206 On certain Products obtained 



difference will he 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 gases 5 but 

 they probably consist of different mixtures of carbonic oxide, 

 with light and heavy hydrocarbonate and defiant gases, if in- 

 deed (as I much doubt) there be any real boundary by which 

 the composition of these three last gases can be defined. 



If the process of distillation which I have now described he 

 stopped when the oily matter begins to acquire a brown co- 

 lour, and when the production of acetic acid is less perceptible, 

 the matter in the retort will be found, when cold, to have assumed 

 a solid consistence. In this state it resembles either pitch or 

 asphaltum, according to the degree of heat it has undergone after 

 it became capable of solidifying. 



I will describe this substance as it appears when it first be- 

 comes solid, the reason of which will soon be apparent. 



Previously to its arriving at this state, it bears a consideralile 

 resemblance to maltha, being of a consistence intermediate be- 

 tween that of petroleum and asphaltum: but I did not completely 

 examine its chemical properties in this condition, because they 

 appeared not to differ from what might be expected, and its 

 history will be sufficiently full without it. In the solid state it 

 is brilliant and shining, and breaks with a conchoidal fracture 

 and som.e external resemblance to obsidian. It has a pungent 

 burning taste, and the well known smell of wood smoke. It is 

 heavier than the specimen of asphaltum with which I compared 

 it, having a specific gravity of 1*254, while that of the asphaltum 

 was 1"202. It is fusible and readily inflammable, burning with 

 a white flame. It is electric, and exhibits the same electricity as 

 the resinous bodies. When heated in an open vessel, it smokes; 

 and if kept in fusion till it ceases to smoke, it at length ceases to 

 be fusible, and is ultimately converted into a coal. During this 

 progress it becomes more brilliant and less fusible, its fracture 

 also from conciioidal becomes more splintery, and it puts on the 

 appearance of asphaltum so accurately that the eye cannot de- 

 tect the difference. Its specific gravity also diminishes, and its 

 chemical properties vary in the way I am now about to detail. 



1 have described the perfect solubility of the tar in alcohol. 

 The softest specimens of the pitch are nearly as soluble, leaving 

 only a small residuum, which is infusible and powdery. The 

 harder specimens become in proportion less soluble, and leave a 

 larger residuum ; and those which have been the longest ex- 

 posed to heat scarcely give a stain to the alcohol, resembling in 

 this respect th^ driest specimens of asphaltum. The analogy is 

 here very apparent ; for asphaltum may approach more or less to 

 petroleum, and the various specimens of it are found to exhibit 



various 



