420 THE CANADIAN NATURALIST. [D 



ec. 



vertible, be simply different modes of motion of the molecules of 

 matter, precisely as mechanical motion is a motion of its mass ? 

 Thus was born the dynamic theory of force, first brought out in 

 any completeness by Mr. Grove, in 1842, in a lecture on the 

 " Progress of Physical Science," delivered at the London Institu- 

 tion. In that lecture he said : " Light, heat, electricity, mag- 

 netism, motion, are all convertible material affections. Assuming 

 either as the cause, one of the others will be the effect. Thus 

 heat may be said to produce electricity, electricity to produce 

 heat ; magnetism to produce electricity, electricity magnetism ; 

 and so of the rest." ® 



A few simple experiments will help us to fix in our minds the 

 great fact of the convertibility of force. Starting with actual 

 visible motion, correlation requires 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 mo- 

 tion is not destroyed when it strikes, but is only changed in di- 

 rection. 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 evidence of which 

 is that a piece of phosphorus placed upon the lead is at once in- 

 flamed. 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 cylindei' 

 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.^ When this piece of hard rub- 

 ber, for example, is rubbed with the fur of a cat, it is at once elec- 

 trified ; and now if it be caused to communicate a portion of its 

 charge to this glass plate, to which at the same time wc add the 

 mechanical motion of rotation, the strong sparks produced give 

 evidence of the conversion. 



So, ton, taking heat as the initial force, motion, light, electrici- 

 ty m.iy be produced. In every steam-engine the steam which 

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



