THEORY. 187 



replaced immediately by another, equivalent for 

 equivalent. Thus,, & force A, being given in action, 

 as soon as it ceases to act, we see it replaced im- 

 mediately by its equivalent of another force B. 

 And when the force B ceases to manifest itself, 

 it is immediately replaced by its equivalent of 

 another force C, D, or E, or the force A re-ap- 

 pears. 



Hence we see one kind of motion (friction) 

 transformed into heat or electricity, according to 

 the substance submitted to experiment. By rub- 

 bing wood we produce heat, by rubbing glass 

 or resin, electricity. Again, motion (pression) is 

 transformed into electricity, when we press the 

 angles of certain crystals, such as Iceland spar. 

 In the same way heat can be transformed into 

 motion (steam-engine), into electricity (thermo- 

 electric currents), into light, into chemical action, 

 etc. And each of these new kinds of motions 

 (forces) generated may in its turn be transformed 

 into heat or into any of the others. By heating 

 water, the molecules of this body are put into 

 motion ; by heating a bar of antimony soldered to 

 a bar of bismuth, a certain amount of the vibra- 

 tion called heat is transformed into its equivalent 

 of electricity. The same transformation takes 

 place when crystals of Tourmaline or Boracite are 

 heated. 



When we heat a platinum wire by means of an 



