46 (io) 



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 hete- 

 rogeneous particles. 9 When 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 pre- 

 cisely 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 met- 

 als is heated, electricity is developed. If the two metals 

 be bismuth and antimony, as represented in this dia- 

 gram, 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. Moreover, 

 heat and light have the very closest analogy ; exalt the 

 rapidity with which the molecules move and light ap- 

 pears, the difference being only one of intensity. 



