COPPER BROMIDE AND ALUMINIUM CHLORIDE. 231 



showed that transmission began at 0.502/, and increased very gradually to 

 about 0.527/x. From this point on the solution was quite transparent. The 

 strip corresponding to the most concentrated solution recorded relatively weak 

 transmission of the parts of the spectrum less refrangible than 0.554/1 . 



The negative of which plate 21 (a) is a reproduction was taken with a 

 Cramer trichromatic plate. The photographic strips correspond to the five 

 more concentrated solutions of the complete set of ten. Obviously the strip 

 nearest to the scales pertains to the solution which contained the greatest 

 amount of aluminium chloride. The depth of the cell was here also 1.41 cm. 

 The time of exposure for the Nernst glower was two minutes. The strip cor- 

 responding to the solution of concentration 1.375 showed that transmission 

 began weakly at 0.473/<, approached gradually its full value, and continued 

 complete nearly to the end of the plate. The photographic record began at 

 about 0.552/e for the strip pertaining to the most concentrated solution. 

 The general outline suggested by the ends of photographic strips of plate 

 21 (a) (which ends show the beginnings of transmission) curves in a direction 

 exactly opposite to that of the contour outlined by the first five strips of plate 

 21 (6). In other words, for the more concentrated solutions of the series, the 

 successive increments of absorption decreased as the concentration of the 

 aluminium chloride increased. By means of the two-compartment cell, eye 

 observations were made on the solutions in pairs. The results obtained 

 photographically were confirmed in all respects. One of the most important 

 facts noted was that as the concentration of the dehydrating agent increased 

 in arithmetical progression, the successive increments of the absorption re- 

 gion, which comprised the ultra-violet, first increased and then subsequently 

 decreased. Stated otherwise, the curve of absorption apparently possessed 

 a point of inflection. The existence of the comparatively long region of 

 weak transmission of the green for the 6th solution was established visually 

 as well as photographically. As far as the red end of the spectrum was 

 concerned, every solution absorbed the red a little more than the next less 

 concentrated member of the series. It was only possible to obtain a value 

 for the mean displacement of the red or orange end of the spectrum. For the 

 aqueous solution which contained only copper bromide, and for the most 

 concentrated member of the set, transmission of about the same intensity 



i 



began, respectively, at 0.670/1 and at 0.645,. Therefore, since there were 

 ten solutions in the series, the average displacement of the red end of the 

 spectrum was 10 A. U. 



The lowerings of the freezing-point of water produced by the solutions 

 which contained copper bromide and aluminium chloride are recorded in the 

 table on the following page. 



The solution of concentration 1.925 changed from reddish-brown to pea- 

 green as the temperature fell from room value to the neighborhood of 50. 



