MINERALOGY. 319 



Diamonds are the most valuable of Mineral productions, 

 and are iu great estimation for ornamental Jewellery, as 

 likewise for the purpose of cutting Glass, and they are used 

 by Lapidaries to engrave the harder gems. Carbon is obtain- 

 ed from Charcoal in almost as pure a state as from the Dia- 

 mond, but it is always accompanied with either Hydrogeu 

 or Water. Diamonds are found in India, South America, 

 the Isle of Borneo, &c, they frequently occur in veins, or 

 among the sand of rivers. 



The two foregoing substances. Sulphur and Carbon, in 

 greater or lesser proportion, enter into the composition of all 

 combustible Minerals ; Plumbago or Black Lead is composed 

 of ninety parts of Carbon, it is of a glistening metallic lustre, 

 unctuous to the touch, soft and not very brittle, the principal 

 use to which it is applied is the manufacture of those Pencils 

 generally known by the name of Black Lead Pencils ; a 

 name probably derived from the lead coloured mark they 

 leave upon paper : but no lead enters into the composition- 

 it is about twice as heavy as water ; and cannot support com- 

 bustion by itself. NapLha is a combustible substance which 

 with Petroleum are comprehended under the term of Mineral 

 Oil ; they are both liquid, highly inflammable,and lighter than 

 water ; Naptha is transparent and almost colourless, it burns 

 with a blue flame, gives out much smoke, and a penetrating 

 odour when burning, and does not leave any residium. 



Petroleum, is of a substance somewhat thicker than com- 

 mon Tar, of a reddish brown colour, it possesses a strong, 

 unpleasant odour, burns with a thick black smoke and leaves 

 * residue like common black coal. It is found in various 



