Dr MacCulloch on Peat. 
SOS 
striking, by selecting the simplest and extreme cases, namely, 
by placing in a parallel light the severah bituminous substan- 
ces and those products of vegetable matter already described, 
which are analogous to peat, and are produced by the action of 
fire. 
It will be sufficient, in considering the bitumens, to enume- 
rate the three most prominent varieties, namely, asphaltum, pe- 
troleum, and naphtha ; the others may be considered as interme- 
diate states or modifications of these. These substances are seve- 
rally analogous to the pitch, the tar, and the essential oil of wood, 
and the changes which they undergo on the application of heat 
are in every respect similar. Thus, if petroleum be distilled, it 
is converted into naphtha and pitch, while the former, by a fur- 
ther application of heat, is decomposed, and furnishes hydro- 
carbonic gas. The asphaltum, by distillation, gives out petro- 
leum, becoming gradually friable, until at length charcoal re- 
mains behind, as in the distillation of bistre. There is, more- 
over, in both cases, an uninterrupted transition from the essen- 
tial oil to the charcoal. Thus naphtha, when of a brown co- 
lour, passes gradually into petroleum, as the oil of wood does 
into tar ; and thus, during the prolonged distillation, both of 
petroleum and of tar, the oil gradually becomes thicker, while 
the matter in the retort, in both cases, becomes first tenacious, 
then brittle, and lastly friable, when a further continuance of 
the heat at length converts it into charcoal. 
But, with this general analogy, there are important differen- 
ces in the two sets of substances, united to some chemical cha- 
racters common to both. There is, in the first place, a striking 
difference in the taste and smell, by which they can readily be 
distinguished ; while, even the mixtures of the two, can be re- 
cognized by an union of the sensible qualities appertaining to 
both. These differences are most remarkable in the more fluid 
varieties, but are sufficiently sensible in all. The action of al- 
cohol, of ether, acetic acid, the alkalies, and the mineral acids, 
on both these classes of substances, in a great measure resemble 
each other. Certain differences will nevertheless be found ; but 
it is unnecessary here to enter minutely into the subject, as the 
detail is not required for the purposes at present in view. The 
most remarkable distinction in their chemical relations is the fol- 
