and the Dispersion of Opaque Bodies. 113 



2. Hydrated Peroxide of Lead. 



By the electrolytic decomposition of a solution of oxide of 

 lead in potash at the positive pole, a series of colours is obtained 

 which is commonly used to represent Nobilr's rings. The solu- 

 tion for this purpose is prepared by boiling litharge with* a strong 

 alkaline lye. As, however, comparatively little lead goes into so- 

 lution, and operations with strong alkaline fluids are attended with 

 inconvenience, it is more judicious to prepare the solution in the 

 following manner: — 50 grms. of acetate of lead dissolved in 

 water are poured, whilst being stirred, into a solution of 50 grms. 

 of tartar and 35 grms. of hydrate of soda, and after the precipi- 

 tate has disappeared is diluted with water to 500 cubic centims. 

 This liquid still gives good results when three-fourths of the lead 

 have been precipitated by the electrolytic process. 



As a positive electrode I have always used a platinum plate 

 about 1 decimetre square, which is preferable for the measure- 

 ments, as other metals might become oxidized by the active 

 oxygen ; the negative pole consists of two small plates of lead 

 or platinum, arranged on both sides of the platinum plate at 

 equal distances from the edges and bent sharply downwards. 

 This system, after insulation of the conducting-wires, it is easy 

 to regulate so that the positive platinum plate shall be uniformly 

 coloured. With these dimensions of the apparatus a small weak 

 Bunsen's element is sufficient to produce the layers. 



It appears to be generally considered that the body which 

 gives the interference-colours is peroxide of lead ; but the far 

 lower specific gravity (the statements of the density of PbO 2 vary 

 between 8'903 and 8*933) which I found as the result of nume- 

 rous concordant experiments induced me to investigate it more 

 closely. It followed that the coloured layers on the positive-pole 

 plate, whatever solutions were applied, are never PbO 2 , but a 

 hydrated oxide of definite composition, which only with difficulty 

 entirely loses its water at a higher temperature, without at the 

 same time any oxygen disappearing. In a thicker layer it forms 

 a compact, bright, blue-black body, which adheres firmly to the 

 platinum, and is by no means hygroscopic, so that its specific 

 gravity is easily determined. Three determinations, which dif- 

 fered very little in their results, gave, as the mean, for the den- 

 sity of hydrated peroxide of lead 



5 = 6-169, 

 a number which is very different from the specific gravity of the 

 dry hydrated peroxide of lead. 



For the investigation of the refraction and dispersion, it is pru- 

 dent to place the spectroscope in front of the decomposition-appa- 

 ratus with glass sides; in this way, since the liquid is colourless, 



Phil Mag. S. 4. Vol. 40. No. 265. Aug. 1870. I 



