du feu.  Et telle est la propension naturelle et prodigieuse de la manganese a la vitrification, qu’on n’a pu parvenir encore a reduire son regule en un seul culot; on trouve dans le creuset plusieurs petits boutons, qui forment 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 evaporated from them.  On the other hand, coaly substances are destitute of fusibility and volatility, in proportion as they have been exposed to greater degrees of heat, and to other circumstances favourable to the dissipation of their more volatile and fluid parts.