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substance was explained, the former being built up from within upon definite 

 lines, the latter being fashioned by external action. The study of the geo- 

 metric forms of minerals by careful measurements of the angles of their 

 facets by the Abbe Haiiy early in the present century resulted in a system of 

 crystallography, showing the relation which these angles bear to the axes of 

 the crystal around which the molecules are arranged with such order and 

 precision. All known crystals are now arranged under six systems, accord- 

 ing to the positions of their axes. The diamond is referred to the cubic 

 system, in which there are three equal axes at right angles to each other. 

 This the lecturer illustrated by drawing on the blackboard diagrams of the 

 simple cube and its various modifications in the octohedron and dodecahedron, 

 common forms of the diamond. The diamond is the hardest mineral known ; 

 no other body can scratch it, yet it can be easily pulverised, and, like other 

 crystalline minerals, pieces can be chipped oif , provided the cut be made along 

 the plane of cleavage, i.e. in a line with one of its faces. In this way diamonds 

 are reduced in size before the operation of grinding the stone iuto the shape 

 required by the jeweller. The only useful application of the diamond is for 

 cutting glass, but of late years an amorphous and tough variety known as bort, 

 or carbonado, has been set on the edges of boring tools, which arrangement 

 has enabled the engineer to bore the hardest rocks. The optical properties of 

 the diamond, combined with its hardness, give the value to the stone. The 

 beautiful adamantine lustre of a fine, well-cut diamond arises from its very 

 high refracting power, and Newton, without any knowledge of its chemical 

 nature, but reasoning entirely from the results of his experiments on other 

 highly refractory bodies, such as essential oils, surmised that the diamond 

 might be a combustible substance. Mr. Rudler described several experiments 

 that had been made by wealthy persons to discover what this stone really 

 was, but it was not until Smithson Tennant, an English chemist, burnt a 

 diamond in oxygen, and, after weighing the gaseous product, discovered that 

 an equivalent of carbonic acid gas remained corresponding to the weight of 

 the stone consumed, thus proving that the diamond is pure crj'Stallised 

 carbon. The progression in the amount of carbon from wood to diamond 

 was explained. Lignite, a woody coal, contains from fifty to seventy per 

 cent, of carbon ; house coal from seventy to ninety ; anthracite, a homoge- 

 neous coal, contains sometimes as much as ninety-eight per cent, of carbon ; 

 graphite, or blacklead, contains only a trace of impurities ; and the diamond 

 is absolutely pure carbon. From these and other important facts which have 

 been brought to light from the examination of diamonds, an organic origin is 

 claimed for this stone. The chief diamond localities are India, Brazil, Borneo, 

 Australia, and recently South Africa, in which colony they were discovered 

 about fifteen years ago at Kimberly, on the Vaal River. The Indian fields 

 are the oldest, and most of the finest stones have been found there, but many 

 good stones have lately come from the Cape, which yields the largest supply. 

 From a study of the mode of occurrence of diamonds in the soft rock of these 

 South African mines, a volcanic origin has been suggested, and the recent 

 experiments in this direction by Mr. Hannay, of Glasgow, who has produced 

 diamonds artificially, has strengthened this idea. Hannay found that when a 

 gas containing carbon and hydrogen is heated under pressure in the presence 

 of certain metals, its hydrogen is attracted by the metal and carbon set free in 

 the clear transparent form of the diamond. 



The lecture was illustrated by several excellent diagrams and by a beautiful 

 series of models of historic diamonds, and by several native diamonds, kindly 

 lent by Mr. E. W. Streeter, of New Bond Street. The interest of the exhibi- 

 tion was much enhanced by the information relating to the histories, pos- 

 sessors, and values of these gems, aiforded by Mr. Abbot, who was so good 

 as to attend the meeting in charge of this valuable collection. 



