the two characteristic Constants oj the JEther of Space. 467 



while magnetomotive force is ejt ; so the ratio of the two 

 potentials is the inverse ratio of the charges. 



Every electrical expression which can be interpreted 

 dynamically necessarily involves two of the electric or 

 magnetic quantities, — one of each except when a mere 

 ratio is involved, — otherwise the setherial constants could 

 not cancel. 



If the constants are omitted and their unknown dimensions 

 not attended to or allowed for, we get impossible equations 

 •expressing electric and magnetic quantities in terms of 

 ordinary mechanical units ; so that the relation between 

 .Aether and matter is masked. If they are always retained, 

 the equations are true in all respects,, and it is easy to 

 interpret them in any artificial and temporary system of 

 units that may be convenient. The two recognized artificial 

 systems are necessary until the nature of the constants 

 ix and K is known : and even afterwards they will probably 

 still be used for practical purposes. 



All that we at present certainly know about these constants 

 is derived from Maxwell's theory, and can be expressed 

 thus : — that if the following operators are applied to any 

 suitable electric vector this relation holds, 



v-'+mk|;=o. 



"Whence it follows that the product fiK. is the reciprocal 

 of the square of: the velocity of wave propagation, or 



Well-known facts in electrostatics suggest that K has 

 some kind of elastic significance ; while //-, which dominates 

 magnetic phenomena, simulates more the properties of 

 inertia. Yet neither property can belong to ordinary 

 matter : they both "belong to vacuum : matter can only 

 modify them, as it can onlv modify the velocity of light. 



4-7T 



Hence it becomes natural to suggest that ^- may be the 



transverse elasticity or rigidity of the a?ther ; while 47T//, 

 may lie its density. (See ' Modern Views of Electricity,' 

 Appendix p.) 



Reasons are given in the Phil. Mag. for April 1907, p. 488, 

 rand in my book ' The Ether of Space,' for the probability — 

 assuming aether to be the raw material out of which electrons 

 are made and that it is incompressible — that the density of 

 aether is excessively great : in fact possibly of the order 

 10 12 grammes per c.c. 



2K2 



