498 The Causes of Osmotic Pressure. 



Thus a direct, molecular explanation of the molecular 

 lowering of vapour-pressure in solutions is available, and 

 might be made the starting-point of the molecular theory of 

 solutions. 



It may be urged against the theory here outlined that it 

 makes no provision for the facts of the " dissociation " of 

 salts in watery solution, so beautifully discovered by Arrhenius, 

 and worked out so consistently by Ostwald and the other 

 enthusiasts of the new Physical Chemistry ; but it is easy to 

 provide for the facts of dissociation by stipulating that the 

 word molecule is to include any mass whose translational 

 kinetic energy is on the average that of a molecule of a gas at 

 the same temperature. The electrically charged ion, if it is 

 really a separate kinetic entity, must, according to the kinetic 

 theory, be a molecule under the above stipulation ; and thus 

 the requirements of the dissociation theory of watery solutions 

 of salts are met. Of course we should expect such dissociated 

 ions as CI and Na to be very mobile, and to escape with ease 

 from the surface of a solution of NaCl ; but as soon as they 

 get free from the liquid water which is the cause of their 

 dissociation, they will combine with avidity to form NaCl, 

 which then returns at once to the solution in the manner 

 described above. Thus dissociation presents no difficulty in 

 this connexion. Evidently the very marked dissociating 

 power of water is associated with its generally exceptional 

 character; if water were not exceptional, it ought to be a 

 somewhat difficultly liquefiable gas, whereas its high molecular 

 force constrains it to liquefy and so produce a dense aggre- 

 gation of swiftly moving molecules; that these should tear 

 apart the atoms of salts with their known electrical contrast 

 is a strange but fruitful discovery in physical chemistry. 

 Whatever the actual cause of the dissociating power of water 

 may be, its existence has been established almost beyond cavil, 

 and offers no difficulty in osmotic theory, as here presented. 



Such interesting cases of association as those investigated 

 by Prof. S. Pickering would, according to the theory above 

 outlined, need to be treated, each on its own merits, as 

 belonging to those exceptions which, accepted grudgingly at 

 first, are ultimately found to afford the best opening into new 

 regions of law. 



Melbourne, Sept. 1897. 



