10 Dr. MacCulloch on certain products 



resin and the wood pitch which I have been describing. To this 

 admixture, and not to that of adventitious charcoal produced in 

 combustion, is the black colour of common pitch owing. 



The analogy between this wood pitch and the bitumens is 

 equally striking, and the preceding history of these compounds will 

 throw light on the several varieties of the bituminous substances. 



Assuming the tar as the medium form, it is seen that when ex- 

 posed to heat it gives over oil, and that pitch remains. Thus, 

 petroleum yields naphtha and asphaltum ; and thus too, asphaltum 

 exhibits all the gradations which I have described in the pitch, its 

 properties varying in a similar manner, according to its particular 

 state. In the process of distillation the principal difference will be 

 found to consist in the relative quantities of acetic acid and ammonia, 

 which they severally yield ; the former chiefly characterizing the 

 wood tar, and the latter the petroleum. From the same chemical 

 cause which produces this effect arises also the difference in the 

 nature of the inflammable gasses which are produced from these 

 different substances. 



The sensible qualities of the bitumens (their taste and smell) are 

 in all states utterly and entirely different from those of the vegetable 

 tar. Petroleum is also much less soluble in alcohol, and further 

 differs from the vegetable tar in being perfectly soluble in naphtha. 

 In their solubilities in oil of turpentine they resemble each other, as 

 well as in their habitudes with acetic acid and the alkaline lixivia, 

 although the vegetable tar will be found the more readily soluble of 

 the two. I need not repeat the circumstances in which the essential 

 oil of wood differs from naphtha. It is a sufficiently characteristic 

 one, that it forms no union with this latter. 



It has been already shown that the difference between the pitch 

 and asphaltum is considerable, when the former is in its first state, 

 particularly with regard to its solubility in alcohol. 



