IN THE PRESENCE OF STRONGLY HYDKATED SALTS. 53 



Fig. 14 is the curve for magnesium chloride having a depth of 21 1 = 

 20 mm., and the corresponding water-curve. The data from which these 

 curves are plotted are given in table 8. 



Fig. 15 is the curve for magnesium chloride having a depth of 1 cm., also 

 obtained by the "differential" method. These data are taken from table 9. 



Fig. 16 is the curve for aluminium sulphate having a depth of 21 1 = 

 20 mm., and the corresponding absorption curve for water. 



Fig. 17 is the curve for aluminium sulphate having a depth of 11 1 = 

 10 mm., and the corresponding water-curve. 



Fig. 12 shows the relative absorption of water and of the solution of cal- 

 cium chloride having a concentration of 5,38 normal and a depth of 20 mm. 

 The corresponding water-curve is marked throughout by the symbol H 2 0. 

 The solution is the more transparent from 0.9/* to nearly 1/*. The water then 

 becomes the more transparent over a short region of wave-lengths. From 

 1.05,1* to 1.2/* the solution is the more transparent. In this region the solu- 

 tion becomes as much as 25 per cent more transparent than the pure water, 

 as can be seen by comparing the points on the "water" curve with the corre- 

 sponding points on the curve for the solution which are vertically above the 

 points on the water-curve. The water becomes appreciably more trans- 

 parent only at and near the bottom of the "water-band" having a wave- 

 length of approximately 1/*. This is the effect that we would expect to get 

 if the dissolved substance exerted a "damping" effect on the absorption of 

 light by water. 



It will be recalled that the salts which do not form hydrates show, in 

 aqueous solution, practically the same absorption as the corresponding 

 amount of water. It would, therefore, seem reasonable to account for the 

 differences in the case of nonhydrating and strongly hydrating salts as due 

 to the water of hydration, or the water that, in this case, is combined with 

 the calcium chloride. 



The curves in fig. 13 are for a smaller depth of the same solution of cal- 

 cium chloride. This figure brings out the same general relations as was 

 shown in fig. 12. The water-curve in the region 1.25/* is above that of the 

 solution, showing that water in this region for the shallower depths of solu- 

 tion is more transparent than the solution. The additional feature brought 

 out by this figure is the water-band in the region 1.4 to 1.5/*-. After the first- 

 named water-band is passed the solution becomes more transparent than the 

 water and remains so until the wave-length 1.42 is reached. Here both the 

 solution and the water are practically opaque, as is shown by both the curves 

 approaching the abscissas. 



The curve for magnesium chloride having a depth of 20 mm. is almost 

 exactly a duplicate of that for calcium chloride having the same depth. 

 Practically the only difference worthy of mention is in the region from 1.0/* 

 to 1.1/*. In the case of magnesium chloride the water remains the more 

 transparent over this region of wave-lengths. In the case of calcium chlo- 

 ride the solution is the more transparent over this region. The difference 



