606 SCIENCE PROGRESS 



The regular distribution of the diffusing substance is clearly 

 established by these observations. It is not so easy apparently 

 to ascertain what diffuses and how diffusion takes place. 



The forces at work within a solution must be such as to give 

 rise to very complicated conditions and we are only beginning 

 to take these into account. The fact that water itself is not a 

 simple substance has not yet met with general recognition ; the 

 disturbances that are effected in the water when substances are 

 dissolved in it have scarcely been considered. Of late years, in 

 science, as in ordinary life, fashion has ruled the day and there 

 has been a tendency to adopt extremes. "Authority," exercised 

 through text-books and fostered by our examination system, 

 dictates the fashion. Students of chemistry all the world over 

 have been led to profess their belief in the doctrine of electro- 

 lytic, ionic dissociation, much for the same reason that they 

 have turned up their trousers, not because the practice is rational 

 but because it is conventional ; so it comes that attention has 

 been concentrated almost entirely on the dissolved substance. 

 But the electrolytic dissociation hypothesis does not help us in 

 the least in dealing with diffusion. 



There has been moreover an unfortunate tendency to regard 

 liquids from the point of view of gases and to suppose that 

 what is true of one is true of the other; the analogies the two 

 states present have — improperly, I contend — been regarded as 

 similarities. 



If a closed porous vessel provided with a gauge be enclosed 

 in a vessel containing air at atmospheric pressure, the gauge 

 indicates no pressure within the vessel — not because there is 

 no pressure within it but because the pressures without and 

 within are in equilibrium ; because the air particles pass into 

 and from the vessel with equal rapidity. If hydrogen be intro- 

 duced into the outer atmosphere, the gauge rises, showing an 

 increase of pressure within the porous vessel. Graham has 

 taught us that hydrogen molecules move four times as rapidly 

 as oxygen molecules and it is because hydrogen molecules get 

 in so much faster than air particles get out that the pressure 

 rises at first ; but it falls gradually again to zero, because air 

 particles continue to pass out into the hydrogen ; ultimately the 

 mixture has the same composition within and without the porous 

 vessel. If the vessel be of such material that only one of the 

 gases can pass through its walls, the pressure to which the 



