182 Lord Kelvin 



on 



the vapour, it presents, so far as we know, no optical appear- 

 ance to demonstrate the pressure of condensed vapour of 

 water upon it : but enormous differences of electric con- 

 ductance, according to the density of the vapour surrounding 

 it, prove the presence of water upon the surface of the glass, 

 or among the interstices between its molecules, of which 

 electric conductance is the only evidence. Rayleigh has 

 himself expressed this viewiin a recent article, " Investigations 

 on Capillarity," in the Philosophical Magazine *. From the 

 estimates of the sizes of molecules of argon, hydrogen, 

 oxygen, carbonic oxide, carbonic acid, ethylene (C 2 H 4 ), and 

 other gases, which we shall have to consider (§47 below), 

 we may judge that in all probability if we had eyes micro- 

 scopic enough to see atoms and molecules, we should see in 

 those thin films of Rayleigh and Rontgen merely molecules 

 of oil lying at greater and less distances from one another, 

 but at no part of the film one molecule of oil lying above 

 another or resting on others. 



§ 29. A very important and interesting method of estim- 

 ating the size of atoms, founded on the kinetic theory of 

 gases, was first, so far as I know, thought of by Loschmidt f 

 in Austria and Johnstone Stoney in Ireland. Substantially 

 the same method occurred to myself later and was described 

 in ' Nature/ March 1870, in an article % on the " Size of 

 Atoms " already referred to, § 26 above, from which the 

 quotations in §§ 29, 30 are taken. 



" The kinetic theory of gases suggested a hundred years 

 " ago by Daniel Bernoulli has, during the last quarter of a 

 " century, been worked out by Herapath, Joule, Clausius, and 

 4 ' Maxwell to so great perfection that we now find in it 

 " satisfactory explanations of all non-chemical " and non- 

 electrical " properties of gases. However difficult it may be 

 " to even imagine what kind of thing the molecule is, we 

 '" may regard it as an established truth of science that a gas 

 " consists of moving molecules disturbed from rectilinear paths 

 "and constant velocities by collisions or mutual influences, so 

 '" rare that the mean length of nearly rectilinear portions of 

 " the path of each molecule is many times greater than the 

 " average distance from the centre of each molecule to the 

 " centre of the molecule nearest it at any time. If, for 

 " a moment, we suppose the molecules to be hard elastic 



* Phil. Mag, Oct. 1899, p. 337. 



f Sitzungsberichte of the Vienna Academy, Oct. 12, 1865, p. 395. 

 % Eeprinted as Appendix (F) in Thomson and Tait's ' Natural Philo- 

 sophy,' part. ii. p. 499. 



