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'* when a plate of transparent mica or talc, when heated red 

 '^hot, is separated into multitudes of thin layers, each of 

 "which, of inconceivable thinness, is found to he highly 

 " transparent, while the entire plate assumes the lustre of a 

 " plate of silver." 



Mr. Dancer sent for exhibition several pieces of talc, which 

 in places, by the action of the blowpipe, had been heated to 

 redness ; the films were thereby separated, and the raised or 

 blistered portion gave a metallic lustre like silver. 



Mr. W. C. Unwin believed that the metallic lustre was due 

 not to pigment but to the reflection of light from internal 

 surfaces of the scales through the transparent outer layer. 

 The light so reflected appeared to be modified in two ways — 

 by the ribs or striae ; it was dispersed by them so that the 

 scales were lustrous at various angles, and it was also in some 

 cases coloured by interference caused by them. Iridescence 

 appeared to be also produced in some scales by the thinness 

 of the laminae through which the light was refracted causing 

 interference. Mr. Unwin exhibited a number of specimens 

 to illustrate his arguments. 



Mr. Dale referred to beautifully coloured films which 

 arise upon various chemical solutions, the metallic brilliancy 

 of which may arise from similar causes. 



Mr. Sidebotham observed that the metallic appearance 

 was not due to any colouring matter in the scales, as chemical 

 agents, which destroy the coloured scales, have no eflect 

 whatever on these metallic ones ; he also mentioned a curious 

 polarizing effect produced by crossing the metallic scales of 

 Plusia bractea. 



Mr. Sidebotham exhibited the metallic scales from Plusia 

 orichalcea, Plusia bractea, Plusia festucae, Plusia concha, &c., 

 illustrative of his remarks. 



It was ultimately resolved that the discussion should be 

 adjourned so as to enable the proposer of the subject, and other 

 gentlemen not present, to express their views. 



