t autant de culots separes. Dans la mine de manganese native, elle
n'est point en une seule masse; elle est disposee egalement en plusieurs
culots separes, et un peu aplatis, comme ceux que l'art produit;
beaucoup plus gros, a la verite, parce que les agens de la nature
doivent avoir une autre energie, que ceux de nos laboratoires; et cette
ressemblance si exacte, semble devoir vous faire penser que la mine
native a ete produite par le feu, tout comme son regule. La presence
de la chaux argentee de la manganese, me permettroit de croire que la
nature n'a fait que reduire cette chaux. Du reste, cette mine native
est tres-pure, et ne contient aucune partie attirable a l'aimant. Cette
mine, unique jusqu'a ce moment, vient, tout comme les autres manganese
que j'ai decrites, des mines de fer de _Sem_, dans la vallee de
_Viedersos_, en Comte de Foix."--_Journal de Physique, Janvier 1786_.]

We come now to the _second_ species of inflammable bodies called oily or
bituminous. These substances are also found variously mixed with mineral
bodies, as well as forming strata of themselves; they are, therefore, a
proper subject for a particular examination.

In the process of vegetation, there are produced oily and resinous
substances; and, from the collection of these substances at the bottom
of the ocean, there are formed strata, which have afterwards undergone
various degrees of beat, and have been variously changed, in consequence
of the effects of that heat, according as the distillation of the more
volatile parts of those bodies has been suffered to proceed.

In order to understand this, it must be considered, that, while immersed
in water, and under insuperable compression, the vegetable, oily, and
resinous substances, would appear to be unalterable by heat; and it is
only in proportion as certain chemical separations take place, that
these inflammable bodies are changed in their substance by the
application of heat. Now, the most general change of this kind is in
consequence of evaporation, or the distillation of their more volatile
parts, by which oily substances become bituminous, and bituminous
substances become coaly.

There is here a gradation which may be best understood, by comparing the
extremes.

On the one hand, we know by experiment, that oily and bituminous
substances can be melted and partly changed into vapour by heat, and
that they become harder and denser, in proportion as the more volatile
parts have evap