1883.] Sir William Thomson on the Size of Atoms. 185 



WEEKLY EYEXING MEETING, 



Friday, February 2, 1883. 



George Busk, Esq. F.Pi.S. Treasurer and Yice-President, 



in the Chair. 



Sir William Thomson, LL.D. F.E.S. 



The Size of Atoms. 



Four lines of argument founded on observation have led to the con- 

 clusion that atoms or molecules are not inconceivably, not immeasur- 

 ably small. I use the words " inconceivably " and "immeasurably" 

 advisedly. That which is measurable is not inconceivable, and 

 therefore the two words put together constitute a tautology. Wo 

 leave inconceivableness in fact to metaphysicians. Xothing 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 smallness, or in our views and appreciation and 

 numerical expression of the magnitude. The general result of the 

 four lines of reasoning to which I have referred, founded respectively 

 on the undulatory theory of light, on the phenomena of contact 

 electricity, on caj)illary attraction, and on the kinetic theory of gases, 

 agrees in showing that the atoms or molecules of ordinary matter 

 must be something like the l-10,000,000th, or from the 1-I0,0b0,000th 

 to the 1-100, 000,000th of a centimetre in diameter. I speak somewhat 

 vaguely, and I do so, not inadvertently, when I speak of atoms and 

 molecules. I must ask the chemists to forgive me if I even abuse the 

 words and apply a misnomer occasionally. The chemists do not 

 know what is to be the atom ; for instance, whether hydrocfen eras 

 is to consist of two pieces of matter in union constituting one molecule, 

 and these molecules flying about ; or whether single molecules each 

 indivisible, or at all events undivided in chemical action, constitute 

 the structure. I shall not go into any such questions at all, but 

 merely take the broad view that matter, although we may conceive it 

 to be infinitely divisible, is not infinitely divisible without decompo- 

 sition. Just as a building of brick may be divided into parts, into a 

 part containing 1000 bricks, and another part containing 2500 bricks, 

 and those parts viewed largely may be said to be similar or homo- 

 geneous ; but if you divide the matter of a brick building into spaces 

 of nine inches thick, and then think of subdividing it farther, you 

 find you have come to something which is atomic, that is, indivisible 

 without destroying the elements of the structure. The question of 



