178 Lord Kelvin 



on 



events we regard ether as utterly continuous and structureless. 

 It may be found in some future time that ether too has a 

 molecular structure, perhaps much finer than any structure 

 of ponderable matter ; but at present we neither see nor 

 imagine any reason for believing ether to be other than 

 continuous and homogeneous through infinitely small con- 

 tiguous portions of space void of other matter than ether. 



§ 24. The first suggestion, so far as we now know, for 

 estimating the dimensions of molecular structure in ordinary 

 matter was given in 1805 by Thomas Young *, as derived 

 from his own and Laplace's substantially identical theories 

 of capillary attraction. In this purely dynamical theory he 

 found that the range of the attractive force of cohesion is 

 equal to ?>T\K\ where T denotes the now well-known 

 Young's tension of the free surface of a liquid, and K 

 denotes a multiple integral which appears in Laplace's 

 formulas and is commonly now referred to as Laplace/s K y 

 as to the meaning of which there has been much controversy 

 in the columns of ' Nature ' and elsewhere, Lord Rayleigh 

 in his article of 1890, " On the Theory of Surface Forces f/* 

 gives the following very interesting statement in respect to 

 Young's estimate of molecular dimensions : — 



§ 25. u One of the most remarkable features of Young's 

 "treatise is his estimate of the range a of the attractive force 

 "on the basis of the relation T=laK. Never once have 

 " I seen it alluded to ; and it is, I believe, generally supposed 

 " that the first attempt of the kind is not more than tw r enty 

 " years old. Estimating K at 23000 atmospheres, and T at 

 " 3 grains per inch, Young finds that ' the extent of the 

 " ' cohesive force must be limited to about the 250 millionth 

 " 'of an inch [10 -8 cm.] ' ; and he continues, ' nor is it very 

 " ' probable that any error in the suppositions adopted can 

 " ' possibly have so far invalidated this result as to have made 

 a ■ it very many times greater or less than the truth ' . . . . 

 " Young continues : — ' Within similar limits of uncertainty, 

 u i we may obtain something like a conjectural estimate of 

 " ' the mutual distance of the particles of vapours, and even 

 " ' of the actual magnitude of the elementary atoms of liquids, 

 " ' as supposed to be nearly in contact with each other ; for 

 " ' if the distance at which the force of cohesion begins is 

 " ' constant at the same temperature, and if the particles of 



* "On the Cohesion of Fluids," Phil. Trans. 1805 ; Collected Works, 

 vol. i. p. 461. 



t Phil. Mag. vol. xxx. 1890, p. 474, 



