196 CONTINUITY. 



an active part for many years, dating from a quarter of a 

 century, in promoting this view, I may not be considered an 

 impartial judge) that it is now proved that all these forces are 

 so invariably connected inter se and with motion as to be 

 regarded as modifications of each other, and as resolving 

 themselves objectively into motion, and subjectively into that 

 something which is supposed to produce or resist motion, and 

 which we call force. 



I may perhaps be permitted to recall a forgotten experi- 

 ment, which nearly a quarter of a century ago I showed at 

 the London Institution, an experiment simple enough in itself, 

 but which then seemed to me important from the consequences 

 to be deduced from it, and the importance of which will be 

 much better appreciated now than then. 



A train of multiplying wheels ended with a small metallic 

 wheel, which, when the train was put in motion, revolved with 

 extreme rapidity against the periphery of the next wheel, a 

 wooden one. In the metallic wheel was placed a small piece of 

 phosphorus, and as long as the wheels revolved the phosphorus 

 remained unchanged ; but the moment the last wheel was 

 stopped, by moving a small lever attached to it, the phosphorus 

 burst into flame. My object was to show that while motion 

 of the mass continued heat was not generated, but that when 

 this was arrested, the force continuing to operate, the motion 

 of the mass became heat in the particles. The experiment 

 differed from that of Rumford's cannon-boring and Davy's 

 friction of ice in showing that there was no heat while the 

 motion was unresisted, but that the heat was dependent on 

 the motion being impeded or arrested. We have now become 

 so accustomed to this view, that whenever we find motion 

 resisted we look to heat, electricity, or some other force as the 

 necessary and inevitable result. 



It would be out of place here, and treating of matters too 

 familiar to the bulk of my audience, to trace how, by the 

 labours of Oersted, Seebeck, Faraday, Talbot, Daguerre, and 

 others, materials have been provided for the generalisation 

 now known as the correlation of forces or conservation of 

 energy ; while Davy, Rumford, Seguin, Mayer, Joule, Helm- 

 holtz, Thomson, and others (among whom I would not name 



