above related, but I cannot forbear remarking on some false hypotheses 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 asphaltum 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 naphtha. 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 substances to suppose, that the change consists in the alteration of the relative proportions of the hydrogen and carbon, but whether this is
