CONCLUDING REMARKS. 165 



To take another and different example : A piece of zinc 

 dissolved in dilute sulphuric acid gives somewhat less heat 

 than when the zinc has a wire of platinum attached to it, and 

 is dissolved by the same quantity of acid. The argument is 

 deducible that, as there is more electricity in the second than 

 in the first case, there should be less heat ; but as, according 

 to our received theories, the heat is a product of the electric 

 current, and in consequence of the impurity of zinc electricity 

 is generated in the first case molecularly, by what is called 

 local action, though not thrown into a general direction, there 

 should be more of both heat and electricity in the second 

 than in the first case, as the heat and electricity due to the 

 voltaic combination of zinc and platinum are added to that 

 excited on the surface of the zinc, and the zinc should be, as 

 in fact it is, more rapidly dissolved ; so that the extra heat 

 and electricity are produced by extra chemical force. Many 

 additional cases of a similar description might be suggested. 

 But although it is difficult, and perhaps impossible, to restrict 

 the action of any one force to the production of one other 

 force, and of one only yet if the whole of one force, say 

 chemical action, be supposed to be employed in producing its 

 full equivalent of another force, say heat, then as this heat is 

 capable in its turn of reproducing chemical action, and in the 

 limit, a quantity equal or practically all but equal to the 

 initial force : if this could at the same time produce inde- 

 pendently another force, say magnetism, we could, by adding 

 the magnetism to the total heat, get more than the original 

 chemical action, and thus create force or obtain perpetual 

 motion. 



The term Correlation, which I selected as the title of my 

 Lectures in 1843, strictly interpreted, means a necessary 

 mutual or reciprocal dependence of two ideas, inseparable 

 even in mental conception : thus, the idea of height cannot 

 exist without involving the idea of its correlate, depth ; the 

 idea of parent cannot exist without involving the idea of off- 

 spring. The word itself had not been previously used ; and 

 although, as I have said, I object to the introduction of new 

 terms without strong reason, there are a vast variety of 

 physical relations which cannot certainly be so well expressed 

 by any other term. The extent to which it has been since 



