in Borrondale. 175 
being heated in a crucible, with nitre, or 
other substances affording a great quantity of 
oxygen, its texture is weakened, and it is 
finally decomposed, the carbon disappearing, 
and a little ochreous matter being left behind. 
By keeping it for some time immersed in 
melted sulphur, the black-lead becomes im- 
pregnated with sulphur; and being first re- 
duced into slices of about one twentieth of an 
inch in thickness (the proper size for pencils), 
it may thus be brought to the degree of hardness 
required. Pencils treated in this way may be 
known by the sulphureous odour, and phos- 
phorescent light, emitted on rubbing the point 
upon a moderately heated iron. 
By an account drawn up in 1804, the stock 
then on hand was valued at £54,000, and the 
annual consumption about £3500; the best 
black-lead was then sold at 35s. per pound ; 
since that time the price has been from 30 to 
45 shillings, and no doubt the consumption 
has greatly increased. And this mine which 
200 years ago, was valued at fifteen shillings 
and fourpence; has lately, on assessing the ~ 
property tax, been estimated at two thousand 
seven hundred pounds sterling a year. 
Keswick, 
Noy. 28, 1816. 
