1886.'] on Experiments shoioing Dilatancy. 355 



the late Professor Maxwell, to whom we owe so much of our definite 

 knowledge of the fundamental physics. Maxwell insisted on the 

 fact, that even if each of the physical properties could be explained by 

 a special tether, it would not advance philosophy, as each of these 

 aethers would require another aether to explain its existence, ad in- 

 finitum. Maxwell clearly contemplated the existence of one medium, 

 but it was a medium which would cause not one but all the physical 

 properties of matter. His writings are full of definite investigations 

 as to what the mechanical properties of this aether must be to account 

 for the laws of gravitation, electricity, magnetism, and the trans- 

 mission of light, and he has i)roved very clear and definite jn'operties, 

 although, as he distinctly states, he was unable to conceive a mecha- 

 nism which should possess these properties. 



As the result of a long-continued effort to conceive a mechanical 

 system possessing the properties assigned by Maxwell, and further, 

 which would account for the cohesion of the molecules of matter, 

 it became apparent that the simplest conceivable medium — a mass of 

 rigid granules in contact with each other — would answer not one but 

 all the known requirements, provided the shape and mutual fit of the 

 grains were such, that while the grains rigidly j)reserved their shape, 

 the medium should possess the apparently paradoxical or anti-sponge 

 jiroperty of swelling in bulk as its shape was altered. 



I may here remark, that if aether is atomic or granular, that it should 

 be a mass of grains holding each other in position by contact like 

 the grains in the sack of corn is one of only two jjossible con- 

 ceptions ; the other being that of La Sage, or the corpuscular theory 

 that the grains are free like bullets moving in space in all directions. 



Nor, in spite of its paradoxical sound, is there any great difficulty 

 of conceiving the swelling in bulk. When the grains are in contact, 

 it ajjpears at once that the mechanical projjerties of the medium must 

 be to some extent afi'ected by the shape and fit of the grains. And 

 having arrived at the conclusion that in order to act the part of 

 aether, this shape and fit must be such that the mass could not 

 change its shape without changing its volume or space occupied, the 

 next thing was to see what possible shape could be given to the 

 grains, so that while these rigidly preserved theii* shape, the medium 

 might possess this property of dilatancy. 



It was obvious that the grains must so interlock, that when any 

 change of shape of the mass occurred, the interstices between the 

 grains should increase. This would be possessed by grains shaped to 

 tit into each other's interstices in one particular arrangement. 



In an ordinary mass of brickwork or masonry well bonded without 

 mortar, the blocks fit so as to have no interstices ; but if the pile be in 

 any w^ay distorted, interstices appear, which shows that the sjDace 

 occupied by the entire mass has increased (slioivu hy a model). 



At first it appeared that there must be something special and sys- 

 tematic, as in the brick wall, in the fit of the grain of aether, but 

 subsequent consideration revealed the striking fact, that a medium 



