. SPECTROSCOPIC EVIDENCE FOR THE HYDRATE THEORY. 239 



results of this nature were sometimes corrected by the negative taken on 

 the trichromatic plates, and they were always brought to light by the obser- 

 vations with the spectroscope. 



The other cause for the apparently anomalous behavior of the contour 

 of the ultra-violet region of absorption is the gradual superposition of the 

 bands belonging to the two colored salts. For example, when calcium 

 chloride was added to the solution containing copper bromide, the absorp- 

 tion is complicated by the production in the solutions of calcium chloride 

 and copper chloride. Of course the compound absorption spectra of solu- 

 tions of the kind just mentioned can be distorted by the peculiarities of 

 photographic processes, as well as the simpler cases. 



The well-known color changes which take place when fairly large variations 

 are made in the temperature of the solutions are in complete agreement with 

 the present theoiy. For example, when a red solution of cobalt is sufficiently 

 warmed it becomes blue. As already explained the blue and red colors corre- 

 spond, respectively, to wide and narrow bands, or, in terms of the theory, to 

 small hydration and to relatively large hydration. But we know from other 

 lines of evidence that the complexity of the hydrates decreases with rise in 

 temperature, and thus we have another illustration of the wide applicability 

 of the theory. 



When a solution of cobalt chloride has been made deep blue at room 

 temperature, by the addition of some one of the strong dehydrating agents, 

 it turns red when cooled to the neighborhood of its freezing-point. The 

 explanation of this phenomenon in terms of the theory is obvious. The 

 corresponding color changes for the copper salts can be explained in exactly 

 the same manner as has been done for cobalt chloride. 



Before concluding the discussion of the interpretation of the spectroscopic 

 data, it is desirable to emphasize the fact that all the color changes investi- 

 gated take place very gradually and continuously, and that there are not 

 sudden variations in the bands and regions of absorption. Perhaps the most 

 striking color change of all is that from blue to green manifested by copper 

 chloride. The one solution is blue because there is no absorption band in 

 the violet, and yet there is a strong band in the red. The other solution is 

 green because a band has pushed its end out of the ultra-violet into the visi- 

 ble spectrum, and absorbed the violet and perhaps weakened the blue. 



The band in the red has likewise encroached on the visible spectrum. 

 The apparently abrupt change from blue to green is only due to the fact 

 that the visible spectrum is limited by the sensitivity of the retina to a certain 

 region of wave-lengths. // we could see distinctly into the ultra-violet as far 

 as 0.200/'.,ie would probably be less hasty in writing about mixtures o} the colors 

 oj certain ions and molecules. What is meant by the color oj an ion? Uow do 

 absorption bands mix colors? 



