Royal Institution of Great Britain^ 465 



tained in a small copper cup, the part, or facette, to be polished 

 beincf left to protrude. 



A flat circular plate of cast-iron is then charged with the powder 

 produced during the abrasion of the diamonds ; and by this means 

 a tool is formed that is capable of producing the exquisite lustre so 

 much admired on a finely-polished gem. Those diamonds that are 

 unfit for working, on account of the imperfection of their lustre or 

 colour, are sold for various purposes under the technical name of 

 Bort. Stones of this kind are frequently broken in a steel mortar 

 by repeated blows, xmtil they are reduced to a fine powder, which 

 is used to charge metal plates, of various kinds, for the use of 

 jewellers, lapidaries, and others. Bort, in this state of preparation, 

 is incapable of polishing any gems; but it is used to produce flat 

 surfaces on rubies and other precious stones. 



It was also shown that fine drills are made of small splinters of 

 bort, which are used for drilling small holes in rubies and other 

 hard stones, for the use of watch-jewellers, gold and silver wire- 

 drawers, and others who require very fine holes drilled in such 

 substances. These drills are also used to pierce holes in china, 

 where rivets are to be inserted ; also for piercing holes in artificial 

 enamel teeth, or any vitreous substances, however hard. 



A paper, published by Dr. Wollaston in the Philosophical 

 Transactions of the Royal Society, on the nature and properties of 

 the cutting diamond, was referred to as fully explaining the requisite 

 properties in the natural diamond to suit it to that purpose. In 

 this it was clearly shown, that it is only in the natural state the 

 diamond can be depended upon to cut or divide plates or panes of 

 glass ; and even then a particular crystallization is absolutely 

 requisite. 



These observations were illustrated by various diamonds, which 

 proved the truth of Dr. Wollaston's remarks. The Lecture con- 

 cluded by exhibiting some beautiful specimens of steel, orna- 

 mented in a most curious manner by Mr. John Barton, of the 

 Royal Mint. The most beautiful display of prismatic colours were 

 seen, produced by divisions 2000 in an inch, the lines being made 

 with a natural diamond. It would be impossible to convey the 

 smallest idea of the exquisite beauty and play of colours that are 

 produced by these means. Several other specimens of dividing 

 were laid before the company, tending to elucidate the value of the 

 diamond when applied to eflect certain purposes in the arts. 



Friday y June Sth, 

 Some observations on the recently proposed improvement of the 



