the Weights of Atoms. 179 



" ' steam are condensed when they approach within this 

 " ' distance, 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 * 

 " ' [*025 x 10~ 8 cm.] . It is true that the result of this calcu- 

 " ' lation will differ considerably according to the temperature 

 " ' of the substances compared. . . . This discordance does not 

 " ' however wholly invalidate the general tenour of the con- 

 " ' elusion . . . 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 ' [between • 1 25 x 10~ 8 and *025 x 10 ~ s 

 "of a cm.]. This passage, in spite of its great interest, has 

 " been so completely overlooked that 1 have ventured briefly 

 '* to quote it, although the question of the size of atoms lies 

 " outside the scope of the present paper " 



§ 26. The next suggestion, so far as 1 know, for estimating 

 the dimensions of molecular structure in ordinary matter, is 

 to be found in an extract from a letter of my own to Joule 

 on the contact electricity of metals, published in the ' Pro- 

 ceedings ' of the Manchester Literary and Philosophical 

 Society f, Jan. 21, 1862, which contains the following 

 passage : — " Zinc and copper connected by a metallic arc 

 ; " attract one another from any distance. So do platinum 

 " plates coated with oxygen and hydrogen respectively. 

 " 1 can now tell the amount of the force, and calculate how 

 " great a proportion of chemical affinity is used up electrically, 

 ;i before two such discs come within 1/1000 of an inch of 

 "one another, or any less distance down to a limit within 

 ;i which molecular heterogeneousness becomes sensible. This 

 4 * of course will give a definite limit for the sizes of atoms, or 

 " rather, as I do not believe in atoms, for the dimensions of 

 " molecular structures." The theory thus presented is some- 

 what more fully developed in a communication to ' Nature ' 



* Young here, curiously insensible to the kinetic theory of gases, 

 supposes the molecules of vapour of water at 60 Q Fahr. to be within 

 touch (or direct mutual action) of one another ; and thus arrives at 

 a much finer-grainedness for liquid water than he would have found if 

 he had given long enough free paths to molecules of the vapour to 

 account for its approximate fulfilment of Boj 7 le's law. 



t Reproduced as Art. 22 of my ' Electrostatics and Magnetism.' 



N2 



