DISCUSSION OF EVIDENCE. 187 



differ very greatly from that of so much pure water, since, when the 

 solvent is not combined with the dissolved substance, it is difficult to see 

 how either could affect appreciably the absorbing power of the other. 



When we turned to the strongly hydrated salts, very different results 

 were obtained. As examples of this class of substances we studied 

 calcium and magnesium chlorides and aluminium sulphate. Take the 

 results for a 5.3 normal solution of calcium chloride. The solution is 

 more transparent from 0.9 /* to 1 /*. It is again the more transparent 

 from 1.05/i to 1.2/*, being as much as 25 per cent more transparent 

 than the solution. For the longer wave-lengths the water is in general 

 the more transparent until 1.42/x is reached, when both water and 

 solution become equally opaque. Similar results were obtained with 

 magnesium chloride. 



Aluminium sulphate presents this peculiarity, that at 1 p. the solu- 

 tion is more transparent than the water. The obvious explanation of 

 these surprising results seems to be that they must be due to some 

 action of the dissolved substance on the solvent. Jones and Anderson 1 

 showed that the solvent can have a marked effect on the absorbing 

 power of the solution in that solvent, even when the solvent itself had 

 no absorption in the region in question. 



A large number of examples of " solvent bands" were discovered by 

 Jones and Strong. 2 They found many non-absorbing solvents which 

 affected the absorption of the dissolved substance, and could even dis- 

 tinguish between certain organic solvents and their "iso" compounds 

 by the "solvent bands" which manifested themselves. This action 

 seems to have been satisfactorily explained as due to a combination of 

 the solvent with the dissolved substance forming solvates. The sol- 

 vate theory enables us to account for many facts which apparently 

 could not be satisfactorily explained by the theory of electrolytic dis- 

 sociation alone, as we have seen. The same theory seems to aid us in 

 explaining the facts just described. Those compounds which do not 

 form hydrates, or which form only very simple hydrates, such as 

 potassium chloride and the like, show results such as would be expected. 

 Their solutions are not more transparent than so much pure water. 

 In general, the absorption of such solutions is of the same order of 

 magnitude as that of the water in which they are dissolved. We shall 

 see that it came out in later work that solutions of only slightly hy- 

 drated salts are more opaque than pure water at the centers of the 

 absorption bands. This, however, does not affect at all the conclu- 

 sions drawn above. It is only the hydrated salts whose solutions are 

 appreciably more transparent than so much pure water. How does 

 the solvate theory explain these facts? 



The combined water seems to have less power to absorb light than 

 free water. This would account for the above facts. The presence 



Carnegie Inst. Wash. Pub. No. 110. z lbid., Nos. 130 and 160. 



