1888 - 89 .] Sir W. Thomson on Constitution of Matter. 697 
smallest distances there is repulsion, and at greater distances attrac- 
tion ; ending with infinite repulsion at infinitely small distance, and 
with attraction according to Newtonian law for all distances for 
which this law has been proved. He suggested numerous transi- 
tions from attraction to repulsion, which he illustrated graphically 
by a curve, — the celebrated Boscovich curve, — to explain cohesion, 
mutual pressure between bodies in contact, chemical affinity, and all 
possible properties of matter — except heat, which he regarded as a 
sulphureous essence or virtue. It seems now wonderful that, after 
so clearly stating his fundamental postulate which included inertia, 
he did not see inter-molecular motion as a necessary consequence of 
it, and so discover the kinetic theory of heat for solids, liquids, and 
gases ; and that he only used his inertia of the atoms to explain the 
known phenomena of the inertia of palpable masses, or assemblages 
of very large numbers of atoms. 
§16. It is also wonderful how much towards explaining the 
crystallography and elasticity of solids, and the thermo-elastic pro- 
perties of solids, liquids, and gases, we find without assuming more 
than one transition from attraction to repulsion. Suppose, for in- 
stance, the mutual force between two atoms to he zero for all dis- 
tances exceeding a certain distance, I, which we shall call the radius of 
the sphere of influence ; to be repulsive when the distance between 
them is <£; zero when it is =•-£; and attractive when it is >£ : 
and consider the equilibrium of groups of atoms under these con- 
ditions. 
A group of two would he in equilibrium at distance £ ; and only 
at this distance. This equilibrium is stable. 
A group of three would be in stable equilibrium at the corners of 
an equilateral triangle of sides £ ; and only in this configuration. 
There is no other configuration of equilibrium except with the three 
in one line. There is one, and there may he more than one, con- 
figuration of unstable equilibrium, of the three atoms in one line. 
§ 17. The only configuration of stable equilibrium of four atoms is 
at the corners of an equilateral tetrahedron of edges £. There is one, 
and there may be more than one, configuration of unstable equi- 
librium of each of the following descriptions : — 
(1) Three atoms at the corners of an equilateral triangle, and one 
at its centre. 
