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modifications, and that, entering into combination with other 

 elements, it forms part of an immense number of chemical com- 

 pounds. In considering the uses of carbon it will be convenient 

 to confine ourselves to those of the free element. Of the various 

 modifications of this it seems natural to consider the diamond 

 first. 



This occupies a very high position among precious stones and 

 has been greatly valued from very ancient times. It possesses an 

 extraordinary lustre and exceeds all other gems in hardness. It 

 occurs in various parts of the world such as India, Australia, 

 Borneo, Brazil, and South Africa. In ancient times India was the 

 chief diamond country. In the 18th century the Brazilian fields 

 became known and the yield continued to increase up to the early 

 fifties of the 19th century. The existence of diamonds in South 

 Africa was known or suspected long ago, but it was not till 1867 

 that the first Cape diamond reached Europe. During the next 

 thirty years (1867-97) diamonds to the value of ^90,000,000 were 

 exported from South Africa. The Kimberley mines had by the 

 end of 1904 yielded ten tons of diamonds. This figure is given by 

 Sir William Crookes. But diamonds are not usually sold by the 

 ton, a very different unit of weight is used, namely, the carat. 

 This unit has not the perfect definiteness that belongs to the 

 Imperial pound or to the gramme, but has varied a little with 

 time and place. In 1877 a syndicate of jewellers of London, Paris, 

 and Amsterdam fixed the carat as 205 milligrams, equalling 

 3.163 grains. To realise somewhat better the value of the carat 

 it may be noted that 



1 ounce avoirdupois = 138.3 diamond carats. 



I ounce troy = 151. 1 diamond carats. 



1 sovereign weighs almost exactly 39 diamond carats 



A new threepennypiece weighs almost exactly 6.9 diamond 

 carats. 



The fourth part of a carat is called a grain, but this diamond 

 grain is clearly not equal to the grain used in dispensing medicines. 

 Occasionally it may happen that a diamond is found so perfect 

 in shape that it can be set as a gem without any preparation, but 

 this is very rare. Cutting and polishing are required to bring out 

 the full beauty of the stone. Before attempting to outline these 

 processes it will be well to note (1) that in hardness the diamond 

 far exceeds all other naturally occurring substances, (2) that this 

 hardness does not prevent some degree of brittleness, (3) that 

 the diamond can readily be split or cleaved in certain directions, 

 namely, along planes parallel to the faces of the octahedron, which 

 is its primary form of crystallisation. In former times it was not 

 realised that hardness and brittleness could go together, and it 

 was supposed that a diamond could not be broken on an anvil by 

 a hammer. In fact Pliny says that Indian and Arabian diamonds 

 are of such " indomitable and unspeakable hardness " that the 

 attempt to break one on an anvil results in the shattering of anvil 

 and hammer alike. This, of course, is absurd, and Sir Thomas 



