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stance depends. principally on' the relation existing ‘between ' its 
elasticity and its ‘density. ‘I'he former helps while the latter hinders 
waye passage. Now asa general rule the elasticity of a solid in 
relation to its density is much higher than in the case of liquids 
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 (say 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 fouris 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 waive of light through the Ether is more than a million times 
greater than that of a wave of sound through air. By’ the above 
law then the elasticity of the Ether should be at least a million 
million times greater than of air. Soitis 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, i.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 if the density of the 
Ether were as great as that of the air (it is infinitely less), its 
elasticity being at least a million million times greater, its resist- 
ance to pressure ought also to be a miliion million times greater ; 
i.e.'15 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 suffers 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 actas the Ether does, none whatever which 
