CONDUCTION THROUGH GASES 151 



observed mass of the electron may be regarded 

 as an effect due to the electro-magnetic inertia 

 of its electric charge. Representing the atoms of 

 ordinary matter as made up of negative electrons 

 scattered in space round some central positive 

 nucleus, it becomes possible to explain their mass 

 by the electro-magnetic properties of an electric 

 charge. To explain other phenomena, it is 

 supposed that the electrified corpuscles — the 

 electrons — are in rapid orbital or oscillatory 

 motion within the atom : that, for example, the 

 electrons whirl round the nucleus in their orbits 

 as the planets swing round the sun, and thus we 

 get a first picture of the atom which has been 

 filled in in much detail by later research. 



Mass or inertia is the most permanent and 

 characteristic property of matter, and having 

 explained mass as due to electricity in motion, the 

 physicist may well ask the metaphysical question : 

 has matter any objective reality ; may not its very 

 essence be but a form of disembodied energy ? 

 And here the philosophical speculation of 1904 

 is in accord with the mathematical principle of 

 relativity of to-day. On that principle, matter 

 and energy are of the same nature, and both 

 intimately bound up with the properties of that 

 combined space-time which is more real than 

 either time or space independently. 



An attempt to obtain a more vivid picture of 

 the electro-magnetic field was made by J. J. 

 Thomson by means of the conception of tubes of 

 force, a conception which we owe to the instinctive 

 insight of Faraday. A small electrified body, 

 carrying, let us suppose, a negative charge, is well 

 known to attract other bodies in the neighbour- 



