1 66 The Aether as an Elastic Solid. 



*> j3*> 7*> denote the proportions in which lines parallel to the 

 axes of strain are altered ; then if the solid be initially strained 

 in a way defined by given values of a, (3, y, by forces applied to 

 its surface, and if waves of distortion be superposed on this 

 initial strain, the transmission of these waves will follow exactly 

 the laws of Fresnel's theory of crystal- optics, the wave-surface 



being 



* 



q q 



There is some difficulty in picturing the manner in which 

 the molecules of ponderable matter act upon the aether so as to 

 produce the initial strain required by this theory. Lord 

 Kelvin utilized* the suggestion to which we have already 

 referred, namely, that the aether may pervade the atoms of 

 matter so as to occupy space jointly with them, and that its 

 interaction with them may consist in attractions and repulsions 

 exercised throughout the regions interior to the atoms. These 

 forces may be supposed to be so large in comparison with those 

 called into play in free aether that the resistance to compres- 

 sion may be overcome, and the aether may be (say) condensed 

 in the central region of an isolated atom, and rarefied in its 

 outer parts. A crystal may be supposed to consist of a group 

 of spherical atoms in which neighbouring spheres overlap each 

 other ; in the central regions of the spheres the aether will be 

 condensed, and within the lens-shaped regions of overlapping 

 it will be still more rarefied than in the outer parts of a solitary 

 atom, while in the interstices between the atoms its density 

 will be unaffected. In consequence of these rarefactions and 

 condensations, the reaction of the aether on the atoms tends 

 to draw inwards the outermost atoms of the group, which, 

 however, will be maintained in position by repulsions between 

 the atoms themselves; and thus we can account for the pull 

 which, according to the present hypothesis, is exerted on the 

 aether by the ponderable molecules of crystals. 



* Baltimore Lectures (ed. 1904), p. 253. 



