702 Mr. S. 0. Bradford on the 



particles, and the force opposing solution will decrease as- 

 the cohesion of the solute particles and their adhesion to 

 the solvent, or " cohesion pressure, " increases. This is the 

 rule already found by Traube. 



The capacity for hydration, regarded as a function of 

 the molecular forces, must be related to the adhesion of 

 solute to solvent. This property, therefore, should follow 

 the order of Traube's cohesion pressure and the opposite 

 order of the solubilities of substances which increase the 

 surface tension of the solvent, as in series (III.). Similarly 

 solutes which increase the surface tension of the solvent 

 must increase the internal pressure, and since their com- 

 pressibility decreases with increase of internal pressure, the 

 order of the compressibilities of solutions (IV.) will be 

 the inverse of that of the cohesion pressures, or directly as 

 that of the solubilities of the solutes. As Gilbault [J 897] 

 has pointed out, the decrease in compressibility of solutions 

 of salts does not suggest dissociation of ions, which ought to 

 be accompanied by great expansion owing to the release of 

 chemical combination. If the diminution of vapour pressures 

 and depression of freezing-points of solutions are relaled to 

 increase in surface tension, the effects should be in the same 

 order as in (V.) and (VI.). 



The last series quoted by Traube gives the order of the 

 molecular volumes of salts. As has previously been noted, 

 it was first suggested by Nicol that molecular volume 

 is related inversely to molecular cohesion. The apparent 

 molecular volume of a substance may be supposed to include 

 the actual volume occupied by a molecule, together with the 

 space in which it is free to move, which corresponds to 

 Traube 5 s co-volume. It is natural to regard this as inversely 

 proportional to the forces of molecular attraction. It follows 

 that those salts which have the larger molecular volume will 

 be the more soluble, as is found to be the case. However, 

 the apparent molecular volume of a substance in any 

 condition will depend on the internal pressure to which it 

 is subjected in that condition, consequently changes in 

 molecular volume will occur with change of state, whether of 

 the pure substance or by solution. Therefore substances in 

 solution will generally he subject to a different intrinsic 

 pressure than in the pure state. And surrounding each 

 particle of solute, whether in true or colloidal solution, there 

 will be a thin skin in which the intrinsic pressure will be 

 intermediate between that of solute and solvent. The relations 

 between compressibility, molecular volume, surface tension, 

 and other properties have been investigated by a number of 



