256 Mr Purvis, The influence of dilution on the colour, etc. 



into MnOs and O, or into MnOa and O2. In such dilute solutions 

 the tension between the molecules of water surrounding the ions 

 must have been very considerable, and it may have been so great 

 as to destroy the MnOj condition, so that further changes took 

 place which became visible in changes of colour, in changes in the 

 width of the bands and in the liberation of oxygen ; and these 

 changes would be accelerated by the vibrations of light. The 

 intermediate stages of the changes might be represented by the 

 narrowing of the bands 1, 2 and 3 and the widening of the 

 bands 4 and 5, and these might correspond to changes of the 

 MnOj ions into MnOg and ions. The complete change in colour 

 from the well known permanganate tint to a light brownish yellow 

 colour in the more diluted solutions, the disappearance of the 

 absorption bands, and the increased general absorption represented 

 the complete change into MnOg and Oo. The MnOg would be 

 dissolved in the colloidal condition, and it would be represented by 

 the complete disappearance of the bands, and the increased general 

 absorption, whilst the oxygen partly escaped. 



DESCRIPTION OF PLATE XI. 



It has not been possible to reproduce all the original photographs comparable 

 with them in clearness and precision. The reproductions on the plate are from 

 photographs of original negatives enlarged one and a half times. The shaded parts 

 represent light issuing through the solutions ; the light bands represent the absorp- 

 tion bands ; and the lighter parts in the more refrangible regions represent general 

 absorption. 



1. Barium permanganate, 154 solution. 



2. do. do. 5 solution. 



These solutions were made in the dark, and they were kept out of contact with 

 the light during the experiments. They correspond to paragraph G in the notes. 



3. Barium permanganate, 154 solution. 



4. do. do. 5 solution. 



These solutions remained fully exposed to the action of sunlight. They corre- 

 spond to paragraph B in the notes. 



It will be noticed that the weak band at \570 is almost obliterated, although it 

 can be easily seen in the original photographs and in the enlargements therefrom. 

 The narrowing of the bands at XX 546, 524 and the widening of those at XX 504, 486 

 in the diluted solutions when compared with those in the stronger solutions are 

 weU marked. 



5. Zinc permanganate, 5 solution made in the dark and kept in the dark. 



6. do. do. 154 do. do. do. 



7. do. do. 154 do. fully exposed to the action of light. 

 These solutions correspond to paragraph L in the notes. 



8. Zinc permanganate, 5 solution made in the dark and kept in the dark. 



9. do. do. 310 do. do. do. 



10. do. do. 310 do. fully exposed to the action of light. 



These solutions correspond to paragraph M in the notes. 



The band at X 570 is only just visible; but the narrowing of the bands at 

 XX 546, 524 and the widening of those at XX 504, 486 when compared with those in 

 the stronger solutions are easily distinguishable. The general absorptions of the 

 solutions 6 and 7 do not show very marked differences : but there is a little 

 difference in those of the solutions 9 and 10. 



