192 HYDRATES IN AQUEOUS SOLUTION. 



CUPRIC BROMIDE. [See plates 3 (c), 6, and 7 (a).] 



The concentrations of the solutions photographed were 0.175, 0.350, 

 0.394, 0.437, 0.481, 0.525, 0.568, 0.612, 0.656, 0.700, 0.874, 1.049, 1.224, 

 1.399, and 1.574. The first, second, sixth, and tenth solutions, as well as 

 those having greater concentrations than the tenth, form a series in which 

 the successive concentrations differ by the same amount, namely, by 0.175. 

 The remaining solutions were interpolated in the series just mentioned, and 

 the successive increments of concentration thus produced are all equal to 

 0.044, i. e., to one-quarter of the greater common difference 0.175. In this 

 case the concentration of the mother-solution was 2.186. The solutions 

 when in the plane parallel cell, which was adjusted at a depth of 2 mm., varied 

 from almost no color for the most dilute solution, through different shades 

 of clear green and yellowish-green, to a very dull brownish-green for the solu- 

 tion of concentration 1.574. In the bottles, of course, the colors were much 

 deeper than in the cell, and the more concentrated solutions of the set photo- 

 graphed, together with the mother-solution, were apparently opaque to light. 



In plate 6, as usual, the photographic strips showing the absorption spectra 

 of the solutions of concentration 0.175 and 1.574 are adjacent, respectively, 

 to the numbered scale and to the comparison spark spectra. The fifteen 

 strips succeed one another in the order of increasing concentration. The 

 most dilute solution transmitted an extremely faint trace of the line at 3250.5, 

 but nothing of shorter wave-length. Due regard being had for the successive 

 differences in concentration, it is seen from the spectrogram that the region 

 of absorption which includes the ultra-violet increases in a perfectly regular 

 way for about the first twelve solutions. The thirteenth strip begins to show 

 weak absorption at about 0.517;<, and the fourteenth brings out the presence 

 of this band very noticeably. The long, penumbra-like region which extends 

 from about 0.45/^ to 0.53/t on the fourteenth strip is entirely absent from 

 the fifteenth. The shortest wave-length recorded by the spectrogram for 

 the most concentrated solution is approximately 0.543/z. The last three 

 or four photographic strips, which correspond to the more concentrated solu- 

 tions, indicate the presence of a region of weak general absorption in the 

 orange and red. In drawing conclusions from spectrograms of solutions of 

 copper bromide, care must be taken to avoid errors that might arise from 

 the maxima and minima of sensitivity of the photographic emulsions, espe- 

 cially when Seed orthochromatic films are involved. [See plate 1 (6).] 



The statements made above are substantiated for the five most concen- 

 trated solutions by plate 3 (c), the negative for which was a trichromatic 

 Cramer plate. The cell was adjusted to a depth of 2 mm. as before, and the 

 length of each exposure to the light from the Nernst glower was two minutes. 

 That the absorption in the orange-red is weak, in general, is shown by the 



