IN THE PRESENCE OF STRONGLY HYDRATED SALTS. 55 



explained in terms of any other suggestion that has thus far been made. 

 In a word, the solvate theory of solution as proposed by Jones about a dozen 

 years ago, 1 to supplement the theory of electrolytic dissociation in order that 

 we might have a theory of the real solutions which we use in the laboratory, 

 and not simply a theory of ideal solutions as the theory of electrolytic dis- 

 sociation alone must be regarded, has served good purpose in explaining the 

 phenomena that have been previously observed in connection with the 

 absorption of light by solutions of dissolved substances. 



We are inclined to explain the phenomena recorded in this paper by means 

 of the same theory. For solutions of those substances which have been 

 shown by entirely different methods not to hydrate to any appreciable 

 extent, the absorption of light by the solution and by a layer of water equal 

 in depth to that of the water in the solution, is the same almost to within the 

 limit of experimental error. 



For those substances which have been shown to form complex hydrates, 

 however, the absorption of light by their solutions and by a layer of water 

 equal in depth to that of the water in the solution is very different. The 

 water in these solutions is usually more opaque to light than the solution 

 or, in other words, a solution is more transparent than the water that is 

 present in the solution. 



The most rational explanation of this phenomenon appears to be that the 

 part of the water that is combined with the dissolved substance has a smaller 

 power to absorb light than pure, free, uncombined water. The fact that 

 we are able to detect the difference between the water in the solution and 

 pure water, by its action on light, we regard as good evidence that water in 

 the solution is different from pure, free water. This difference, it seems to 

 us, can be readily accounted for by the theory that a part of the water 

 present in the solution is in combination with the dissolved substance. 



We have carried out similar investigations with aluminium nitrate, but the 

 concentration of the strongest solution that could be obtained was not suffi- 

 ciently great to show the phenomenon in question. We therefore do not 

 incorporate the results obtained with this substance. That the solutions 

 must be very concentrated to show clearly the phenomenon with which we 

 are dealing is seen from the results given in table 10. Here the solutions of 

 the three salts in question that were used are more dilute than those for 

 which the results are tabulated in tables 8 and 9. An examination of table 

 10 will show that the phenomenon in question does not manifest itself to 

 anything like the same extent as with the more concentrated solutions. 

 This is exactly what we would expect in terms of the solvate theory of solu- 

 tions. The more concentrated the solution the larger the total amount of 

 the water present combined with the dissolved substance. If combination 

 between water and the dissolved substance explains the facts recorded in this 

 paper, then the larger the amount of water present that is combined with 

 the dissolved substance the more pronounced the phenomenon in question. 



1 Amer. Chem. Journ., 23, 89 (1900). 



