184 THE LIMITATIONS OF SCIENCE 



principle to electrons, we shall need a fourth postulate. 

 We must assume that a quantity of electricity is 

 atomic in character; either the charge of electricity 

 associated with what we call an atom of matter is an 

 invariable, or, as it is now usually expressed, elec- 

 tricity is an entity which may be divided into invariable, 

 equal, and indivisible parts called electrons. As a 

 corollary to this postulate, the ponderable mass of an 

 atom is a variable depending on its velocity. It has 

 been shown that the constancy of electric quantity and 

 the variability of mass is not a necessary assumption. 

 All experiments, which involve both these quantities, 

 include them in the form of a simple ratio, e / m ; where 

 e is the quantity of electricity and m the mass of the 

 electron. It is evident that any value may be given 

 to this ratio by supposing either one of its members 

 to remain constant and the other to vary, or by sup- 

 posing both to vary in opposite fashions, and it is just 

 as reasonable to keep m constant and to let e vary with 

 the velocity, as to make the contrary assumption. This 

 is certainly possible until we have experimental evi- 

 dence which will determine our decision, and this evi- 

 dence is not likely ever to be at our disposal. 



If we attempt to estimate the results which follow 

 from the postulates of relativity, the first would be that 

 Professor Einstein has proved that we cannot theoret- 

 ically measure in space and time moving bodies exactly 

 by the laws which apply to bodies at rest. The first 



