
Electric Origin of Rigidity and Consequences. 421. 
corresponds to the inertia of the magnet, and the electric 
doublet to the two magnetic poles. Consider our doublet in a 
spherical atom of mass m and radius a, and suppose this atom 
vibrating in the field of electric force due to all the other 
atoms, the str ength of this field being 4%e/K. The period is 
27 (moment of inertia / moment of force)? 
= a?m2/5 y= ? fy, ee cg 2 
= eK = 27a/ (30 $7 o s/Ka)?. 
tole 
But as we must suppose only one-third of the energy per 
unit volume associated with the field of force in any one 
direction, we must in the last expression replace o? by o*/3, 
and we get on numerical reduction with s=2a, 
Qala(@ipK)3. 9.2. «4 oe 3 OQ) 
_ The expression (8) on numerical simplification reduces to 
the same as (9) except that 2°1 for 27/3 replaces 2°0. Thus 
we see that what I called the mechanical period of vibration 
of an atom in my paper on Spectra, is capable of the simple 
electrical interpretation which it must have if rigidity at 
absolute zero is of electric origin. If this fundamental 
period is the same at all temperatures, then, as regards actions 
so rapid as the vibrations of the atom, the whole electric energy 
must act as if it retained the electrostatic form and the same 
value as at absolute zero. Note that we have found the 
electric doublet forcing the material atom to vibrate. For 
other actions we shall find that with rising temperature more 
and more of the originai stock of electrostatic energy at 
absolute zero appears in the form of electrokinetic energy. 
The double interpretation of atomic vibrations as mecha- 
nical and electrical is another instance of a certain duality 
which must accompany the attempt to treat the properties of 
matter and electricity as identical. Maxwell’s electromagnetic 
theory has led to the ctirrent fruitful speculations on the 
electric constitution of matter. The older mechanical theory 
of light, of which Kelvin is the ieading living cultivator, has 
as its logical goal a material theory of electricity. Both 
roads lead ultimately to the same junction, and duality such 
as that just discussed must be the rule. The connexion 
between the vibration of atoms here discussed and the trans- 
latory vibrations of the monatomic metal molecules at their 
melting-point, has been pointed out in “The Cause of the 
Structure of Spectra.” 
