures of carbonic oxide, with light and heavy hydrocarbonate 

 and olefiant gasses, if indeed, (as I much doubt) there be any real 

 boundary by which the composition of these three last gasses can be 

 defined. 



If the process of distillation which I have now described be 



