MATTER AND ETHER
touch by means of electrified bodies, whether this universe can be set in motion by bodies which are not electrified, is a question on which we have as yet no decisive evidence.
Let us for the moment confine ourselves to the case of electrified bodies, the fact that when these move they have to set some of the ether in motion must affect their apparent mass: for exactly the same reason that the apparent mass of a body is greater when it is immersed in water than when it is in a vacuum; when we move the body through the water we have to set in motion not merely the body itself but also some of the water around it, in some cases the increase in the apparent mass of the body due to this cause may be much greater than the mass of the body itself, this is the case, for example with air bubbles in water which behave as if their mass were many hundred times the mass of the air enclosed in them. In the case of the electrified bodies we may picture to ourselves that the connection between them and the ether around them is established in the following way, we may suppose that the lines of electric force which proceed from these charged bodies and pass through the ether, grip as it were some of the ether and carry it along with them as they move; by means of the laws of electricity we can calculate the mass of ether gripped by these lines in any portion of space through which they pass. The results of this calculation can be expressed in a very simple way. Faraday and Maxwell have taught us to look for the seat of the potential energy of an electrified system in the space around the system and not in the system itself, each portion of space possessing an amount of this energy for which Maxwell has given a very simple expression. Now it is remarkable that if we calculate the mass of the ether gripped by the lines of electric force in any part of the space surrounding the charged bodies we find that it is exactly proportional to the amount of potential energy in that space, and is given by the rule that if this mass were to move with the