THEORY. 187 
replaced immediately by another, equivalent for 
equivalent. Thus, a 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 H, 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) 1s 
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 
