1870.] BARKER— ON VITAL AND PHYSICAL FORCES. 419 



meant attraction ; and by actual energy, motion. It is in the latter 

 sense that we shall use the word force in this lecture ; and we shall 

 speak of the forces of heat, light, electricity and mechanical motion, 

 and of the attractions of gravitation, cohesion, chemism. 



From what has now been said, it is obvious that when we speak 

 of the forces of heat, light, electricity or motion, we mean simply 

 the different modes of motion called by these names. And when 

 we say that they are correlated to each other, we mean simply that 

 the mode of motion called heat, light, electricity, is convertible 

 into any of the others, at pleasure. Correlation therefore implies 

 convertibility and mutual dependence and relationship. 



Having now defined the use of the term force, and shown that 

 forces are correlated which are convertible and mutually dependent, 

 we go on to study the evidences of such correlation among the 

 motions of inorganic nature 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 alreadv hinted, the time was when these forces were believed 

 to be various kinds of imponderable 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 themselves. 

 Gradually, however, the idea of the indestructibility 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 offeree. 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 f\ict which distinguished authority has 

 pronounced the most important discovery of the present century.^ 

 These experiments distinctly proved that when any one of these 

 forces disappeared, another took its olace ;that when motion was ar- 

 rested, for example, 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^ — 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 magaificent theory. What 

 can produce motion but motion itself? luto what can motion be 

 converted, but motion ? May not these forces, thus mutually con- 



