68 Scientific Proceedings, Royal Dublin Society. 



were obtainable to render numerical results of value, yet the com- 

 parative test was easily applied, and would probably throw some 

 light on the nature of the mineral. 



Comparison with Orthoclase. — The orthoclase used was from 

 Belleek, and also from Co. Dublin ; respectively red and white in 

 colour — subtranslucent. The unknown crystals were transparent 

 and colourless. 



The orthoclase fused first, and gathered into transparent beads 

 of glass containing large bubbles. At a much higher temperature 

 the unknown mineral rounded and turned milk-white in colour, 

 developing no bubbles. 



The experiment was more than once repeated. The unknown 

 mineral had evidently a much higher melting point than orthoclase, 

 and its behaviour in other respects, also, differentiated the two 

 substances decisively. 



Comparison with Topaz. — Fragments of clear topaz and the 

 unknown mineral. Both rounded simultaneously, and both turned 

 milk-white. The topaz, however, emitted a gas which raised 

 blisters and blue bubbles on the melting surface of the fragments. 

 On the breaking of these bubbles, threads of glass were thrown 

 about the hob, and the gas attacking the platinum deposited rings 

 of colour around. Probably the gas contained in these bubbles is 

 fluorine, liberated at the high temperature employed. The coloured 

 rings, fluoride of platinum. No such phenomena occurred with 

 the unknown mineral. They probably contained no fluorine. 



Comparison with Quartz. — Clear rock crystal showed a much 

 greater resistance to the temperature of the hob, only fusing at 

 the extreme limit of endurance of the platinum itself. 



The melting point of the unknown mineral was therefore fixed 

 as above that of orthoclase, and below that of quartz. 



These results, together with the knowledge of its angles already 

 gathered from the microscope, led me to think for a time that I 

 might be dealing with a new speoies. More accurate crystallo- 

 graphic measurements were desirable. 



By manipulating one solitary crystal, obtaining extinctions 

 along its faces, and measuring its angles, its orthorhombio charac- 

 ter, both by symmetry and elasticity, was determined beyond doubt. 



Its specific gravity was now taken by Professor Sollas' method. 

 It was found to be 2-58. 



