52 ABSORPTION SPECTRA OF SOLUTIONS. 



short wave-lengths. In the red, the limit of transmission for the most 

 concentrated solution of set A is at A 6400, and for the most dilute at 

 A 6550. For B the corresponding figures are A 7250 and X 7330. 



COPPER BROMIDE IN WATER MOLECULES CONSTANT. (See Plate 39 A.) 



The concentrations of the solutions, beginning with the one whose 

 spectrum is adjacent to the numbered scale, were 1.50, 1.22, 0.91, 0.68, 

 0.52, 0.415, and 0.335; the depths of absorbing layer were 3, 4, 6, 9, 13, 

 18, and 24 mm., respectively. 



No data giving the dissociation of copper bromide were at hand, and 

 hence it was assumed to be the same as that of copper chloride. This 

 assumption is perhaps not absolutely correct, but the change in dissocia- 

 tion with change in concentration is undoubtedly so nearly the same that 

 the conditions of "molecules constant" were fulfilled in the series as given 

 to a very high degree of accuracy. 



The concentrations and depths of cell are exactly the same as those 

 used in making B of Plate 31; hence the two spectrograms serve well for 

 comparing the absorbing powers of copper chloride and copper bromide. 



The limits of transmission for the first and seventh solutions in the 

 region of shorter wave-lengths are A 5400 and A 3990, respectively, those 

 in the red being A 6550 and A 6400. It appears, therefore, that the bromide 

 absorbs more strongly in the violet than does the chloride, while the two 

 have about the same absorbing power in the red. It might be argued 

 that since the copper bromide molecule is heavier than that of copper 

 chloride, we should expect both absorption bands of the former to be 

 shifted towards the red; and that taking this shift into account the result 

 would indicate a greater absorbing power for the bromide throughout. 

 If this were so, then we should expect that, with decrease in concentration, 

 the absorption bands of the bromide would move towards the violet, as 

 referred to the same bands for the chloride. The reason for this is that 

 with decrease in concentration the salts become more and more strongly 

 dissociated, and, hence, in dilute solutions the spectra ought to resemble 

 each other more and more closely. Now, the violet edge of the region 

 of transmission moves towards shorter wave-lengths by about the same 

 amounts for the two salts, when the changes in concentrations are the 

 same. The red band seems to move a little more towards the violet in the 

 bromide than in the chloride, from the measurements given; but on super- 

 imposing the two negatives the two seemed identical, except for the strip 

 corresponding to the most concentrated bromide solution, which shows 

 more absorption, due to the large amount of general absorption of this 

 solution. As stated before, measurements on the limits of transmission in 

 the case of absorption bands, having such hazy edges as the one we are 

 dealing with here, are liable to very considerable errors. 



COPPER BROMIDE IN METHTL ALCOHOL BEER'S LAW. (See Plate 40.) 



The concentrations of the solutions used in making the negative for 

 A, beginning with the one whose spectrum is adjacent to the numbered 

 scale, were 0.089, 0.071, 0.056, 0.045, 0.036, 0.028, and 0.022, the corre- 



