Scientific Lectures. 235 



A few simple experiments will help ns to fix in onr minds the great 

 fact of the convertibility of force. Starting with actual visible 

 motion, correlation reqnires 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 

 received upon this anvil would be simply changed in direction, if 

 allowed to fall upon this bar of lead, is converted into heat; the evi- 

 dence of which is that a piece of phosphorus placed upon the lead is 

 at once inflamed. 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, as in the case of friction. If this cylinder 

 containing water or alcohol be caused to revolve rapidly between the 

 two sides of this wooden rubber, the heat due to the arrested motion 

 will raise the temperature of the liquid to the boiling point, and the 

 cork will be expelled. But motion may also be converted into 

 electricity. Indeed electricity is always the result of friction between 

 heterogeneous particles. * AVhen this piece of hard rubber, for 

 example, is rubbed with the fur of a cat, it is at once electrified ; and 

 now if it be caused to communicate a portion of its charge to this 

 glass plate, to which, at the same time, we add the mechanical motion 

 of rotation, the strong sparks produced give evidence of the con- 

 version. 



So, too, taking heat as the initial force, motion, light, electricity 

 may be produced. In every steam engine the steam which leaves 

 the cylinder is cooler than that which entered it, and cooler by 

 exactly the amount of work done. The motion of the piston's mass 

 is precisely that lost by the steam molecules which batter against it. 

 The conversion of heat into electricity, too, is also easily effected. 

 When the junction of two metals is heated, electricity is developed. 

 If the two metals be bismuth and antimony, as represented in this 

 diagram, the currents flow as indicated by the arrows ; and by 

 multiplying the number of pairs, the effect may be proportionately 

 increased. Such an arrangement, called a thermo-electric battery, 

 we have here ; and by it the heat of a single gas burner may be 

 made to move, when converted, this little electric bell engine. More- 



♦ Id., ib., Am. ed., p. 33, et t^eq. 



