1869.] Coal Washing. ' 487 



The explanation of the phenomena is probably as follows : — In 

 the case of the moving line, the light-emitting plane in the opal is 

 somewhat broad, and has the property of giving out at one end, 

 along its whole height and for a width equal to the breadth of the 

 band, say, red hght ; this merges gradually into a space emitting 

 orange, and so on throughout the entire length of the spectrum, or 

 through that portion of it which is traversed by the moving line in 

 the instrument ; the successive pencils (or rather ribbons) of emitted 

 Ught passing through all degrees of refrangibility. It is evident 

 that if this opal is slowly passed across the slit of the spectrum 

 microscope, the sht will be successively illuminated with light of 

 gradually increasing refrangibihty, and the aj^pearance of a moving 

 luminous line will be produced ; and if transmitted hght is used for 

 illumination the reversal of the j^henomena will cause the production 

 of a black line moving along a coloured field. A diagonal hue will 

 be produced if an opal of this character is examined in a sloping 

 position. 



The phenomenon of a spiral line in relief, roUing along as the 

 opal is moved, is doubtless caused by modifying planes at different 

 depths and connected by cross planes ; I can form a mental picture 

 of a structure which would produce this effect, but scarcely clear 

 enough to enable me to describe it in words. 



It is probable that similar phenomena may be seen in many, if 

 not all, bodies which reflect coloured light after the manner of 

 opals. A magnificent specimen of lumacelH, or fiery limestone, 

 from Italy, kindly presented to me by my friend David Forbes, 

 shows two sharp, narrow, and parallel bands in the red. I have 

 also observed similar appearances in mother-of-pearl. The effects 

 can be imitated to a certain extent by examining " Newton's rings," 

 formed between two plates of glass, in the spectrum microscope. 



II. COAL WASHING. 

 By F. C. Danvers, A.I.C.E., M.S.E. 



In most mining operations it has been the practice, from the 

 earliest times of which we have record, to wash the minerals 

 obtained from beneath the surface of the earth in water, before 

 subjecting them to further purification by means of fire, for the 

 l^urpose of separating a portion of the earthy matters, with which 

 they are invariably mixed, from the ore. In former years this 

 was accomplished in a very rude manner, which doubtless involved 

 the loss of a not inconsiderable amount of pure ore. Dr. Percy, in 

 his valuable work on Metallurgy, gives an historical notice of the 



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