160 BLACK LEAD. 
the jet obtained is dry and brittle ; and cannot be used 
in the forming of trinkets. But, if the texture be unc- 
tuous the fibre acquires a considerable degree of soft- 
ness, is susceptible of being properly wrought, and re- 
ceives a perfect and beautiful polish. 
A fictitious kind of jet is made of glass ; and several 
varieties of mineral pitch, and cannel coal, are imposed 
upon ignorant purchasers for jet. 
When jet is once set on fire it burns with a green 
flame, and continues to burn for a considerable time, 
exhaling a strong bituminous smell. If the heat be 
rendered greater, it melts. 
GRAPHITE FAMILY. 
223. BLACK LEAD, or PLUMBAGO, is an inflam- 
mable mineral, which consists of carbon, or charcoal (48), 
combined with iron, in the proportion of about nine parts of 
the former to one of the latter. 
It is of dark iron-grey colour, with a strong metallic lustre, 
and so soft that it is easily scratched with a knife. To the 
touch it is soft and greasy ; and, when handled, it stains the 
fingers. In weight it is about twice as heavy as water. 
The name of black lead has very improperly been 
given to this substance from its appearance only, as it 
has no alliance whatever with lead. It is usually found 
in kidney-shaped lumps of various size, and occurs in 
several countries of Europe, but no where of such 
excellent quality as in Borrowdale, Cumberland, where 
it has the name of tvadd. The vein of black lead lies 
between strata of slate, and is from eight to nine feet 
thick. This mine is not opened more than once every 
three or four years, the quantity thus obtained being 
found fully sufficient for the demand. The only other 
mine of black lead in Britain is in Ayrshire, Scotland. 
Artists in water-colours, if deprived of this mineral, 
would find great difficulty in making their sketches ; as 
the marks that are erroneously made with it are more 
easily expunged than those of almost any other sub- 
stance. Hundreds of thousands of pencils are every 
