1907.] 



MAGIE— THEORY OF SOLUTIONS. 139 



water which enters into it) becomes less as the dikition increases, 

 and in most cases even attains a neg^ative vaUie, so that the heat 

 capacity of a dihite sohition is less than that of the water which 

 enters into it. It is manifestly inadmissible to ascribe a negative 

 heat capacity to the solute or to any part of it. It is further impos- 

 sible to account for the diminished heat capacity by ascribing it to 

 a change in the extent of dissociation due to the rise of temperature 

 during the determination of the heat capacity, for this change has 

 been proved to be so small as to be negligible in this connection. 

 We are therefore forced to believe that the solute, or some part 

 of it, interacts with the surrounding water in such a way as to 

 diminish the heat capacity of the water. 



Thomsen also found that the apparent volume of the solute 

 (that is, the difference between the volume of the solution and that 

 of the water which enters into it) becomes less as the dilution in- 

 creases, and in a very few cases even attains a negative value. In 

 these cases the volume of the solution is less than that of the water 

 which enters into it. It is manife^ly impossible to ascribe this 

 result to the shrinking of the solute alone, or to an insertion of the 

 dissociated ions in the interstices between the molecules of water. 

 A similar inference follows to that already drawn from the behavior 

 of the heat capacities, that the solute, or some part of it, interacts 

 with the surrounding water in such a way as to diminish the volume 

 of the water. 



We are thus naturally led to speculate on the mode of inter- 

 action between the solvent and the solute. Tammann assumes that 

 the eft'ect of the solute extends throughout the solution, so as to 

 produce an interior pressure in it, by reason of which the heat 

 capacity of the solution is altered. He undertakes to calculate the 

 amount of the change by the aid of a thermodynamic formula for 

 the specific heat, involving the specific heat of the solvent under 

 a pressure equal to the assumed interior pressure and a correction 

 term containing the temperature coefficients of volume and pres- 

 sure. Owing possibly to the difficulty of obtaining accurate data 

 for the necessary calculations, the agreement of Tammann's calcu- 

 lated values with those obtained by observation is not good, though 

 the general course of the observations is fairly well represented. 



