52 ABSORPTION OF LIGHT BY WATER CHANGED 



DISCUSSION OF THE RESULTS. 



An examination of the tables of data for potassium chloride, ammonium 

 chloride, and ammonium nitrate that is, for those substances which, in 

 aqueous solutions, combined with very little water, as was demonstrated by 

 the freezing-point method, shows that for all wave-lengths studied the solu- 

 tion, and water of the same depth as the water in the solution, have prac- 

 tically the same transmission. The dissolved substance does not combine 

 with the solvent water, and the water in the solution has almost exactly the 

 same effect upon light as so much pure water would have. This is exactly 

 what would be expected from our knowledge of the absorption of light by 

 dissolved substances and by the solvent. When we began this work we 

 supposed, as others had done, that the water in the solution, whether it was 

 combined with the dissolved substance or not, would have the same power 

 to absorb light as so much pure solvent water. We shall now see that such 

 is not the case. 



The results for the above-named substances were not plotted in the form 

 of curves, since the curve for water and for the solution would practically 

 coincide with one another, the dissolved substance having very little absorp- 

 tion over the region of wave-lengths studied in this investigation. 



When we turn to the data in tables 8 and 9 very different relations mani- 

 fest themselves. These are the data for calcium chloride, magnesium chlo- 

 ride, and aluminium sulphate, that is, for salts which, in aqueous solution, 

 are strongly hydrated, as was shown by the earlier work in this laboratory. 1 

 The solution in these cases is often more transparent than the same amount 

 of water that is contained in the solution. 



That these relations may appear the more clearly, the results obtained for 

 the above-named salts are plotted as curves in figs. 12 to 17. Fig. 12 is the 

 curve for calcium chloride having a depth of 20 mm. This was obtained by 

 dividing the deflection produced by 21 mm. of the solution by that pro- 

 duced by 1 mm. of the solution. On the same sheet we have the curve for 

 water having a depth equal to that of the water in the calcium chloride. 

 This curve for water was also obtained by the "differential" method, i. e., by 

 dividing the deflections produced by the deeper solution by those obtained 

 with the more shallow solution, the difference in the depths of water in the 

 two cases being just equal to the depth of water in 20 mm. of the solution in 

 question. Fig. 13 is the curve for calcium chloride with a depth of layer of 

 10 mm. (11 1). The data from which the curve was plotted are contained 

 in table 9. The smaller depth of solution was used, so that the water-band 

 between 1.2/x and 1.3jU would come out more distinctly. The results for this 

 solution, like those for all the others, are compared with the absorption of 

 a depth of water equal to that of the water in the solution. The absorption 

 of the water, in this as in all other cases, was obtained by the "differential" 

 method. 



1 Cam. Inst. Wash. Pub. 60. 



