1 3*7 '3

I could not persuade myself that the minerals, on whichthe above experiments' were made, could be such as arecalled among us black-lead, till some of the finest black-lead of our pencil-makers, weighing one hundred andsixty-eight grains, ip three pieces, having been kept of amoderately strong red heat on a scorifying dish for threehours,with the common precaution of covering the vesselfor a time, lest the matter should crackle, and some par-ticles be thrown off from it in substance; I found it re-duced to about an hundred and twenty grains, and all thepieces changed on the outside to a sparkling rusty browncalx, of which a considerable part was attracted by a mag-netic bar, the internal parts continuing of the same appear-ance as at first. Being then broken into smaller pieces,and exposed to a like heat for two hours, it suffered thesame change as before, and was reduced to about sixtygrains. Being further broken, and calcined with a mo-derate red heat for ten hours, it was diminished to thirtygrains; and by a repetition of this operation, to twelvegrains, or a fourteenth part of its first weight.

The remarkable dissipation, in these experiments, of asubstance which in close vessels resists intense fires, maybe somewhat illustrated by the known property of charcoal,which when excluded from the action of the air, whetherby being inclosed in a vessel, or mixed with clay into amass, remains unconsumed and unaltered in the fire.Masses of black-lead seem to calcine and suffer a dissipationonly on the surface; the internal part remaining long un-changed, unless the mass be broken, or the calx rubbed off,so as that fresh surfaces may be exposed to the air. Thecommon black-lead melting-pots, made of clay and thecoarser kinds of black-lead powdered, like those made ofclay and charcoal powder, lose their external blacknesswith part of their weight, and thus have their staining

quality