THE SIZE OF ATOMS. 



\Friday evening Lecture bejore the Royal Institution of Grea 

 Britain^ February 3, 1883 (Proc. R. I. vol. x. p. 185).] 



FOUR lines of argument founded on observation 

 have led to the conclusion that atoms or molecules 

 are not inconceivably, not immeasurably small. 

 I use the words " inconceivably " and " immeasur- 

 ably " advisedly. That which is measurable is not 

 inconceivable, and therefore the two words put 

 together constitute a tautology. We leave in- 

 conceivableness in fact to metaphysicians. Nothing 

 that we can measure is inconceivably large or 

 inconceivably small in physical science. It may 

 be difficult to understand the numbers expressing 

 the magnitude, but whether it be very large or 

 very small there is nothing inconceivable in the 

 nature of the thing because of its greatness or 



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