248 Mr Purris, The injiuence of dilution on the colour 



each solution contained the same amount of permanganate. The 

 solutions were placed in their respective cells or troughs, one of 

 which was 5 mm. long, a second one was lo4 mm. long, and a 

 thiixi one was 810 mm. long. The comparative solutions were, 

 therefore, made by diluting the strongest tme o0"8 and &2 times 

 respectively. 



The apparatus used in these experiments has been previously 

 described* by the author in some experiments on the absorption 

 spectra of solutions of various salts of didymium and erbium. The 

 source of light was a Welsbach burner without the glass, whose 

 rays passed through a small circular aperture and thence carried 

 through the solutions by a quartz lens ; and the emerging light 

 was focussed on the slit of the spectroscope. The times of exposure 

 vaiied from 1, 3 and 5 minutes: but the same length of time was 

 allowed for each series of comparative observations. In order 

 to make the comparison as strict as possible, blank tubes con- 

 taining distilled water and of exactly the same lengths and 

 diameters as those containing the solutions were placed in front 

 of the slit when photographs were taken of the stronger solutions 

 contained in the 5 mm. cell. By this arrangement, the light mys 

 passed through the same thickness of water. 



In the preliminary series of observations photographs were 

 taken immediately after the solutions were made ; and although 

 there appeared to be no ditierence in the colour of the solutions, 

 there were ditierences in the width of the bands as well as in the 

 amount of general absorption of light in the more refrangible 

 parts of the spectrum. After the solutions had stood for some 

 houi-s changes in the colour of the dilute solutions were also 

 observed. Fresh solutions of the permanganates oi barium and 

 potassium were then made, and they were allowed to stand two 

 days when photographs of the absorption spectra and changes in 

 the colour were noticed. 



The following notes describe these observations ; and in order 

 to shorten the description it should be stated that the phrases 

 "5 solution," '' 154 solution," and '"olO solution" mean that there 

 was the same amount of permanganate in the tubes whose lengths 

 were 5 mm., 154 mm., and 810 mm. respectively. 



The bauds observed were \\ 570, 546, 524, 504, 4SGf, and 

 they are numbered in the following notes so that 



X 570 = 1, 



546 = 2, 



524 = 3, 



504 - 4, 



486 = 5. 



* Proe. Camb. Phil. Soc. Vol. xii. pt. iii. p. 200. 

 t Leeoq de Boisbaudrau, Spectres hn)nneu.r, p. 109. 



