CH. XVL] COHESION 155 



rise to part of phenomena recently discovered under 

 the name of radio-activity. But instead of suppos- 

 ing that their violence of ejection is due to velocity 

 previously possessed by them, it is possible to suppose 

 that they are driven away by intrinsic static force, 

 so that their previous energy was potential ; and 

 this is the form of hypothesis favoured by Lord 

 Kelvin. See Phil. Mag. for March 1902. 



Molecular Forces, Cohesion. 



But there is another kind of adhesion or cohesion of 

 molecules, not chemical but what is called molecular. 

 This occurs between atoms not possessing ionic or 

 extra charges, but each quite neutral, consisting of 

 paired-off groups of electrons. At any moderate 

 distance the force of attraction between paired elec- 

 trons will be next to nothing, but at very minute 

 distances it may be very great ; ultimately becoming 

 almost indistinguishable from chemical combination, 

 except that the cling will be a weak cling at a 

 multitude of points instead of an intense cling at 

 only one. 



Consider the outer surface of an atom consisting 

 of a regular group of interleaved electrons of alter- 

 nately opposite sign. Its equipotential surfaces will 

 be dimpled or corrugated or pimply sheets, which at 

 a little distance away will be almost plain ; but the 

 dimples will increase rapidly in depth and become 

 like the cover of a mattrass, when something less 

 than molecular distance something approaching the 

 internal electron distances apart is reached. 



Two such atoms will therefore tend to settle down 

 with their equipotential surfaces adjusted into uni- 

 formity, the pimples of the one fitting into the hollows 

 of the other ; and this is the state of things suggested 



