CHAPTER V. 



THE ABSORPTION OF LIGHT BY WATER CHANGED IN THE 

 PRESENCE OF STRONGLY HYDRATED SALTS, AS SHOWN BY 

 THE RADIOMICROMETER NEW EVIDENCE FOR THE SOLVATE 

 THEORY OF SOLUTION. 



The use of the radiomicrometer in studying the absorption spectra of cer- 

 tain substances has already been discussed by Jones and Guy. 1 The radio- 

 micrometer was used for studying the absorption spectra of solutions 

 rather than the grating spectrograph and the photographic plate, because it 

 enabled us to measure not only the positions of the different lines and bands, 

 but also to study quantitatively their intensity; and also because it made 

 possible the study of the absorption spectra of solutions over a much greater 

 range of wave-lengths than the photographic method. 



In building a radiomicrometer adapted to this work that is, with suffi- 

 cient sensibility and with a short period one of the greatest difficulties, as 

 already mentioned, was to obtain copper wire free from iron. This was a 

 necessity, since the presence of an appreciable quantity of iron in the copper 

 gave rise to a "magnetic control" which rendered the instrument unstable 

 and the zero-point inconstant. This difficulty was for the most part over- 

 come, due to the kindness of Messrs. Leeds and Northrup of Philadelphia 

 and of R. W. Paul of London. Both of these houses furnished us with 

 copper wire so free from iron that the "magnetic control" could easily be 

 regulated. By means of this wire and the thermo-electric junction already 

 described, a most sensitive radiomicrometer was built, which at the same 

 time had a very short period, and with this instrument work was done with 

 salts of neodymium and praseodymium, the results of which were recorded 

 in the Physikalische Zeitschrift. 2 



ABSORPTION OF FREE AND COMBINED WATER. 



At the beginning of the academic year 1912-13 the absorption spectra 

 of solutions of a large number of salts of different metals were mapped out 

 and compared with the absorption of water, using the same depths of water 

 as the water in the various solutions. The depth of water in the solution 

 was determined from the concentration of the solution and from its specific 

 gravity. It was soon found that the absorption of the solution was less, and 

 in many cases very much less than that of the layer of water having a depth 

 equal to the depth of the water in the solution. 



The above result is directly at variance with everything that was known 

 at the time. The dissolved substance could not have less than no absorp- 

 tion of light, the assumption having been made up to this time that in an 



1 Phys. Zeit., 13, 649 (1912). 2 Ibid., 13, 651 (1912). 



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