MATTER AND FORCE 67 



The processes of chemistry are examples of these. Long 

 thinking and experimenting has led philosophers to con- 

 clude that matter is composed of atoms from which, 

 whether separate or in combination, the whole material 

 world is built up. The air we breathe, for example, is 

 mainly a mechanical mixture of the atoms of oxygen and 

 nitrogen. The water we drink is also composed of oxy- 

 gen 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. 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 two 

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

 num, not permitting them to touch each other in the 

 liquid. I connect the two upper ends of the strips by 

 a piece of copper wire. The wire is now the channel of 

 what, for want of a better name, we call an "electric cur- 

 rent." 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. Before you is a series 

 of ten vessels, each with its pair of metals, and I wish 

 to get the added force of all ten. The arrangement is 

 called a voltaic battery. I plunge a piece of copper wire 

 among these iron filings; they refuse to cling to it. I 



