^58 
Bcietitific LitelUgence, [no. 2, new series. 
what constitutes the value of good Graphite and what causes the 
deterioration of that from India. For this purpose it is necessary 
to notice the different uses to which this substance is applied. 
These are 1st for making pencils, 2nd to diminish the friction of 
machinery, 3rd to make fire proof crucibles, 4th to hhck-lead grates 
as it is called. For all these purposes except the last, it is evident 
that purity is essential. 
The best pencils were formerly made of the Borrowdale Gra- 
phite, obtained from a mine which is now exhausted. This kind 
was pure and compact, and sold readily at about 40s. a pound. 
It was sawn into thin slices, these were inserted into the groove of 
one-half of Cedar Pencils and the superiluous part filed off, then 
the other half was glued on to the filled up half of the pencil. It 
is evident that for such a purpose only the purest specimens were 
of any value, for the presence of a bit of quartz or of an ore of 
iron or of any other metal would injure the tools and fracture 
the thin slices or slender prisms of Graphite, and if they did not 
produce any of these eff*ects, they would be very inconvenient at 
the point of a pencil for whatever purpose employed. Indeed if 
it had not been for the discovery, that finely-powdered Graphite can 
by an extreme degree of pressure be rendered nearly as compact 
as the best natural Graphite, w^e should have been without any 
more good drawing pencils, But the Graphite for grinding though 
in small pieces, must be pure, or otherwise the grinding mills be- 
come injured, besides pencils made with it being unfit for use. 
So also finely powdered Graphite is required for mixing with 
fatty substances in order to diminish the friction of machinery. 
It is equally evident, that this must be of the purest kind, or 
otherwise the machinery in which it was prepared, or that to which 
it was applied would be equally injured. Thus also if Graphite 
as an infusible substance, is required for mixing with the more in- 
fusible kinds of clay for making the best crucibles, the Graphite 
must be without impurities, as these would diminish the melting 
point and render the crucibles useless. 
Th^ Graphite or Plumbago, black-lead as it is commonly call- 
ed and used for polishing grate?', is an impure sub3tance, but if 
