190 HYDRATES IN AQUEOUS SOLUTION. 



than anywhere else in the visible spectrum, with the exception that a faint 

 narrow absorption band was just noticeable at 0.636/*. Transmission ceased 

 at about 0.588^. 



CUPRIC CHLORIDE. [See plates 3 (6), 4 (a), and 5.] 



The concentrations of the solutions were 0.239, 0.318, 0.557, 0.795, 1.113, 

 1.272, 1.431, 1.590, 1.749, 2.067, 2.385, 2.703, 3.021, 3.339, 3.657, and 3.976. 

 The successive differences in concentration were 0.079, 0.239, 0.239, 0.318, 

 0.159, 0.159, 0.159, 0.159 for the first nine solutions, and 0.318 for all the rest. 

 The last number belongs to the mother-solution of copper chloride. 



The color of the most dilute solution was a delicate blue. Then, as the 

 concentration increased, the color of the solutions passed through various 

 shades of greenish-blue, of bluish-green, of light green, and finally the mother- 

 solution was a deep, dark green. 



Plate 5 will be considered first. The solution whose concentration was 

 0.239 normal has its absorption spectrum shown by the photographic strip 

 nearest to the numbered scale. The remaining strips succeed one another 

 in the order of increasing concentration, so that the strip nearest to the 

 comparison spectrum corresponds to the mother-solution. The effective 

 depth of the cell was 0.67 cm. Ninety seconds and eighty seconds were the 

 respective times of exposure of the Nernst glower and of the spark. 



The spectrogram shows two regions of absorption, the one in the ultra- 

 violet, violet-blue, etc., and the other in the orange. Absorption in the region 

 last named is to be inferred from the fact that the ends of the photographic 

 strips corresponding to the more concentrated solutions do not lie in a straight 

 line at right angles to the length of the spectrogram, but recede from the end 

 of the plate as the concentrations increase. 



If there has been no strong selective absorption in the yellow-orange, 

 then all the photographic strips would have faded out at the same distance 

 from the end of the plate, due simply to the lack of sensitivity of the Seed 

 negative films to light of relatively small refrangibility. The most dilute 

 solution transmitted a faint trace of the spark line at wave-length 3250.5, 

 but nothing of shorter wave-length. The absorption of the solution of con- 

 centration 0.318 was very little more extended than that of the most dilute 

 member of the series. 



This is due to the fact that these two solutions did not differ greatly in 

 concentration. To obtain the clearest view of the variation of absorption 

 with concentration, it is best to fix the attention only upon those photographic 

 strips that correspond to the solutions whose concentrations differed by the 

 same amount, e. g., by 0.318. In other words, omit the first, second, third, 

 sixth, and eighth strips from consideration for a moment. It is then seen 

 that a smooth curve is presented by the limits of the region which absorbs 



