20 8 Annals of the Philosophical Club 



was offensive. Sir Joseph Hooker said the white ant had 

 been introduced from Africa into St. Helena only about 

 fifteen years ago. 



Oct. 3ist, 282nd meeting. In reply to a question about 

 the principle of Edison's electric light, Dr. Siemens said 

 that he had only heard of it that day, but understood that 

 two iridium wires had their ends formed into cups, which 

 were brought near together in opposition in the middle of 

 a small glass globe, partially exhausted. Thus a current 

 of high intensity, passing between them, formed a zone of 

 light. It was, however, a question whether the edges of 

 the cups might not be dissipated. 



Sir W. Grove said that zinc in a minutely pulverized state 

 was largely deposited on the walls of the containing vessel 

 from two zinc points in vacuo. Professor Frankland gave 

 his experience that after working for months with platinum 

 points, though they were not sensibly diminished, a deposit 

 of the metal had discoloured the containing tube. With 

 aluminium points there was no apparent diminution of 

 substance, but no discoloration of the tube. 



1879. Feb. 27th, 286th meeting. Professor Maskelyne 

 exhibited a large and richly-coloured emerald from Santa 

 F6 de Bogota (Colombia), attached to a characteristic 

 piece of the rock in which it occurs. The crystal looked as 

 if it had been chipped at the edges by ill-usage, which, 

 however, could not be the case, for he had himself carefully 

 detached the matrix in which it had been all but embedded. 

 Two other crystals of emerald in the British Museum, which 

 he had also freed from the enclosing rock, had the same 

 chipped character. His interpretation was that the emerald 

 had not been formed in the rock where it now occurred. 

 This was a limestone of Cretaceous age, with calcite and 

 dolomite crystals, some pyrites, and a large amount of 

 powdery carbonaceous material, quite unlike the mica- 

 schists in which it is elsewhere found. Possibly this chipped 

 character may explain the story that the emerald in 

 Colombia is soft when first found, but is hardened by 

 exposure. 



