COPPER CHLORIDE AND CALCIUM BROMIDE. 219 



for plate 16 (6). The angle of the cell was about 11.7' and the absorbing 

 layer varied linearly in depth from zero to 0.11 mm. Each of the three suc- 

 cessive exposures with the Nernst glower had a duration of two minutes. 



The contour of the regions of absorption was essentially the same as 

 recorded by the negative of plate 16 (a) as by the film of plate 16 (6). Both 

 emulsions gave the wave-length 4525 for the more refrangible minimum of 

 absorption. The maximum of absorption for the band in the green was at 

 0.500/<, according to the trichromatic plate. Therefore, the two spectrograms 

 differed by about 150 A. U. in the wave-lengths which they give for the 

 absorption band in the green. Since this band is wide and diffuse, it is fail- 

 to assign the number 0.508/< as the approximate position of the middle of 

 the band. Plate 16 (a) shows correctly the presence of weak general absorp- 

 tion in the red. The spectrograms for the wedge-shaped layers of solution 

 proved beyond question that the band in the green, as indicated by plates 

 14 (a), 14 (6), and 15 (b) for the more concentrated solutions, had actual 

 existence and was not due solely to the weak regions of sensibility of the 

 photographic emulsions. The tw r o spectrograms of plates 16 (a) and 16 (b), 

 being placed side by side, illustrate with unusual clearness the part which the 

 photographic plate can play in producing spurious results. This is especially 

 noticeable in the region of the spectrum between 0.55/t and 0.63/*. 



To serve as a check on the data obtained by the photographic method, as 

 well as for the sake of greater completeness, eye observations were made 

 on the spectra of several of the solutions. The cell with the upper and 

 lower compartments was used ; hence, the length of the absorbing layer was 

 2.5 cm. for each solution. The solution of concentration 0.254 of calcium 

 bromide absorbed the red only a little more than the solution which con- 

 tained none of this dehydrating agent. These two solutions transmitted 

 the yellow and green with equal intensity, at least as far as the eye could tell. 

 The more concentrated solution absorbed almost all the violet, whereas the 

 less concentrated one readily transmitted this color. This accounts for the 

 fact that the former solution appeared bluish-green in the cell, whereas the 

 latter had a clear blue tint. The solution of concentration 0.508 absorbed 

 the red just a little more than the one of concentration 0.254. The change 

 was too small to admit of quantitative determination with the dispersion 

 used. For both solutions faint transmission began in the neighborhood of 

 0.639/(. The yellow and green are not as bright for the more concentrated 

 solution as for the less concentrated one. Also, the former absorbed consid- 

 erably more of the blue than the latter. Quantitatively, the same changes 

 were observed when the spectrum of the solution of concentration 0.763 

 was compared with that of the solution of concentration 0.50S. The stronger 

 solution of this pair cut off almost all of the blue and very appreciably weak- 

 ened the green. 



