1901 - 2 .] Lord Kelvin on Molecular Dynamics of a Crystal. 215 
constraining guidance (§ 16) that we are sure of these equili- 
briums being stable. It is quite certain, however, that without 
guidance the monatomic assemblage would be stable for the small 
density corresponding to the point m of 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 iv, 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 intermedi- 
ate 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 assemblage), or either of the 
maximums m" (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 m" (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 a stable configuration ; though 
perhaps not the only stable configuration. 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 
