x PREFACE TO THE FIFTH EDITION. 



adopted this view, the former with a fair approximative 

 attempt at numerical calculation, but no one of these 

 philosophers seems to have connected it with the in- 

 destructibility of force. A passage in the writings of 

 Dr. Roget, combating the theory that mere contact of 

 dissimilar bodies was the source of voltaic electricity, 

 philosophically supports his argument by the idea of 

 non-creation of force. 



As I have introduced into the later editions of my 

 Essay abstracts of the different discoveries which I 

 have found, since my first lectures, to bear upon the 

 subject, I have been regarded by many rather as the 

 historian of the progress made in this branch of thought 

 than as one who has had anything to do with its initia- 

 tion. Every one is but a poor judge where he is him- 

 self interested, and I therefore write with diffidence ; 

 but it would be affecting an indifference which I do not 

 feel if I did not state that I believe myself to have 

 been the first who introduced this subject as a gene- 

 ralised system of philosophy, and continued to enforce 

 it in my lectures and writings for many years, during 

 which it met with the opposition usual and proper to 

 novel ideas. 



Avocations necessary to the well-being of others 

 have prevented my following it up experimentally, to 

 the extent that I once hoped ; but I trust and believe 

 that this Essay, imperfect though it be, has helped 

 materially to impress on that portion of the public 

 which devotes its attention rather to the philosophy of 

 science than to what is now termed science, the truth 

 of the thesis advocated. 



