86 



MINERAL GALLBfey. 



tinguished by the letters a to h. For the use of the student 

 there is published an Index to the names of all the numerous 

 mineral species and varieties represented in the collection, 

 with references to the table-cases in which the specimens are 

 placed.* 



The system of classification, which includes not only ores, 

 but all known minerals, is not easy for a general visitor to 

 follow, and it is thus convenient to indicate here the positions 

 in the gallery of those minerals — as, for instance, precious and 

 ornamental stones, and metallic ores — which have an interest 

 for all; for details, reference must be made to the Mineral 

 Guide. Most of these minerals occur in crystals, the forms of 



Fig. 58. — The Colenbo Diamond, from South Africa. 

 Preseuted by Professor John Euskin. (Natural size.) 



Native 

 Elements. 



Sulphides. 



which were at one time supposed to be modifications of six 

 chief types. 



I. In cases 1 and 2 are the pure metals, as Copper, Silver, 

 Gold, and Platinum; and non-metals, as Sulphur, Graphite, 

 and Diamond. The large symmetrical South African " Colenso " 

 Diamond (fig. 58), weighing 130 carats, presented by the late 

 Professor Euskin, is worthy of special attention (case Ig). 



II. The next six cases contain minerals which have mostly a 

 metallic lustre and consist of metals in chemical combination 

 with elements of the sulphur or arsenic groups. 



Argentite (3d) is an important silver-ore, containing 87 per 

 cent, of silver and 13 of sulphur. 



Blende (4b) is a valuable zinc-ore, and contains 67 per cent, 

 of zinc and 33 per cent, of sulphur. 



* " The Student's Index to the Collection of Minerals." Price twopence. 



