A KINETIC THEORY OF MATTER. 243 



mechanical model, to the case of light travelling 

 in the direction of the lines of force in a 

 magnetic field. 



In these illustrations and models we have 

 different portions of ideal rigid matter acting 

 upon one another, by normal pressure at mathe- 

 matical points of contact of course no forces of 

 friction are supposed. It is exceedingly interest- 

 ing to see how thus, with no other postulates 

 than inertia, rigidity, and mutual impenetrability, 

 we can thoroughly model not only an elastic 

 solid, and any combination of elastic solids, but 

 so complex and recondite a .phenomenon as the 

 passage of polarised light through a magnetic 

 field. But now, with the view of ultimately dis- 

 carding the postulate of rigidity from all our 

 materials, let us suppose some to be absolutely 

 destitute of rigidity, and to possess merely 

 inertia and incompressibility, and mutual im- 

 penetrability with reference to the still remaining 

 rigid matter. With these postulates we can 

 produce a perfect model of mutual action at a 

 distance between solid particles, fulfilling the 



R 2 



