148 Lord Kelvin on 



each of the diagrams, because for infinitesimal deviations 

 each atom experiences forces only from its twelve nearest 

 neighbours, and these forces are each of them zero for 

 equilibrium. It may conceivably be that each of the 

 maximums of w, whether for the monatomic or the diatomic 

 assemblage, is stable without guidance. But it seems more 

 probable that, for assemblage I. and law of force 2, the 

 intermediate maximum m! (close to a minimum) is unstable. 

 If it is so, the assemblage left to itself in this configuration 

 would fall away, and would (in virtue of energy lost by waves 

 through ether, that is to say, radiation of heat) settle in stable 

 equilibrium corresponding to the maximum m (single assem- 

 blage), or either of the maximums m n (single assemblage), or 

 m'" (double assemblage). It is also possible that for law of 

 force 1 the maximum m! for the single assemblage is unstable. 

 If so, the system left to itself in this configuration would fall 

 away and settle in either of the configurations m (single 

 assemblage) or wi" (double assemblage). Or it is possible 

 that with either of our arbitrarily assumed laws of force 

 there may be stable configurations of equilibrium with the 

 atoms in simple cubic order (§ 21 below): and in double 

 cubic order ; that is to say, with each atom in the centre 

 of a cube of which the eight corners are its nearest neighbours. 



§ 18. It is important to remark further, that certainly a 

 law of force fulfilling the conditions of § 13 may be found, 

 according to which even the simple cubic order is n stable 

 configuration ; though perhaps not the only stable configura- 

 tion. The double cubic order, which has hitherto not got as 

 much consideration as it deserves in the molecular theory of 

 crystals, is certainly stable for some laws of force which would 

 render the simple cubic order unstable. Meantime it is 

 exceedingly probable that there are in nature crystals of 

 elementary substances, such as metals, or frozen oxygen or 

 nitrogen or argon, of the simple cubic, and double cubic, and 

 simple equilateral, and double equilateral, classes. It is also 

 probable that the crystalline molecules in crystals of compound 

 chemical substance are in many cases simply the chemical 

 molecules, and in many cases are composed of groups of the 

 chemical molecules. The crystalline molecules, however 

 constituted, are, in crystals of the cubic class, probably 

 arranged either in simple cubic, or double cubic, or in simple 

 equilateral, or double equilateral, order. 



§ 19. It will be an interesting further development of the 

 molecular theory to find some illustrative cases of chemical 

 compound molecules (that is to say, groups of atoms presenting 

 different laws of force, whether between two atoms of the 



