174 On a Possible Structure for tlie Ether. 



dissociated with extreme ease, the dissociation of ether is unknown 

 and hypothetical, save as represented by its apparent results. 



"Nevertheless, it must be the case that the slight, almost infinites- 

 imal, shear, which goes on in a light wave, is of the nature of 

 incipient and temporary electrical separation .... It appears 

 possible that a sufficiently violent E.M.F., applied to the ether by 

 some method unknown to us at present, must be the kind of influence 

 necessary to shear it beyond the critical value and leave its 

 components permanently distinct ; such constituents being opposite 

 electric charges, which, when once thoroughly separated, only 

 combine to form matter, and do not recoil into ordinary ether 

 again." 



Let me make one more quotation, immediately following, relating to 

 gravitation : — 



"Every attempt at separation of this kind, even if no stronger 

 than exists in ordinary light', [is] accompanied by a longitudinal 

 force — Maxwell's pressure .... If the disturbance could be made 

 so extreme as to result in permanent dislocation, this pressure might 

 leave behind it, as permanent residue, a longitudinal pressure 

 [or tension] extending throughout space." 



There seems to be a necessary connexion between transverse and 

 longitudinal stresses, the one being \ujv times the other. If we 

 re-estimate Maxwell's data for luminous vibrations, as given in his 

 article " Ether '' in the Encv. Brit. (Collected Works, vol. 2, p. 7(37), on 

 the basis of a reasoned high estimate of ether density, ignoring the guess 

 of that period that in the brightness near the Sun the amplitude of a 

 light vibration might possibty be as great as one-hundredth of a wave- 

 length, for this was only an upper limit and it is surely bound to be 

 much smaller, we can proceed thus : — 



Let a be the maximum amplitude of shear of a light wave, y = a cos 

 p(x — vt), near the Sun; where the luminous energy is nearly 2 ergs 

 per c.c. ; and let u be the maximum speed of elastic recovery ; 



then - =]hi — 2tt- . 



v X 



The energy \pu- = 2 ergs per c.c, 

 so, if p = 10 12 , u = 2 X 10~ 6 cm. per sec, and a = 10" 17 A. 

 Hence, expressing conditions near the sun in Maxwell's manner (/oc. cit.) 7 



Energy per cubic centimetre = ^py 2 a 2 // = 2 ergs. 



Greatest tangential stress per sq. cm. — pv 2 ap — 6 X TO 16 dynes. 



Coefficient of rigidity of ether = pv 2 = 10 33 c.g.s. 



Density of ether =p =10 12 ,. 



It will be observed that pv 2 ap is the same as pur, which is an 

 expression for the travelling momentum of a light-beam. 



