that every stroke added confiderably to the quantity of air; and when water was admitted to it, juft fo much remained unabforbed as had been added by the explosions. I then took about an hundred explosions of the fame jar, in a larger quantity of alkaline air; after which, fo much of it remained unabsorbed by water, that I could examine it with the greateft certainty. It neither affected common air, nor was affected by nitrous air, and was as ftrongly inflammable as any air that I had ever procured.

These experiments appear to me to furnish matter for much fpeculation, and farther experimental inquiry. Till this be done, all conjecture concerning them must be very much. at random. I therefore defer making any at prefent.

122

SEC

SECTION XIV.

Experiments and Obfervations on CHARCOAL, first published in the Philofophical Tranfactions, vol. LX. p. 211.

Among the original experiments, publifhed in the History of Electricity, was an account of the conducting power of charcoal. This fubftance had been confidered by electricians, in no other light than that of more perfectly baked wood, which is known to be no conductor of electricity. I have even heard of attempts being made to excite it; and though those attempts were ineffectual, the failure of fuccefs was attributed to other caufes than that of charcoal being no electric fubftance; fo fixed was the perfuafion, that water and metals were the only conducting, fubftances in nature. The confideration of the chymical properties of charcoal, which are, in many respects, remarkably different from those of the wood from which it is made, might have led philofophers to fufpect, that fince, after its being reduced to a coal, it was become quite another thing from what it was before, it might poffibly differ from

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from wood in this property; but this confideration had not been fufficiently attended to.

In the account of my former experiments on charcoal, I obferved, that there were very great differences in the conducting power of charcoal, and particularly of wood-charcoal, though I could not determine on what circumstances in the preparation, &c. those differences depended. I therefore expreffed a wifh, that fome person, who had conveniencies for making chymical experiments, would profecute the inquiry, as one that promised, not only to afcertain the cause of the conducting power of charcoal, but perhaps of conducting power univerfally. Not hearing that any chymift or electrician has attended to this bufinefs, I have, at length, refumed the fubject, though not with every advantage that I could have wished. I have, in a great measure, however, infucceeded in the principal object of my quiry; and I fhall now lay before this fociety the refult of my experiments and obfervations.

I fhall begin with correcting a mistake I lay under at the time that I made the former experiments. Having been informed by perfons, who attend the making of pit-charcoal, that it was confiderably increased in bulk after the procefs; I imagined that all other substan

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ces received an increase of bulk, when they were reduced to a coal; but the first experiments that I made, convinced me of my miftake. All vegetable substances are confiderably contracted in all their dimensions, by the process of coaling, and the more perfect this process is (that is, as will be explained hereafter, the greater is the heat that is applied in the course of it) the greater is the diminution. I have even reduced pieces of wood to little more than one-fourth of their original length and breadth, in a common fire, by the use of a pair of hand-bellows only. And this was the cafe equally with wood of the firmest texture, as ebony; that of a middle texture, as oak; and that of the loofeft, as fir, &c.

As moisture (and, I believe, small degrees of heat or cold) affects wood much more fenfibly across the fibres than along them, it might have been fuppofed, that when wood was reduced to a coal, by the application of a greater degree of heat, the fame rule would have been obferved; but I found very little difference in this refpect. To afcertain this circumstance, I took from the fame board, two pieces, each 2 inches in length. In one of them, the fibres were divided, in the other they were not; and after coaling them thoroughly together, in the fame crucible, I found that the former measured

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measured 2.05 inches, and the latter 2.15. Their conducting power could not be distinguished.

A more particular account of the degree, in which wood is fhortened in coaling, will be feen afterwards, when the variations in this respect. are compared with the variations in the power of conducting electricity.

To my great furprize, I found animal fubftances not reduced in their dimenfions by the process of coaling. This, at least, was the cafe with fome pieces of ivory, feveral inches in length, and a piece of bone. They bore a very intense heat for many hours, and came out of the crucible confiderably diminished in weight, but hardly fo much as diftorted in their shape, as is remarkably the cafe with wood, and, I believe, all vegetable substances.

In examining mineral fubftances, I found that my information, mentioned above, was juft. Coals are very much enlarged in their dimenfions by charring; but the experiment must be made with great care, to judge of this circumstance; for, unlefs the operation be very flow, the coal will retain nothing of its former shape, having been made, in fome measure, fluid by the heat. The infide of all pieces of