ACTUAL AND POTENTIAL AGENCY. 71 



years before. We may thus render muscular force latent in a 

 stretched bowstring, raised cannon-ball, or other instrument, for 

 any length of time. This latent force is generally spoken of as 

 potential energy, while the active force exertable at any moment 

 by the flying arrow or falling ball constitutes its actual or dynamic 

 energy. Thus the actual energy of my arm becomes the potential 

 energy of the crossbow, reappears as the actual energy of the 

 projectile, and is finally not lost, but dissipated in the form of 

 heat. The point, then, I wish particularly to impress upon you 

 is, that the actual mechanical energy manifested in the falling 

 together, or springing together, of two separated bodies, the ball 

 and the earth, the cord and the bow, is only a liberation of the 

 potential energy stored up in them at the moment they were 

 pulled apart from one another. 



(75.) Let us now turn our attention to an altogether different 

 kind of pulling apart, namely, the pulling apart of oxygen and 

 hydrogen from their state of chemical combination. I am here 

 repeating the well-known experiment of the electrolytic decom- 

 position of water. By means of a galvanic battery I am tearing 

 asunder the previously combined oxygen and hydrogen, and col- 

 lecting the two gases in separate cylinders. Now, what is the 

 nature of this separation, and how is it brought about ? Seizing 

 one link in the chain of actions as a starting-point, let us consider 

 first of all the combustion of zinc in the battery-cell ; for in all 

 ordinary batteries the direct or indirect oxidation of zinc is the 

 source of the power obtained. Upon holding this bundle of 

 loose zinc shavings in the blowpipe flame, you see that the 

 metal takes fire from time to time and burns with very great 

 brilliancy, being converted into the white flocculi of oxide of 

 zinc which are now floating about the room. If we introduce 

 the ignited zinc into a vessel of chlorine, it continues to burn, 

 you observe, with even greater intensity than before, producing 

 abundant fumes of chloride of zinc. But we may obtain chloride 

 of zinc more readily by dissolving the metal in hydrochloric 

 acid; and in this case, also, the combination of zinc with chlorine 

 is attended by an evolution of heat. Thus, the solution of 



