232 THE KIDDLE OF THE UNIVEESE. 



substance; it fully occupies the space between the 

 atoms of ponderable matter. 



II. — Ether has probably no chemical quality, and 

 is not composed of atoms. If it be supposed that it 

 consists of minute homogeneous atoms (for instance, 

 indivisible etheric particles of a uniform size), it 

 must be further supposed that there is something 

 else between these atoms, either " empty space " or a 

 third, completely unknown medium, a purely hypo- 

 thetical " inter-ether " ; the question as to the nature 

 of this brings us back to the original difficulty, and 

 so on in infinitum. 



III. — As the idea of an empty space and an action 

 at a distance is scarcely possible in the present 

 condition of our knowledge (at least it does not help 

 to a clear monistic view), I postulate for ether a 

 special structure which is not atomistic, like that of 

 ponderable matter, and which may provisionally be 

 called (without further determination) etheric or 

 dynamic structure. 



IV. — The consistency of ether is also peculiar, on 

 our hypothesis, and different from that of ponderable 

 matter. It is neither gaseous, as some conceive, nor 

 solid, as others suppose ; the best idea of it can be 

 formed by comparison with an extremely attenuated, 

 elastic, and light jelly. 



V. — Ether may be called imponderable matter in 

 the sense that we have no means of determining its 

 weight experimentally. If it really has weight, as is 

 very probable, it must be so slight as to be far below 

 the capacity of our most delicate balance. Some 

 physicists have attempted to determine its weight by 

 the energy of the light-waves, and have discovered 

 that it is some fifteen trillion times lighter than 



