78 FKAGMENTS OF SCIENCE. 



numerous processes in Nature which entirely elude the eye 

 of the body, and must be figured by the eye of the mind. 

 The processes of chemistry are examples of these. Long 

 thinking and experimenting on the materials which compose 

 our world have led philosophers to conclude that matter is 

 composed of atoms from which, whether separate or in com- 

 bination, the whole material world is built up. The air we 

 breathe, for example, is mainly a mixture of the atoms of 

 two distinct substances, called oxygen and nitrogen. The 

 water we drink is also composed of two distinct substances, 

 called oxygen and hydrogen. But it differs from the air in 

 this particular, that in water the oxygen and hydrogen are 

 not mechanically mixed, but chemically combined. In fact, 

 the atoms of oxygen and those of hydrogen exert enormous 

 attractions on each other, so that when brought into sufficient 

 proximity they rush together with an almost incredible 

 force to form a chemical compound. But powerful as is the 

 force with which these atoms lock themselves together, we 

 have the means of tearing them asunder, and the agent by 

 which we accomplish this may here receive a few moments' 

 attention. 



Into a vessel containing acidulated water I dip these 

 two strips of metal, the one being zinc and the other plati- 

 num, not permitting them to touch each other in the liquid. 

 I now connect the two upper ends of the strips by a piece 

 of copper wire. The wire is apparently unchanged, but it 

 is not so in reality. It is now the channel of what, for 

 want of a better name, we call an electric current a power 

 generated and maintained by the chemical action going on 

 in the vessel of acidulated water. What the inner change 

 of the wire is we do not know, but we do know that a change 

 has occurred, by the external effects produced by the wire. 

 Let me show you one or two of these effects. And here it 

 is convenient to operate with greater power than can be ob- 

 tained from a single small pair of strips of metal, and a 



