obtained in the distillation of Wood ^ ^c, 13 



above related, but I cannot forbear remarking on some false hypo- 

 theses which have been held respecting these substances, and their 

 relation to other bodies. It is evident, from considering the products 

 of their decomposition, that the basis of naphtha and of all the 

 intermediate stages of bitumen, down to asphaltum, are carbon and 

 hydrogen, modified by certain small proportions of oxygen and azote. 

 It is in the relative proportions chiefly of these two ingredients that 

 naphtha differs from petroleum, petroleum from maltha, and maltha 

 from asphaltum. If we distill either of these more solid substances 

 with a very gentle heat, we obtain naphtha, in which the proportion 

 of the hydrogen to the carbon is increased to a maximum ratio. 

 If the heat is greater, we obtain a substance of a darker colour, in 

 which that ratio is less ; and, for this reason, the distillation of as- 

 phaltum affords a darker oil than that of petroleum, because its 

 composition cannot be dissolved but in a higher temperature. 



For the same reason also petroleum is easily rectified into naph- 

 tha. Asphaltum, in its ordinary state, contains the two ingredients 

 in a ratio in which the carbon bears a large proportion to the 

 hydrogen, and that ratio is reduced to the minimum, or becomes 

 evanescent, when by the continuance of distillation, charcoal alone 

 remains behind. A large portion of the oxygen, and also of the 

 azote, is disengaged during this process, but not the whole, since 

 the darker compounds still give it over on repeating the process* 

 The naphtha is probably entirely exempt from oxygen. With this 

 view we cannot accede to the notion, that the absorption of oxygen 

 is capable of converting naphtha or petroleum into asphaltum ; or 

 that the harder bitumens originate from the oxygenation of the 

 more liquid. It is more consonant to the nature of these sub- 

 stances to suppose, that the change consists in the alteration of the 

 relative proportions of the hydrogen and carbon, but whether this is 



