328 POPULAR LECTURES AND ADDRESSES. 



up and down, or when I turn it quickly round 

 the vertical diameter, alternately in opposite 

 directions ; that is the nearest representation 

 I can give you of the vibrations of luminiferous 

 ether. 



Another illustration is Scottish shoemakers' wax 

 or Burgundy pitch, but I know Scottish shoe- 

 makers' wax better. It is heavier than water, and 

 absolutely answers my purpose. I take a large 

 slab of the wax, place it in a glass jar filled with 

 water, place a number of corks on the lower side 

 and bullets on the upper side. It is brittle like 

 the Trinidad pitch or Burgundy pitch which I 

 have in my hand you can see how hard it is 

 but when left to itself it flows like a fluid. The 

 shoemakers' wax breaks with a brittle fracture, 

 but it is viscous and gradually yields. 



What we know of the luminiferous ether is that 

 it has the rigidity of a solid and gradually yields. 

 Whether or not it is brittle and cracks we cannot 

 yet tell, but I believe the discoveries in electricity 

 and the motions of comets and the marvellous 

 spurts of light from them, tend to show cracks 





