112 M. W. Wernicke on the Refractive Indices 



condensed on the surface of a metal, and to determine its amount 

 quantitatively. 



To ascertain the specific gravity of the suboxide of copper, the 

 absolute weight and also the loss of weight of a platinum plate 

 in water were determined by several experiments, and it was then 

 coated with a thicker layer of suboxide (being exposed twelve, and 

 from that to thirty-six hours to the action of the current). When 

 the layer becomes opaque, the copper colour changes to a dark 

 violet; the least traces of metallic copper, which in the event of 

 a too strong current might be mixed, would change the beautiful 

 violet into an ugly dark brown. When the layer has reached 

 the desired thickness, the absolute weight and the loss of weight 

 of the whole plate are determined. 



From four experiments with layers of different thicknesses and 

 almost perfectly agreeing in their results, the density of the 

 suboxide of copper (at 15° C.) was found to be 



5 = 5-975. 



To determine the refractive indices, a rectangular sheet of tin- 

 foil, 10*72 by 7*76 centims., was copper-plated, and then a uni- 

 form layer of suboxide of copper deposited on it. 



The increase of weight after the minimum-bands had passed 

 eleven times through the line F amounted to 0*08975 gr., and 

 after seven passages of the band through the line C to 0*08970 gr. 

 From these numbers and equations (8) and (9), together with 

 Fraunhofer's numbers for the wave-lengths of these lines in air, 

 we calculate the refractive exponents for F and C to be 2*963 

 and 2*558. 



A second rectangular sheet of tinfoil, whose surface was 18230 

 square millims., gave for the refractive indices of the lines E and 

 D, after nine and six passages of the minimum-band, the num- 

 bers 2*816 and 2*705 respectively. 



A third plate, 21946 square millims. in surface, gave for B 

 the value 2*534; so that the refractive indices of suboxide of 

 copper (Cu 2 O) are as follows : — 



Fraunhofer's line Refractive index 



B =2*534 



C =2*558 



D =2*705 



E =2*816 



F =2*963 



The refraction, like the dispersion, of suboxide of copper is 

 therefore, apart from the doubtful determinations, the greatest 

 which has hitherto been observed for a solid body. 



