analysis), and finally a larger or smaller volume of nitrogenous gas.
It is possible that the elimination of these gases takes the form of
fractional distillation, and definite compounds may be formed directly
from the wood-tissue or its derivatives, and mingle as they escape. This
is, however, not certain, for the gases, as we find them, are always
mixtures and never pure. In the liquid evolved products, the petroleums,
this is emphatically true, for we combine under this name fluids which
vary greatly in both their physical and chemical characters; some are
light and ethereal, others are thick and tarry; some are transparent,
some opaque; some red, some brown, others green; some have an offensive
and others an agreeable odor; some contain asphalt in large quantity,
others paraffine, etc. Thus they form a heterogeneous assemblage of
liquid hydrocarbons, of which naphtha and maltha may be said to form
the extremes, and which have little in common, except their undefinable
name. The causes of these differences are but imperfectly understood,
but we know that they are in part dependent on the nature of the organic
material that has furnished the petroleums, and in part upon influences
affecting them after their formation. For example, the oil which
saturates the Niagara limestone at Chicago, and--which is undoubtedly
indigenous in this rock, and probably of animal origin, is black and
thick; that from Enniskillen, Canada, is also black, has a vile odor,
probably in virtue of sulphur compounds, and, we have reason to believe,
is derived from animal matter. The oils of northwestern Pennsylvania are
mostly brown, sometimes green by reflected light, and have a pungent and
characteristic odor. These are undoubtedly derived from the Hamilton
shales, which contain ten or twenty per cent, of carbonaceous matter,
apparently produced from the decomposition of sea-weeds, since these are
in places exceedingly abundant, and nearly all other fossils are absent.

The oils of Italy, though varying much in appearance, have usually an