THE SIZE OF ATOMS. 161 



not against one another, but against these ful- 

 crums. There are innumerable say thousands of 

 little particles of dust jammed between the glass 

 surfaces, some of them of perhaps i/3OOOth of a 

 centimetre in diameter, say 5 or 6 wave-lengths. 

 If you lay one piece of glass on another, you 

 think you are pressing glass on glass, but it is 

 nothing of the kind : it is glass on dust. This 

 is a very beautiful phenomenon, and my object 

 in showing this experiment was simply because 

 it gives us a linear measure bringing us down 

 at once to i/ioo,oooth of a centimetre. 



Now I am going to enter a little into detail 

 regarding the reasons that four lines of argument 

 give us for assigning a limit to the smallness of 

 the molecules of matter. I shall take contact 

 electricity first, and very briefly. 



If I take these two pieces of zinc and copper 

 and touch them together at the two corners, they 

 become electrified, and attract one another with 

 a perfectly definite force, of which the magnitude 

 is ascertained from absolute measurements in con- 

 nection \vith the well-established doctrine of contact 



II 



