16 PRESIDENT’S ADDRESS. 
and although for some purposes these are all we require, yet they do 
not tell us very much about the nature of the ether. 
The interest inspired by equations, too, in some minds is apt to be 
somewhat Platonic; and something more grossly mechanical—a model, 
for example, is felt by many to be more suggestive and manageable, 
and for them a more powerful instrument of research, than a purely 
analytical theory. 
Is the ether dense or rare? Has it a structure? Is it at rest or 
in motion? are some of the questions which force themselves upon us. 
Let us consider some of the facts known about the ether. When 
light falls on a body and is absorbed by it, the body is pushed forward 
in the direction in which the light is travelling, and if the body is free 
to move it is set in motion by the light. Now it is a fundamental prin- 
ciple of dynamics that when a body is set moving in a certain direction, 
or, to use the language of dynamics, acquires momentum in that direc- 
tion, some other mass must lose the same amount of momentum; in 
other words, the amount of momentum in the universe is constant. Thus 
when the body is pushed forward by the light some other system must 
have lost the momentum the body acquires, and the only other system 
available is the wave of light falling on the body ; hence we conclude that 
there must have been momentum in the wave in the direction in which it 
is travelling. Momentum, however, implies mass in motion. We con- 
clude, then, that in the ether through which the wave is moving there is 
mass moving with the velocity of light. The experiments made on the 
pressure due to light enable us to calculate this mass, and we find that in 
a cubic kilometre of ether carrying light as intense as sunlight is at the 
surface of the earth, the mass moving is only about one-fifty-millionth of 
amilligramme. We must be careful not to confuse this with the mass of 
a cubic kilometre of ether ; it is only the mass moved when the light passes 
through it ; the vast majority of the ether is left undisturbed by the light. 
Now, on the electromagnetic theory of light, a wave of light may 
be regarded as made up of groups of lines of electric force moving with the 
velocity of light; and if we take this point of view we can prove that the 
mass of ether per cubic centimetre carried along is. proportional to the 
energy possessed by these lines of electric force per cubic centimetre, 
divided by the square of the velocity of light. But though lines of electric 
force carry some of the ether along with them as they move, the amount 
so carried, even in the strongest electric fields we can produce, is but 
a minute fraction of the ether in their neighbourhood. 
This is proved by an experiment made by Sir Oliver Lodge in which 
light was made to travel through an electric field in rapid motion. If 
the electric field had carried the whole of the ether with it, the velocity 
of the light would have been increased by the velocity of the electric 
field. As a matter of fact no increase whatever could be detected, though 
it would have been registered if it had amounted to one-thousandth part 
of that of the field. 
