CONCLUDING REMARKS. 167 



vided the relative motion is the same, and so, without excep- 

 tion, of all other phenomena. The question of whether there 

 can be absolute motion, or, indeed, any absolute isolated 

 force, is purely the metaphysical question of idealism or 

 realism a question for our purpose of little import ; sufficient 

 for the purely physical enquirer, the maxim, ' Denon apparenti- 

 bits et non existentibus eadem est ratio' 



. The sense I have attached to the word Correlation, in 

 treating of physical phenomena, will, I think, be evident, from 

 the previous parts of this Essay, to be that of a necessary 

 reciprocal production ; in other words, that any force capable of 

 producing another may, in its turn, be produced by it nay, 

 more, can be itself resisted by the force it produces, in pro- 

 portion to the energy of such production, as action is ever 

 accompanied and resisted by reaction : thus, the action of an 

 electro-magnetic machine is reacted upon by the magneto- 

 electricity developed by its action. 



To many, however, of the cases we have been considering, 

 the term correlation may be applied in a more strict accord- 

 ance with its original sense : thus, with regard to the forces of 

 electricity and magnetism in a dynamic state, we cannot 

 electrise a substance without magnetising it we cannot mag- 

 netise it without electrising it each molecule, the instant it is 

 affected by one of these forces, is affected by the other ; but, 

 in transverse directions, the forces are inseparable and 

 mutually dependent correlative, but not identical. 



The evolution of one force or mode of force into another 

 has induced many to regard all the different natural agencies 

 as reducible to unity, and as resulting from one force which is 

 the efficient cause of all the others : thus, one author writes to 

 prove that electricity is the cause of every change in matter ; 

 another, that chemical action is the cause of everything ; 

 another, that heat is the universal cause, and so on. If, as I 

 have stated it, the true expression of the fact is, that each 

 mode of force is capable of producing the others, and that 

 none of them can be produced but by some other as an 

 anterior force, then any view which regards either of them as 

 abstractedly the efficient cause of all the rest, is erroneous : 

 the view has, I believe, arisen from a confusion between the 



