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motion but motion itself? Into what can motion be con- 

 verted, but motion ? May not these forces, thus mutu- 

 ally convertible, be simply different modes of motion of 

 the molecules of matter, precisely as mechanical motion 

 is a motion of its mass ? Thus was born the dynamic 

 theory of force, first brought out in any completeness by 

 Mr. Grove, in 1842, in a lecture on the " Progress of 

 Physical Science," delivered at the London Institution. 

 In that lecture he said : " Light, heat, electricity, mag- 

 netism, motion, are all convertible material affections. 

 Assuming either as the cause, one of the others will be 

 the effect. Thus heat may be said to produce electricity, 

 electricity to produce heat ; magnetism to produce elec- 

 tricity, electricity magnetism ; and so of the rest." 8 



A few simple experiments will help us to fix in our 

 minds the great fact of the convertibility of force. 

 Starting with actual visible motion, correlation requires 

 that when it disappears as motion, it should reappear as 

 heat, light, or electricity. If the moving body be elastic 

 like this rubber ball, then its motion is not destroyed 

 when it strikes, but is only changed in direction. But 

 if it be non-elastic, like this ball of lead, then it does 

 not rebound ; its motion is converted into heat. The 

 motion of this sledge-hammer, for example, which if re- 

 ceived upon this anvil would be simply changed in 

 direction, if allowed to fall upon this bar of lead, is 

 converted into heat ; the evidence of which is that a 

 piece of phosphorus placed upon the lead is at once in- 

 flamed. So too, if motion be arrested by the cushion 

 of air in this cylinder, the heat evolved fires the tinder 

 carried in the plunger. But it is not necessary that the 

 arrest of motion should be sudden ; it may be gradual, 



