velocity of light the kinetic energy it would possess would be equal to the electrostatic energy in the portion of space for which we are calculating the mass. Thus the total mass of the ether gripped by an electrical system is proportional to the electrostatic potential energy of that system. Since the ether is only set in motion by the sideways motion of the lines of force and not by their longitudinal motion, the actual mass of the ether set in motion by the electrified bodies will be somewhat less than that given by the preceding rule, except in the special case when all the lines of force are moving at right angles to their length. The slight correction for this slipping of the lines of force through the ether does not affect the general character of the effect, and in what follows I shall for the sake of brevity take the mass of the ether set in motion by an electrified system to be proportional to the potential energy of that system. The electrified body has thus associated with it an etherial or astral body which it has to carry along with it as it moves and which increases its apparent mass. Now this piece of the unseen universe which the charged body carries along with it may be expected to have very different properties from ordinary matter: it would of course defy chemical analysis and probably would not be subject to gravitational attraction, it is thus a very interesting problem to see if we can discover any case in which the etherial mass is an appreciable fraction of the total mass, and to compare the properties of such a body with those of one whose etherial mass is insignificant. Now in any ordinary electrified system, such as electrified balls or charged Leyden jars the roughest calculation is sufficient to show that the etherial mass which they possess in virtue of this electrification is absolutely insignificant in comparison with their total mass. Instead, however, of considering bodies of appreciable size let us go to the atoms of which these bodies are composed, and suppose as seems probable that these are electrical
