PHYSICAL FORCES 135 



the negative electricity. If now the two free ends 

 of the wires be joined, a current will be immediately 

 established, and, by making such a current pass round 

 iron, magnetism is, as before stated, necessarily and at 

 once induced. 



By means of small porcelain jars, conjoined in various 

 series, and each supplied with an acid fluid in which 

 portions of copper and zinc are suspended, powerful 

 generators of electricity (batteries) are formed, the wires 

 from either extremity of which (bearing, to speak popu- 

 larly, different electricities) will, by their approximation, 

 chemically resolve many substances and will emit small 

 sparks at each contact. When these currents are made 

 to pass between small pieces, or delicate filaments, of 

 pure charcoal attached one to the end of each wire, 

 the brilliance we know as " the electric light " will be 

 generated. Such currents also serve to work the elec- 

 tric telegraph, the telephone, and the microphone, which 

 here we can do no more than name. 



Chemical changes between certain bodies may be 

 induced by the mere proximity of similar changes going 

 on in other bodies, and examples of this process are met 

 with in photography. 



When chemical energy takes place with great 

 intensity, it is generally accompanied by light and heat 

 and the quantitative relations before referred to* as 

 existing between them, and between them and magnet- 

 ism and electricity, reveal to us a profound similarity 

 between all known physical energies. 



As to what chemical energy is in itself, we know no 

 more than we do with respect to the other forms of 

 energy. It may be that there is in Nature either one 



* See ante, p. 102. 



