408 Proceedings of the Royal Irish Academy. 



Of the Sidphides, Stibnites, Galena, and Zinc Elende were easily 

 determined, but some of the sulphides were either decomposed or 

 oxidised immediately on fusion, if not before, so that their melting- 

 point could not be satisfactorily determined. Iron pyrites suddenly 

 decomposed at 642° ; whether it is fused at that temperatui'e or not it 

 is impossible to say, as the specimens suddenly became a greyish yellow 

 colour, or rather a substance of that colour was suddenly formed on 

 the ribbon. "What the substance was could not be satisfactorily 

 discovered, as the quantity was so small that it could only be seen 

 under the microscope, but probably it was sulphur. Galena behaved 

 somewhat similarly, only the substance formed was white. Eealgar 

 and Orpiment both changed colour as the temperature was raised, but 

 I noticed that if the temperature was kept constant the colour did not 

 change during the time the temperature remained constant. 



The subject of the sulphides is perhaps the most interesting in 

 connexion with the meldometer when the pyro-chemical side is con- 

 sidered, as the charges that take place can be so much better observed 

 on the ribbon under the microscope, than when observed in connexion 

 with the blowpipe. As this Paper deals exclusively with the points 

 of fusion of minerals, very little can be said in connexion with 

 the sulphides, which nearly all appear to decompose at low tempera- 

 ture. 



A remarkable phenomenon which still remains unexplained was 

 observed to occur in the case of some minerals just before the point 

 of fusion was reached, and was particularly remarked in the case 

 of CuO. 



"When the temperature of fusion was nearly reached, it was 

 observed that round a single grain of the substance under observation, 

 a halo had formed, which increased in size as the temperature con- 

 tinued to rise. If the temperature remained constant the substance 

 did not fuse, but the halo continued to increase in size for a consider- 

 able period, but very slowly after the first thirty seconds, stopping 

 altogether in about two minutes (this was observed in one case with 

 CuO). One is inclined to think that material from the undersurface of 

 the substance formed some compound with the platinum of the ribbon, 

 and that this, being more fusible than the CuO, flooded out on the 

 ribbon, causing the halo appearance. But what combination could 

 occur between CuO and platinum? Again if the substance was 

 viscous, and a very bad conductor of heat, it might have been caused 

 by the undersurface in contact with the ribbon being melted first, and 

 flowing over the ribbon, which actually occurred in the case of olivine. 



