44 (8) 



and mutually dependent, we go on to study the evidences 

 of such correlation among the motions of inorganic na- 

 ture usually called physical forces ; and to ask what 

 proof science can furnish us that mechanical motion, 

 heat, light, and electricity are thus mutually convertible. 

 As we have already hinted, the time was when these 

 forces were believed to be various kinds of imponder- 

 able matter, and chemists and physicists talked of the 

 union of iron with caloric as they talked of its union 

 with sulphur, regarding the caloric as much a distinct 

 and inconvertible entity as the iron and sulphur them- 

 selves. Gradually, however, the idea of 4he indestruct- 

 ibility of matter extended itself to force. And as it 

 was believed that no material particle could ever be 

 lost, so, it was argued, no portion of the force existing 

 in nature can disappear. Hence arose the idea of the 

 indestructibility of force. But, of course, it was quite 

 impossible to stop here. If force cannot be lost, the 

 question at once arises, what becomes of it when it 

 passes beyond our recognition ? This question led to 

 experiment, and out of experiment came the great fact 

 of force-correlation ; a fact which distinguished authority 

 has pronounced the most important discovery of the 

 present century. 6 These experiments distinctly proved 

 that when any one of these forces disappeared, another 

 took its place ; that when motion was arrested, for ex- 

 ample, heat, light or electricity was developed. In short, 

 that these forces were so intimately related or correlated 

 to use the word then proposed by Mr. Grove 7 that 

 when one of them vanished, it did so only to reappear 

 in terms of another. But one step more was necessary 

 to complete this magnificent theory. What can produce 



