238 HYDRATES IN AQUEOUS SOLUTION. 



to cite specific cases. For all the cases examined in which only one colored 

 salt was present, and for all the absorption bands that had definite limits 

 and were not accompanied by long regions of weak general absorption, it 

 was found that as the concentrations of the solutions increased in arithmetical 

 progression, the increments of absorption at one side of the band gradually 

 became less and less. This statement applies both to the solution which 

 contained only one salt and to those that contained a dehydrating agent, 

 together with a colored salt. Since a one-sided region of absorption in the 

 ultra-violet may be looked upon for sake of convenience as the least refran- 

 gible side of the band whose center lies beyond 0.200/t, and hence out in the 

 region of the Schumann waves, and since a one-sided region of absorption 

 in the red is actually the more refrangible side of a band whose center is 

 situated in the infra-red, the preceding idea may be generalized by saying 

 that the boundaries of all the absorption bands studied were concave towards 

 their respective centers or maxima, as the case may be. The fact that the 

 increments of absorption decrease as the concentration increases means 

 that the colored system resists the transfer of its associated water molecules 

 to the dehydrating agent more and more, as the actual number of its asso- 

 ciated water molecules becomes less and less. From the standpoint of the 

 colored salt the process of making up solutions of the same volume, and at 

 the same time increasing the concentration of the salt, is equivalent to taking 

 away some of its molecules of water. The bearing of this on the decreasing 

 increments of absorption is obvious. 



The cases where the spectrograms show that the boundary of the ultra- 

 violet region of absorption was convex towards the shortest wave-lengths are 

 not really at variance with the preceding explanation, since this convexity is 

 due to either of two causes, or to both causes acting simultaneously. One of 

 these conditions is as follows : For a given length of exposure, time of devel- 

 opment, color of light, etc., there is an inferior limit of intensity of light, such 

 that if the intensity falls below this value the sensitized film or plate will 

 show no darkening even if over-developed. Now, some of the solutions 

 had very long, weak regions of general absorption at the edge of the ultra- 

 violet region of complete absorption. Also, the intensity of this penumbra! 

 companion to the main band increased as the concentrations of the solutions 

 of the series increased, at such a rate as to cause the photographic film to 

 record, apparently, increments of absorption that were too large. In other 

 words, for the more dilute solutions the plate would record the light which 

 was only slightly weakened by the general absorption, as if there were no 

 such absorption at all, and then for the more concentrated solutions it would 

 not give any record of the faint light which passed through the penumbral 

 region. The transition from one condition to the other might be either 

 gradual or apparently sudden, according to obvious circumstances. Spurious 



