The Aether as an Elastic Solid. 161 



William Thomson (Lord Kelvin, b. 1824, d. 1908), who 

 devoted much attention to the labile aether, was at one time 

 led to doubt the validity of this explanation of light* ; for when 

 investigating the radiation of energy from a vibrating rigid 

 globe embedded in an infinite elastic-solid aether, he found that 

 in some cases the irrotational waves would carry away a 

 considerable part of the energy if the aether were of the labile 

 type. This difficulty, however, was removed by the observationf 

 that it is sufficient for the fulfilment of Fresnel's laws if the 

 velocity of the irrotational waves in one of the two media is 

 very small, without regard to the other medium. Following up 

 this idea, Thomson assumed that in space void of ponderable 

 matter the aether is practically incompressible by the forces 

 concerned in light-waves, but that in the space occupied by 

 liquids and solids it has a negatiye_CQmpres^biIiljy , so as to give 

 zero velocity for longitudinal aether- waves in these bodies. 

 This assumption was based on the conception that material 

 atoms move through space without displacing the aether: a 

 conception which, as Thomson remarked, contradicts the old 

 scholastic axiom that two different portions of matter cannot 

 simultaneously occupy the same space.J He supposed the 

 aether to be attracted and repelled by the atoms, and thereby to 

 be condensed or rarefied. 



The year 1839, which saw the publication of MacCullagh's 

 dynamical theory of light and Cauchy's theory of the labile 

 aether, was memorable also for the appearance of a memoir by 

 Green on crystal-bptics.H This really contains two distinct 

 theories, which respectively resemble Cauchy's First and Second 

 Theories : in one of them, the stresses in the undisturbed state 



* Baltimore Lectures (edition 1904), p. 214. 



t Ibid. (ed. 1904), p. 411. 



^ Michell and Boscovich in the eighteenth century had taught the doctrine of 

 the mutual penetration of matter, i.e. that two substances may be in the same 

 place at the same time without excluding each other : cf. Priestley's History i., 

 p. 392. 



6 Cf. Baltimore Lectures (ed. 1904), pp. 413-14, 463, and Appendices A and E. 



|j Cambridge Phil. Trans., 1839 ; Green's Math. Papers;?. 293. 



M 



