Recent Theories of Electricity. 217 



attempt to explain the cause for this electrical property of 

 matter any more than I should for its gravitational attributes. 

 Both are fundamental phenomena to be accepted as experi- 

 mental facts until we gain contrary knowledge. Indeed, 

 I have ventured to indulge in this speculation rather with 

 the idea of showing that the recent hypotheses for electricity 

 and matter ; for the aether, protions, and corpuscular light ; 

 for the electromagnetic and other non-Newtonian mechanics, 

 are not necessary. We may still account as adequately for 

 all our experimental facts by a simple addition to the pro- 

 perties of matter and continue to base our theories on 

 mechanical laws. 



So long as the measurement of physical quantities becomes 

 ultimately a matter of measuring mechanical forces, it is 

 advisable to express quantitative physical laws in terms of 

 mechanical formulae. For this reason electricity should be 

 considered a function of mechanical energy rather than the 

 converse. If it be possible to place mechanics on an electro- 

 dynamic basis, it is certain that we may always explain 

 electricity in terms of ponderodynamic laws. As both are 

 possible, it seems far more natural and more rational to con- 

 sider electricity as an attribute of matter than matter as a 

 phenomenon of electricity. 



Before this article is closed there is a point which should be 

 discussed. The close analogy between electromagnetic mass 

 and the apparent increase in mass in hydrodynamic problems 

 has been pointed out by me in a former paper *, by Professor 

 Lorentz, and must have occurred to everyone. But Professor 

 Lorentz indicates a difference between the two which seems 

 to me less essential than he considers it. To be exact, I 

 quote from him f- In the problem of a ball moving in a 

 perfect fluid, " we are able to determine the effective mass 

 m Q + m' (or m + m"), but it would be impossible to find the 

 values of m and m (or m") separately. Now, it is very 

 important that, in the experimental investigation of the 

 motion of the electron, we can go a step farther. This is 

 due to the fact that the electromagnetic mass is not a con- 

 stant, but increases with the velocity.'" This is, if true, 

 important, as it seems to show r an essential difference between 

 these two apparent masses. But it can be shown, as follows, 

 that such an essential difference does not exist. We know 

 that a single electron will maintain a state of rest or of 

 uniform motion in a straight line just as a single ball in an 



* Phil. Mag. vol. xviii. p. 17 (1900). 

 f ' Theory of Electrons/ p. 40. 



