the two characteristic Constants of the yEther of Space. 471 



If a distinct answer can be gained, the experiment is well 

 worth while. 



Perhaps it is not clear why I attach so much importance 

 to a measurement of the aetherial constants and a deter- 

 mination of their dynamical nature. If a positive result 

 could be secured it would he the first positive result which 

 the aether, apart from matter, has yielded, since the funda- 

 mental fact of wave propagation and its definite velocity. 

 This determined the product of the two constants, and was 

 the first step in our knowledge of them. If however by 

 some new phenomenon the two constants could be separately 

 known, a second and even more important step would have 

 been taken towards understanding the aether's structure and 

 real nature. Until these constants are known, its relation to 

 and interaction with ordinary matter must be largely guess- 

 work. Radiation once excited obeys known laws, but of 

 the emission and the absorption of radiation very little is 

 really understood ; and even the refraction or slowing of 

 speed when passing through dense matter appears to be a 

 subject of some difficulty, at least when anything more has 

 to be apprehended than the bare fact and its elementary 

 exposition. 



If the density were known, of course the elasticity would 

 be known too, unless the dynamics* of aether is not merely a 

 variation on Newtonian dynamics but something utterly 

 different. The only way to ascertain the truth on this 

 subject is to try how far the aether can be treated as a 

 substance amenable to ordinary laws. The principle of 

 least action holds for light, and it seems possible that a 

 developed turbulent or vortex sponge theory may account 

 for the aether's elastic rigidity {cf. Appendix E and p. 124 

 of Larmor's ' Mther and Matter/ Also Phil. Mag. for 

 April 1907, p. 503). It is essential however that we 

 know the value of this rigidity. If it is kinetically explicable 

 in the way originally suggested by Lord Kelvin (though 

 afterwards abandoned by him) then the amount of energy 

 locked up in the aether is something prodigious. Some 

 day such a fact as this, when ascertained, may be found 

 to have a bearing on really practical problems. 



