and the Dispersion of Opaque Bodies. Ill 



alternate, which after about five or six hours blend into one another 

 and form a pale brownish red closely resembling metallic copper. 

 If the layer be then observed with the spectroscope, four dark bands 

 will show themselves between Fraunhofer's lines P and B. The 

 layer can be made more than double as thick, and then in the 

 interval named eight or nine minimum-bands obtained. Such a 

 spectrum is, in consequence of the bounding of the dark bands 

 by the high intensities of light of the maxima, far more brilliant 

 than an ordinary absorption-spectrum. The spectrum and also 

 the colours are most splendid when the reflection takes place 

 from silver; for this purpose a thin silvered glass forms the best 

 support for the layer, which moreover conducts sufficiently well 

 and makes possible the examination of the layers in transmitted 

 light. 



Cleansing the surface of the platinum from impurities, which 

 are particularly obstructive to the formation of a uniform layer, 

 is most easily accomplished if it is first covered with copper by 

 means of an alkaline solution of copper. Instead of platinum, 

 ordinary tinfoil may be applied with advantage, which, before 

 the plating, is cleaned by breathing on it and polishing it with a 

 linen pad sprinkled with prepared chalk. The tinfoil is coppered 

 by the current of a weak Bunsen's element in an alkaline copper 

 solution ; the tinfoil covers itself uniformly with a bright copper 

 coating and with condensed hydrogen, which, precipitated simul- 

 taneously with the copper, adheres far better to the plate than it 

 would without this — that is, better than if the tinfoil in a soda so- 

 lution were to be covered with hydrogen. After from ten to fifteen 

 minutes the current is broken, and the copper-covered tinfoil, with- 

 out being washed, is instantly suspended in the above-mentioned 

 solution of copper. The reduction by the hydrogen adhering to 

 the plate soon begins ; the copper is gradually coloured golden- 

 yellow, red, blue, whitish, greenish yellow, and again golden 

 yellow. It takes about fifty minutes for these colours to deve- 

 lope themselves ; the development afterwards proceeds slowly; 

 but it is best to interrupt it at the second golden-yellow, where a 

 minimum near the line F appears in the spectroscope, and to 

 cover the plate afresh with hydrogen if stronger layers are 

 wanted immediately. For this purpose a soda solution of 1*035 

 specific gravity is used, which is first diluted with an equal quan- 

 tity of water ; if the lye or the current be too strong, the suboxide 

 of copper may be reduced to metallic copper. With the pre- 

 scribed data, in ten minutes the layer of suboxide of copper will 

 be covered with a layer of hydrogen, which within an hour re- 

 duces from the solution of copper a layer of suboxide of the thick- 

 ness of half a wave-length. This method may also be used as 

 a convenient lecture experiment, to make visible the hydrogen 



