io CORRELATION OF PHYSICAL FORCES. 



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 

 similar 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, electricity, 

 magnetism, chemical affinity, and motion, are all correlative, or 

 have a reciprocal dependence; that neither, taken abstractedly, 

 can be said to be the essential cause of the others, 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 developed : and that 

 the same must hold good of other forces, it being an irresis- 

 tible 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 view will be given in 

 subsequent parts of this Essay, it would be going too far, at 

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

 term force in reference to them, as meaning that active prin- 

 ciple inseparable from matter which is supposed to induce its 

 various changes. 



The word force and the idea it aims at expressing, might be 

 objected to by the purely physical philosopher on similar 

 grounds to those which apply to the word cause, as it repre- 

 sents a subtle mental conception, and not a sensuous per- 

 ception or phenomenon. The objection would take something 

 of this form. If the string of a bent bow be cut, the bow will 

 straighten itself; we thence say there is an elastic force in the 

 bow which straightens it ; but if we applied our expressions to 



