Heat of Mixture of Substances. 553 



The third and sixth columns or the table contain the corre- 

 sponding values of 1^ calculated by means of the above 

 equation. The internal heat of evaporation of a grm. of 

 water at 20° C. is 561*5 cal., and the value of L 2 therefore 

 561*5 x 18 = 10,110. It will be seen that there is a rough 

 agreement between the two sets of values for the gases NIL, 

 HF, HBr, HI. It seems, therefore, that a molecule of these 

 gases replaces a water molecule on solution. This result is, 

 however, not quite conclusive, as it may happen that hydrates 

 are formed and the resultant molecules occupy such positions 

 relative to the molecules of the solute that the total change 

 in potential energy is the same as if no new molecules were 

 formed and each dissolved molecule replaced a molecule of 

 the solute. 



The calculated heats of solution do not agree, however, 

 with the calculated values in the case of C0 2 , HC1, and Cl 2 . 

 It is usually assumed that C0 2 molecules do not change on 

 solution in water, since the concentration obeys Henry's law. 

 The nature of the disagreement with C0 2 would then indicate 

 that a C0 2 molecule is on the average further away from 

 the surrounding water molecules than a w y ater molecule in 

 pure water. But it also follows (referring to what has gone 

 before) that the volume of occupation of a G0 2 molecule is 

 less than that of a water molecule. These two conclusions 

 do not at first sight agree with one another. But it must 

 be remembered that the volume of occupation of a molecule 

 is the total change in the volume of the liquid when a mole- 

 cule is removed, and it may therefore happen that when a 

 molecule is removed from a mixture a change in the volumes 

 of occupation of the neighbouring molecules takes place, 

 most probably an increase, which changes the magnitude of the 

 volume of occupation of the removed molecule considerably. 

 These results combined, therefore, seem to indicate that a 

 molecule of C0 2 in water is surrounded by two shells of 

 water molecules, the outside one being more dense and the 

 inside one less dense than pure water. In the case of the 

 other two gases Cl 2 and HC1, all we can say is that a 

 dissolved molecule either does not occupy a similar position 

 as a molecule of water or a hydrate is formed. 



Cambridge, Feb. 1, 191 J. 



Phil. Mag. S. 6. Vol. 21. No. 124. April 1911. 2 



