It is owing to the solidity of those natural charred coals, and the want of oil, that they are so very difficult of kindling; but, when once kindled in sufficient quantity, they make a fire which is very durable. There are even some of them which, to common observation, seem to be altogether incombustible. I have of this kind a specimen from a stratum at Stair, which shall be afterwards mentioned. M. Struve, in the Journal de Physique for January 1790, describes a mineral which he calls _plombagine charbonneuse ou hexaëdre_; and gives for reason, _parce qu'elle ressemble extrêmement au charbon de pierre schisteux, ou d'hexaëdre_. He says farther, "Il est très commun, dans une roche qui forme un passage entre les granits et les brèches, qu'on n'a trouvée jusqu'a présent qu'on masses roulées dans le pays de Vaud." He concludes his paper thus: "Ce fossile singulier ne paroît pas appartenir à la Suisse seule. J'ai dans ce moment devant les yeux une substance parfaitement semblable, si on excepte la couleur qui tient le milieu entre le gris de fer et le rouge modéré; elle vient du pays de Gotha de la Friedrischs-grube, proche d'Umneau. On le regarde comme un eisenrahm uni à du charbon de pierre." The specimen which I have from Stair upon the water of Ayr, so far as I can understand, perfectly resembles this _plombagine_ of M. Struve. It consumes very slowly in the fire, and deflagrates like plumbago with nitre. Now this comes from a regular coal stratum; and what is more remarkable, in this stratum is contained a true plumbago, Farther up the country, the Earl of Dumfries has also a mine containing plumbago along with other coal strata; and though the plumbago of these two mines have not all the softness and beauty of the mineral of the same species from Cumberland, they are nevertheless perfect plumbago. I have a specimen of steatetical whinstone or basaltes from some part of Cumberland, in which is contained many nodules of the most perfect and beautiful plumbago. It is dispersed through this stone in rounded masses of all sizes from a nut to a pin's head; and many of these are mixed with pyrites. There is therefore reason to believe that this plumbago had been in fusion. Now, if we consider that every species of coal and every species of plumbago are equally, that is, perfectly combustible, and yield, in burning, the same volatile principles, differing only perhaps a little in the small quantity of fixed matter which remains, we shall be inclined to believe, that they have all the same origin in a vegetable substance; and that they are diversified by some very small composition of other matter. This being allowed, one thing is certain, that it is by the operation of mineral fire or heat that those combustible substances, however composed, have been brought to their present state of coal, although we are ignorant of the circumstances by which their differences and their peculiar chemical and mechanical qualities have been produced.