stance depends principally on the relation existing between its 

 elasticity and its density. The former helps while the latter hinders 

 wave passage. Now as a general rule the elasticity of a solid in 

 relation to its density is much higher than in the case of Hquids 

 and gases. But although we are compelled to think of the Ether 

 as a fluid, or infinitely rare gas, yet in its power of wave-transmis- 

 sion it behaves like a solid, which no other fluid is known to do. 

 I must here, perhaps make one or two dogmatic assertions in 

 illustrating this point, which time will not allow me to prove, and 

 which I must ask you therefore to take for granted. If we regard 

 two substances, the densities of which are the same, the velocities 

 with which a wave [sb,j of sound) would travel through them would 

 be proportional to the square roots of their respective elasticities ; 

 e.g., if the elasticity of one was represented by the number four, 

 and of the other by sixteen, the velocity of the wave in the second 

 would be twice as great as in the first, i.e., as four is to two. Let 

 me put it in another way ; if we find that a wave travels through 

 one medium twice as fast as through another, we know the 

 elasticity of the first is four times as great as in the second, provided 

 of course the densities are the same in each. Well, the velocity 

 of a wave of light through the Ether is more than a million times 

 greater than that of a wave of sound through air. JBy the above 

 law then the elasticity of the Ether should be at least a million 

 million times greater than of air. So it is said to be ; and so far 

 we can easily understand. But another law in physics tells us that 

 the elasticity of a substance is measured by its resistance to pressure. 

 If you have two india-rubber balls, and it is more difficult to indent 

 one than the other,Ji.e. it takes a greater pressure, then that ball is 

 the more elastic of the two, and if thrown to the ground will 

 rebound to a greater height. Whence even it the density of the 

 Ether were as great as that of the air (it is infinitely less), its 

 elasticity being at least a milhon million times greater, its resist- 

 ance to pressure ought also to be a miliion million times greater ; 

 i.e. 16 million million pounds to the square inch, which is about 

 the weight of a cubic mile of granite But this reasoning has 

 landed us in an absurdity, for we have already seen, not only that 

 the Ether offers no resistance at all, or one infinitely small, but that 

 by the nature of the hypothesis it permeates the body itself, which 

 air does not. Our earth rolling onward in its orbit at the rate of 

 66000 miles an hour suffeis no retardation whatever. The two 

 properties of infinite elasticity and infinite permeability, both 

 attaching to the Ether, are in fact utterly opposed to each other 

 and cannot be co-inherent in the same body ; the more elastic a 

 substance is the less permeable it must be. There is, I repeat again, 

 no substance known to act as the Ether does, none whatever which 



