THE SIZE OF ATOMS. 165 



generating more than enough heat to melt each 

 throughout ; just as a large quantity of gunpowder 

 if ignited in any one spot burns throughout with- 

 out fresh application of heat. Hence plates of 

 zinc and copper of 1/300,000, oooth of a centimetre 

 thick, placed close together alternately, form a near 

 approximation to a chemical combination, if indeed 

 such thin plates could be made without splitting 

 atoms." 



In making brass, if we mix zinc and copper 

 together we find no very manifest signs of chemical 

 affinity at all ; there is not a great deal of heat 

 developed ; the mixture does not become warm, 

 it does not explode. Hence we can infer certainly 

 that contact-electricity action ceases, or does not 

 go on increasing according to the same law, when 

 the metals are subdivided to something like 

 i/ioo,ooo,oooth of a centimetre. Now this is an 

 exceedingly important argument. I have more 

 decided data as to the actual magnitude of atoms 

 or molecules to bring before you presently, but 

 I have nothing more decided in giving for certain 

 a limit to supposable smallness. We cannot reduce 



