PREFACE. 



The theory of Electricity adopted throughout these 

 Lessons is, that Electricity, whatever its true nature, is 

 one, not two : that this Electricity, whatever it may 

 prove to be, is not matter, and is not energy j that it 

 resembles both matter and energy in one respect, how- 

 ever, in that it can neither be created nor destroyed. 

 The doctrine of the Conservation of Matter, established 

 a century ago by Lavoisier, teaches us that we can 

 neither destroy nor create matter, though we can alter 

 its distribution, and its forms and combinations, in 

 innumerable ways. The doctrine of the Conservation 

 of Energy, which has been built up during the past 

 half-century by Helmholtz, Thomson, Joule, and Mayer, 

 teaches us that we can neither create nor destroy energy, 

 though we may change it from one form to another, 

 causing it to appear as the energy of moving bodies, or 

 as the energy of heat, or as the static energy of a body 

 which has been lifted against gravity, or some other 

 attracting force, into a position whence it can run down, 

 and where it has the potentiality of doing work. So 

 also the doctrine of the Conservation of Electricity, now 

 growing into shape, 1 but here first enunciated under 

 this name, teaches us that we can neither create nor 

 destroy Electricity though we may alter its distribution, 

 may cause more to appear at one place and less at 

 another, may change it from the condition of rest to 

 that of motion, or may cause it to spin round in whirl- 

 pools or vortices, which themselves can attract or repel 



1 This is undoubtedly the outcome of the ideas of Maxwell and of 

 Faraday as to the nature of Electricity. Since the above was written an 

 elegant analytical statement of the "Doctrine of the Conservation of Elec- 

 tricity " has been published by Mons. G. Lippmann, who had independently, 

 and at an earlier date, arrived at the same view. 



