PREFACE. 



combating the theory that mere contact of dissimilar bodies was 

 the source of voltaic electricity, philosophically supports his argu- 

 ment by the idea of non-creation of force. 



As I have introduced into the later editions of my Essay ab- 

 stracts 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 ini- 

 tiation. Everyone is but a poor judge where he is himself inter- 

 ested, and I therefore write with diffidence, but it would be affect- 

 ing 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 generalised 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 prevent- 

 ed 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. 



To show that the work of to-day is not substantially different 

 from the thoughts I first published on the subject, at a period 

 when I knew little or nothing of what had been thought before, 

 I venture to give a few extracts from the printed copy of my 

 lecture of 1842 : 



Physical Science treats of Matter, and what I shall to-night term its 

 Affections ; namely. Attraction, Motion, Heat, Light, Electricity, Magnet- 

 ism, Chemical-Affinity. When these re-act upon matter, they constitute 

 Forces. The present tendency of theory seems to lead to the opinion that 

 all these Affections are resolvable into one, namely, Motion; however, 

 should the theories on these subjects be ultimately so effectually gener- 

 alised as to become laws, they cannot avoid the necessity for retaining dif- 

 ferent names for these different Affections ; or, as they would then be called, 

 different modes of Motion 



(Ersted proved that Electricity and Magnetism are two forces which act 

 upon each other ; not in straight lines, as all other known forces do, but in 

 a rectangular direction : that is, that bodies invested with electricity, or the 

 conduits of an electric-current, tend to place magnets ut right angles to 

 them ; and, conversely, that magnets tend to place bodies conducting elec- 

 tricity at right angles to them 



