CONCLUDING REMARKS. 183 



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 is 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 at least only infinitely short of the 

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

 dently 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. It has been scarcely, if at all, used by writers on 

 physics, but there are a vast variety of physical relations to 

 which, if it does not in its strictest original sense apply, 

 cannot certainly be so well expressed by any other term. 

 There are, for example, many facts, one of which cannot take 

 place without involving the other ; one arm of a lever can- 

 not be depressed without the other being elevated the finger 

 cannot press the table without the table pressing the finger. 

 A body cannot be heated without another being cooled, or 

 some other force being exhausted in an equivalent ratio to 

 the production of heat ; a body cannot be positively elec- 

 trified without some other body being negatively electri- 

 fied, &c. 



