A KINETIC THEORY OF MATTER. 241 



constituting an elastic solid ; exactly fulfilling 

 the mathematical ideal worked out by Navier, 

 Poisson, and Cauchy, and many other mathema- 

 ticians who, following their example, have en- 

 deavoured to found a theory of the elasticity of 

 solids on mutual attraction and repulsion between 

 a group of material particles. All that can 

 possibly be done by this theory, with its 

 assumption of forces acting according to any 

 assumed law of relation to distance, is done by 

 the gyrostatic system. But the gyrostatic 

 system does besides, what the system of 

 mutually acting material particles cannot do : 

 it constitutes an elastic solid which can have 

 the Faraday magneto-optic rotation of the plane 

 of polarisation of light ; supposing the application 

 of our solid to be a model of the luminiferous 

 ether for illustrating the undulatory theory of 

 'light. The gyrostatic model spring-balance is 

 arranged to have zero moment of momentum as 

 a whole, and therefore to contribute nothing 

 to the Faraday rotation ; with this arrangement 

 the model illustrates the luminiferous ether in 



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