448 Mr. J. H. Jeans on the 



It is clear that there will be no atom of which the valency 

 s is comparable with to. since such an atom would tend to 

 expel the ions of that si on which was in the majority, although, 

 when s is small compared with n, the corrections which we 

 have seen must be applied to the ordinary electrostatic law 

 may be sufficient to counterbalance this tendency. 



An atom of valency s behaves at sufficient distances from 

 its surface like an electrostatic point-charge of electricity se 

 placed at its centre. We may therefore expect that under 

 favourable conditions the atoms will combine into larger 

 systems, namely molecules, a molecule being now defined as a 

 system of ions of which the total charge is zero. 



Thus an atom of valency —2 may combine with another 

 atom of valency +2, or with two atoms each of valency +1, 

 and so on. Or two similar atoms each of valency ± 1 may 

 combine w r ith two ions to form a neutral molecule, though, 

 by hypothesis, a single such atom cannot combine in a stable 

 manner with a single ion. So two atoms of valency +1 

 may combine and form a neutral body, by ejecting the two 

 superfluous ions. 



On this view, a molecule of an element cannot be the same 

 as two (or more) atoms, although the molecule of a compound 

 may or may not be a sum of atoms of elements. But it 

 follows from the definition of a molecule, as a system of which 

 the total charge is zero, that no free ions can be liberated by 

 chemical change. It follows further that the force exerted 

 by a complete molecule at external points will fall off very 

 rapidly as we recede from the molecule. 



Electro statical Forces. — Chemical Affinity, Cohesion. 



§ 37. We may examine the electrostatic field of force more 

 closely as follows. 



With core as origin of coordinates, the potential of an atom 

 upon an ion exterior to it (and at a distance so great that 

 the unknown part of the potential function may be ignored) 

 can be written 



v= «o + Si + s 2 + 



/p rp-> jtO v ' 



where S i5 S 2 , . . . . are spherical surface-harmonics of orders 

 1, 2, ... . 

 Now 



\ \ ~7T r2 ^0 sm d$ = 47r x total convergence of force 

 J J inside atom, 



== — 4:7rse. 



