108 A STUDY OF THE ABSORPTION SPECTRA. 



difficulty is that of the effect of dilution. The greater the dilution the 

 greater the dissociation, and, therefore, according to the theory of Arrhen- 

 ius, for very dilute solutions the U0 2 group should exist in the ionic condi- 

 tion and the absorption spectrum of all the salts should be the same, i. e., 

 the uranyl bands should then occupy the same positions independently of 

 the kind of salt. No effect of this kind is to be noticed, as was shown above 

 under the division describing the effect of dilution. It is intended to use 

 much more dilute solutions in the future. 



Uranyl Bromide in Water. 



In Plate 72, B, we have the absorption spectra of uranyl bromide 

 (crystalline salt, U0 2 Br 2 .H 2 0). The exposures were 1 minute to the Nernst 

 glower with 0.8 ampere and a slit-width of 0.08 mm., and a 3-minute expo- 

 sure to the spark. The depth of the cell was 3 mm. and the concentrations 

 were 1, 0.75, 0.5, 0.33, 0.25, 0.2, and 0.16 normal, the spectrum for the most 

 concentrated solution being that next to the comparison spectrum. 



It will be seen that uranyl bromide gives rise to an absorption very simi- 

 lar to uranyl nitrate, there being a blue-violet and an ultra-violet band. The 

 absorption for the 1 normal solution is complete for all wave-lengths less than 

 X 4470. For 0.75 normal the limits of the blue-violet band are XX 4450 and 

 3900, the ultra-violet band beginning at X 3800. For 0.5 normal the blue- 

 violet band shows considerably larger transmission, having almost com- 

 pletely faded out. Its middle comes at about X 4250. The ultra-violet band 

 gradually recedes towards the shorter wave-lengths as the dilution increases. 



The uranyl bands themselves do not show nearly so prominently as 

 in the case of the chloride and nitrate. The bands are very wide and diffuse. 

 Their approximate positions are: a, 4880; b, 4720; c, 4560; d, 4450; e, 

 4280; /, 4160. 



The spectrogram showing a series to test Beer's law is given in Plate 

 72, A. The concentrations, beginning with the strip nearest the num- 

 bered scale, are 1, 0.75, 0.5, 0.33, 0.25, 0.2, and 0.16 normal, the correspond- 

 ing depths of cell being 3, 4, 6, 9, !2, 18, and 24 mm., respectively. Beer's 

 law is found to hold, the limits of the absorption bands being independent 

 of the above range of concentrations. The ultra-violet band and blue- violet 

 band have a small region of transmission between them which shows in the 

 original film but not on the print from it. This region of transmission is 

 very faint and is quite a sensitive index to any possible deviations from 

 Beer's law. It shows no changes in intensity with change in concentration. 

 The uranyl bands a and b show, although they are very indefinite. Concen- 

 tration does not affect their intensity in the least as far as can be detected. 



Uranyl Sulphate, Temperature Effect. 



A spectrogram (Plate 73, A) was made for a normal solution of uranyl 

 sulphate, the depth of cell being 3 mm. The time of exposure was 90 seconds 

 to the Nernst glower with a current of 0.8 ampere and a slit-width of 0.20 

 mm. The time of exposure to the spark was 6 minutes. Starting with the 

 strip nearest to the numbered scale, the temperatures were 5, 19, 32, 54, 

 67, and 84. 



