134 
which they manage the Lead so as to make it as hard as- 
they like. T believe it is pretty well known that the Chalks 
used in drawing are tempered by the changes from damp to 
dry, which have a similar effect on Plumbago. It is certainly 
most generally used without any alteration, and thoseare most 
certain of making the best pencils, who have the cunning 
to choose the most perfect substance. The harderand finer 
Lead is generally sawn out smallest for the best price, and 
the coarser sawn out larger for commoner purposes, and is 
cheaper, exclusive of the mixtures made from the dust with 
Sulphur, Rosin, &c. which are scarcely worth mentioning. 
This and the more common sort of Lead in dust from va- 
rious places are used for cleaning stoves *, and for making 
crucibles, which will bear a strong heat. 
Plumbago is mostly found included in mixed rocks— 
see the next talle—in pieces from a grain to 40 or 50 
pounds in weight, or larger; there is a piece in the London 
warehouse of 42 pounds and a half weight, consequently 
worth 85/, 
It has a dull dusty or granular outside, but the least 
rub or scratch exposes its bright gray metallic lustre. Its 
fracture is mostly rough and fine, or coarse grained, some- 
times rather plated and even foliated. It feels greasy, as it 
has generally been called, but I submit the word ¢ slippery’ 
to the judgement of the critics. It is, on this account, often 
used as a preventative to the effects of friction, in preference 
to soap. It soils, neatly if required, but does not clog. 
I could never discover it at all crystallized. ; 
The upper figure is from a pure chosen specimen, with 
which I was favoured by the proprietor of the mine, the 
Right Hon. Sir Joseph Banks, Bart. ; and an honest pencil- 
maker, who has attended the sale and seen large quantities 
for many years, assured me it was of the very best sort. 
The shape is as cut out for sale, and the colour is perfect, 
being composed of the substance itself, with the addition of 
gum water. The substance in question is too well known 
to need any more particular mention; it is, however, ne- 
cessary to say that Molybdenum much resembles it, but is 
much brighter, and foliated like Mica, Its specific gravity 
is from 1°987 to 2°2456. 
On analysis it has been found to contain :— 
Carbon or Oxide? of Carbon . 90°9 or 90 
ATODY tap) wigh> ysis +, «kg, eee 
100'0 * 100 
* I understand this dust is occasionally adulterated with micaceous Irom 
—see tal. 64—which is apt to rust, to the great disappointment of the care- 
ful housewife, who considers it as the property of the Black-Lead to pre- 
vent the Iron from rusting. 
