202 ELECTRICAL PHENOMENA [CH. XXL 



they tend to drag it about. All known lines of 

 mechanical force reach from atom to atom, they never 

 terminate in ether ; except indeed at an advancing 

 wave front. At a wave front is to be found one 

 constituent of a mechanical pressure of radiation whose 

 other constituent acts on the source. This is an 

 interesting but essentially non-statical case, and it 

 leads away from our subject. 



As to the nature of an electron, regarded as an 

 ethereal phenomenon, it is too early to express any 

 opinion. At present it is not clear why a positive 

 charge should cling so tenaciously in a mass, while 

 an outstanding negative electron should readily 

 escape and travel free. Nor is the nature of gravi- 

 tation yet understood. When the electron theory 

 is complete, to. the second order, or some higher even 

 order, of small quantities it is complete now to the 

 second order if electrons may be treated as geo- 

 metrical points, it is hoped that the gravitative 

 property also will fall into line and form part of 

 the theory ; at present it is an empirical fact which 

 we observe without understanding ; as has been 

 our predicament not only since the days of Newton 

 but for centuries before : though we did not, before 

 Newton, know its importance in the cosmic scheme. 



Attention has hitherto been chiefly concentrated 

 on the freely moving active negative ingredient, the 

 more sluggish positive charges are at first of less 

 interest, but the behaviour of electrons cannot be 

 fully and properly understood without a knowledge 

 of the nature and properties of the positive con- 

 stituent too. According to Larmor, positive charge 

 must be the mirror-image of negative charge, in 

 essential constitution. 



The positive electron has not, so far as I know, been 



