44 ABSORPTION OF LIGHT BY WATER CHANGED 



aqueous solution the water present absorbs just as much as pure, unconi- 

 bined water. 



It became at once obvious that we could not measure the absorption 

 spectrum of a solution, subtract from it the absorption due to water, and 

 conclude that the remainder was the absorption due to the dissolved sub- 

 stance ; since the water in the solution has very different absorption from an 

 equal amount of pure, uncombined water. 



We then carried out a number of experiments in cells whose depths could 

 be easily and accurately adjusted, with different substances, in the following 

 manner: The absorption spectra of a number of different substances were 

 first measured , then the absorption spectra of water having the same depths 

 of layer as the water in the solutions. For certain substances the pure 

 water was more opaque than the solutions, and for other substances the 

 water was more transparent. The percentage transmission that is, the 

 deflection of the racliomicrometer for the solution, divided by the deflection 

 for water for the first-named substances amounted to more than 100 per 

 cent. Pure water had a different absorption from an equal depth of water 

 in the solution, and since this difference varied from one dissolved substance 

 to another, it is obvious that this method was not the one to be followed. 

 It would be very difficult, not to say impossible, to interpret the results 

 obtained by dividing the radiomicrometer deflections for the solution by 

 those for pure water. We should simply be obtaining the transmission of 

 the solution in terms of pure water, which was not what was desired. 



What we want to know is the actual absorption or transmission of the solu- 

 tion, and then that of pure water having a depth of layer that was just equal 

 to that of the water in the solution. These two sets of results could then be 

 compared with one another. 



HYDRATED AND NONHYDRATED SUBSTANCES. 



In this earlier work we had, however, noted that solutions of those sub- 

 stances which are largely hydrated are more transparent than pure water 

 having the depths of the water in the solutions in question. Solutions of 

 nonhydrated substances, or of only slightly hydrated substances, provided 

 the substances themselves do not absorb light, are not more transparent 

 than pure water having the same depths as the water in the solution. It 

 would seem from this observation that water combined with the dissolved 

 substance had less absorption of light than pure, uncombined water. To 

 test this quantitatively the following procedure was adopted. 



METHOD OF PROCEDURE. 



A solution of the substance in question was prepared of known concen- 

 tration and its specific gravity determined. This solution was placed in one 

 cell set to a depth of say 21 mm. Some of the same solution was then 

 placed in another cell set to a depth of say 1 mm. Light of a known wave- 

 length was then passed through the one solution and the deflection noted. 

 Light of this same wave-length was then passed at once through the other 



