INTRODUCTORY REMAKES. 19 



abstractedly that heat is the cause of electricity, or that elec- 

 tricity is the cause of heat ? Certainly not ; for if either be 

 true, both must be so, and the effect then becomes the cause 

 of the cause, or, in other words, a thing causes itself. Any 

 other proposition on this subject will be found to involve sim- 

 ilar difficulties, until, at length, the mind will become con- 

 vinced that abstract secondary causation does not exist, and 

 that a search after essential causes is vain. 



The position which I seek to establish in this Essay is, 

 that the various affections of matter 'which constitute the 

 main objects of experimental physics, viz., heat, light, elec- 

 tricity, magnetism, chemical affinity, and motion, are all cor- 

 relative, or have a reciprocal dependence ; that neither, taken 

 abstractedly, can be said to be the essential cause of the oth- 

 ers, but that either may produce or be convertible into, any 

 of the others : thus heat may mediately or immediately produce 

 electricity, electricity may produce heat ; and so of the rest, 

 each merging itself as the force it produces becomes devel- 

 oped : and that the same must hold good of other forces, it be- 

 ing an irresistible inference from observed phenomena that a 

 force cannot originate otherwise than by devolution from some 

 pre-existing force or forces. 



The term force, although used in very different senses by 

 different authors, in its limited sense may be defined as that 

 which produces or resists motion. Although strongly inclined to 

 believe that the other affections of matter, which I have above 

 named, are, and will ultimately be resolved into, modes of 

 motion, many arguments for which will be given in subse- 

 quent parts of this Essay, it would be going too far, at pre- 

 sent, to assume their identity with it ; I therefore use the term 

 force in reference to them, as meaning that active principle 

 inseparable from matter which is supposed to induce its vari- 

 ous changes. 



The word force and the idea it aims at expressing might 

 be objected to by the purely physical philosopher on similar 



