BY MEANS OF THE RADIOMICROMETER. 19 



THE ABSORPTION OF LIGHT BY WATER AS AFFECTED BY HYDRATED AND BY 



NON-HYDRATED SALTS. 



It was found by Guy, Shaeffer, and Jones 1 that the absorption of 

 light by water was changed by the presence of hydrated salts which 

 themselves had no absorption. Solutions of non-hydrated salts of 

 equal concentration did not appreciably affect the power of water to 

 absorb light. The results obtained by the above-named authors were 

 considered by them as strong evidence in favor of the solvate theory of 

 solution as proposed in this laboratory about 15 years ago. 



That water of hydration should absorb so differently from an equal 

 amount of uncombined water was regarded as a fact of some impor- 

 tance, especially in its bearing on the solvate theory of solution, and 

 one deserving very careful study. Since the work recorded in the first 

 paper was essentially preliminary in character, it was decided to 

 repeat and extend this work with our improved apparatus, taking 

 advantage of what we had recently learned from experience in con- 

 nection with such work. This seemed all the more desirable in that 

 our earlier results had been interpreted in a different manner by Livens. 2 

 The most recent work gives results confirming the belief that our origi- 

 nal interpretation was correct. 



The earlier work showed clearly that concentrated solutions of such 

 hydrated salts as calcium chloride, magnesium chloride, and aluminium 

 sulphate, which themselves had no absorption over the region of the 

 1/j, and the 1.25/z water-bands, had a very different transmission curve 

 from that of a layer of water equal in depth to the water in the solution 

 in question. For some wave-lengths of light the transparency of the 

 solution was considerably greater than that of an equal amount of 

 pure water. It was concluded that combined water has less power to 

 absorb light than free water. No such marked effect could be noted 

 when studying the absorption of aqueous solutions of such non- 

 hydrated salts as ammonium chloride, ammonium nitrate, and potas- 

 sium chloride, even when the solution was of five normal concentra- 

 tion. For non-hydrated salts the curves for the solution and for water 

 were nearly identical. It was therefore clear that combined water had 

 very different action on light from ordinary free water. 



We have repeated practically all of the earlier work dealing with 

 this phase of our problem, and have also duplicated a very large part 

 of our own bearing upon it. It is a matter of some difficulty to 

 avoid considerable errors in work of this character, and for this reason 

 it was considered desirable to make duplicate measurements as often 

 as possible. That some idea may be gained as to the accuracy with 

 which we could repeat our work, we give in a number of cases the 

 results of such duplicate measurements. 



Carnegie Inst. Wash. Pub. No. 190. Phys. Zeit., 14, 278 (1913). z lbid., 14, 660 (1913). 



