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Scientific Intelligence, [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 Hack-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 405. a pound. 

 It was sawn into thin slices, these were inserted into the groove of 

 one-half of Cedar Pencils and the superfluous 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 effects, 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, we 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. 



The Graphite or Plumbago, black-lead as it is commonly call- 

 ed and used for polishing grates, is an impure substance, but if 



