110 M. W. Wernicke on the Refractive Indices 



of the current have obtained tlie same rings. The only excep- 

 tions to this are the few precipitates which he obtained on gold 

 or platinum. In the following paragraphs I explain the methods 

 by which serviceable interference-layers are best obtained for 

 optical examination, and how their optical constants and specific 

 weight are determined. 



1. Suboxide of Copper. 



In the preparation of the interference-layers of this body two 

 different ways are available. In both cases a solution consisting 

 of 30 grs. hydrate of soda, 60 grs. seignette salts, and 25 grs. 

 blue vitriol in 500 cubic centims. of water, serves as a decompo- 

 sing liquid. Using as cathode a thin platinum plate about 1 deci- 

 metre square, and, as anode, two* copper points at a distance of 2 

 centims. on each side of the middle point of the plate (each pre- 

 senting a surface of a few square millimetres), by means of the 

 current of a smallf Bunsen's element only so much hydrogen 

 is condensed on the platinum plate as will reduce the oxide of 

 copper to suboxide. The surface of the anode may at first be 

 taken a little larger, for the first precipitate is formed slowly ; the 

 anode may afterwards be lessened to the prescribed size, to be 

 certain that no metallic copper is reduced along with the prot- 

 oxide. If the anode be too large, then at first pure protoxide is 

 obtained ; but afterwards the hydrogen, which is developed more 

 abundantly, reduces the oxide either wholly or partially to metal. 

 The limit between the two processes, however, is tolerably wide; 

 and the smallest traces of metallic copper can be recognized by 

 placing iodine on it, which leaves the suboxide perfectly un- 

 changed, whilst it immediately changes the metal into subiodide. 



If the surface of the platinum was sufficiently clean (the 

 cleansing is best done by rubbing the plate with linen cloth 

 soaked with a solution of soda, and by placing the plate for some 

 time as a cathode in an alkaline solution so that it becomes 

 electrolytically covered with hydrogen), the colours soon appear 

 in the following order — gold, brown, purple, blue, &c.J, and re- 

 peat themselves within a few hours three or four times almost un- 

 changed ; as the thickness increases, only pale green and pale red 



* I must here observe that the laws of divided currents in fixed conduc- 

 tors for this and similar fluids are by no means applicable ; the influence 

 of polarization so preponderates that, even by using one point, both sides of 

 the cathode almost equally colour themselves. This is so much more the 

 case the weaker the current and the slower the decomposition. 



t The resistance of alkaline copper solution is so considerable that a 

 small piece of copper or zinc wire accomplishes the same effect as a large 

 Daniell's element, if in both cases equal exciting fluids are applied. 



X All the substances investigated show at first Newton's rings in the 

 transmitted light when the reflection takes place from a metal. 



