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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 be 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 effect 
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 festucse, 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. 
