474 Lord Bayleigh on the 



of great importance ; but, as it lies outside the scope of the 

 present paper, I must content myself with referring the reader 

 to the original memoir and to the account of it by Maxwell*. 



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= ^aK. 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 twenty years' old. 

 Estimating K at 23,000 atmospheres, and T at 3 grains per 

 inch, Young finds f that "the extent of the cohesive force 

 must be limited to about the 250 millionth of an inch ;" and 

 he continues, a nor is it very probable that any error in the 

 suppositions adopted can possibly have so far invalidated this 

 result as to have made it very many times greater or less than 

 the truth. " It detracts nothing from the merit of this won- 

 derful speculation that a more precise calculation does not' 

 verify the numerical coefficient in Young's equation. The 

 point is that the range of the cohesive force is necessarily of 

 the order T/K. 



But this is not all. Young continues : — " Within similar; 

 limits of uncertainty, we may obtain something like a con- 

 jectural 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 steam are condensed when they approach within this dis- 

 tance, it follows that at 60° of Fahrenheit the distance of the- 

 particles of pure aqueous vapour is about the 250 millionth of 

 an inch ; and since the density of this vapour is about one 

 sixty thousandth of that of water, the distance of the particles' 

 must be about forty times as great ; consequently the mutual 

 distance of the particles of water must be about the ten 

 thousand millionth of an inch. It is true that the result of 

 this calculation will differ considerably according to the 1 

 temperature of the substances compared. . . . This discor- 

 dance does not, however, wholly invalidate the general tenour 

 of the conclusion ... and on the whole it appears tolerably' 

 safe to conclude that, whatever errors may have affected the 

 determination, the diameter or distance of the particles of 

 water is between the two thousand and the ten thousand 

 millionth of an inch/'' This passage, in spite of its great 

 interest, has been so completely overlooked that I have 



* < Nature,' vol. x. p. 477 (1874). See also vol. xi. pp. 357, 374. 

 t Works, vol. i. p. 461. 



