August 27, 1909] 



SCIENCE 



269 



ments that this arises entirely from the 

 electric charge, and is therefore due to the 

 ether carried along with the corpuscle by 

 the lines of force attached to it. 



A simple calculation shows that one half 

 of this mass is contained in a volume seven 

 times that of a corpuscle. Since we know 

 the volume of the corpuscle as well as the 

 mass, we can calculate the density of the 

 ether attached to the corpuscle; doing so, 

 we find it amounts to the prodigious value 

 of about 5 X 10'^ or about 2,000 million 

 times that of lead. Sir Oliver Lodge, by 

 somewhat different considerations, has ar- 

 rived at a value of the same order of mag- 

 nitude. 



Thus around the corpuscle ether must 

 have an extravagant density: whether the 

 density is as great as this in other places 

 depends upon whether the ether is com- 

 pressible or not. If it is compressible, 

 then it may be condensed round the cor- 

 puscles, and there have an abnormally 

 great density ; if it is not compressible, then 

 the density in free space can not be less 

 than the number I have just mentioned. 



With respect to this point we must re- 

 member that the forces acting on the ether 

 close to the corpuscle are prodigious. If 

 the ether were, for example, an ideal gas 

 whose density increased in proportion to 

 the pressure, however great the pressure 

 might be, then if, when exposed to the 

 pressures which exist in some directions 

 close to the corpuscle, it had the density 

 stated above, its density under atmospheric 

 pressure would only be about 8 X 10'^°, 

 or a cubic kilometer would have a mass less 

 than a gram; so that instead of being al- 

 most incomparably denser than lead, it 

 would be almost incomparably rarer than 

 the lightest gas. 



I do not know at present of any effect 

 which would enable us to determine 

 whether ether is compressible or not. And 



although at first sight the idea that we are 

 immei-sed in a medium almost infinitely 

 denser than lead might seem inconceivable, 

 it is not so if we remember that in all 

 probability matter is composed mainly of 

 holes. We may, in fact, regard matter as 

 possessing a bird-cage kind of structure in 

 which the volume of the ether disturbed by 

 the wires when the structure is moved is 

 infinitesimal in comparison with the vol- 

 ume enclosed by them. If we do this, no 

 difficulty arises from the great density of 

 the ether; all we have to do is to increase 

 the distance between the wires in propor- 

 tion as we increase the density of the ether. 



Let us now consider how much ether is 

 carried along by ordinary matter, and what 

 effects this might be expected to produce. 



The simplest electrical system we know,, 

 an electrified sphere, has attached to it a 

 mass of ether proportional to its potential 

 energy, and such that if the mass were to 

 move with the velocity of light its kinetic- 

 energy would equal the electrostatic poten- 

 tial energy of the particle. This result can 

 be extended to any electrified system, and 

 it can be shown that such a system binds a 

 mass of the ether proportional to its poten- 

 tial energy. Thus a part of the mass of 

 any system is proportional to the potential 

 energy of the system. 



The question now arises. Does this part 

 of the mass add anything to the weight of 

 the body ? If the ether were not subject to 

 gravitational attraction it certainly would 

 not ; and even if the ether were ponderable, 

 we might expect that as the mass is swim- 

 ming in a sea of ether it would not increase 

 the weight of the body to which it is at- 

 tached. But if it does not, then a body with 

 a large amount of potential energy may 

 have an appreciable amount of its mass in 

 a form which does not increase its weight,, 

 and thus the weight of a given mass of it 

 may be less than that of an equal mass of 



