1901-2.] Lord Kelvin on Molecular Dynamics of a Crystal. 211 
attraction to repulsion. Suppose, for instance, that the mutual 
force between two atoms is zero for all distances exceeding a 
certain distance I, which we shall call the diameter of the sphere 
of influence; is repulsive when the distance between them is <£; 
zero when the distance is = £ ; and attractive when the distance is 
>£ and < I. 
§ 14. Two different examples are represented on the two curves 
of fig. 3, drawn arbitrarily to obtain markedly diverse conditions 
of equilibrium for the monatomic equilateral assemblage (I.), and 
also for the diatomic assemblage (II.). The abscissa ( x ) of each 
Fig. 3. 
diagram, reckoned from a zero outside the diagram on the left, 
represents the distance between centres of two atoms; the or- 
dinates ( y ) represent the work required to separate them from this 
distance to oo . Hence — — represents the mutual attraction at 
ax 
distance x. This we see by each curve is - go (infinite repulsion) 
at distance LO, which means that the atom is an ideal hard 
ball of diameter LO. For distances increasing from LO the force 
is repulsive as far as L61 in curve 1, and 1'55 in curve 2. At 
these distances the mutual force is zero ; and at greater distances 
up to L8 in curve 1, and L9 in curve 2, the force is attractive. 
The force is zero for all greater distances than the last mentioned 
in the two examples respectively. Thus, according to my old 
