180 Lord Kelvin on 



in March 1870, on "The Size of Atoms"*, and in a Friday 

 evening lecture f to the Royal Institution on the same 

 subject on February 3, 1883 ; but to illustrate it, information 

 was wanted regarding the heat of combination of copper and 

 zinc. Experiments by Professor Roberts- Austen and by 

 Dr. A. Gait, made within the last four years, have supplied 

 this want ; and in a postscript of February 1898 to a Friday 

 evening lecture on " Contact Electricity/' which I gave at 

 the Royal Institution on May 21, 1897, T was able to say 

 " We cannot avoid seeing molecular structures beginning to 

 " be perceptible at distances of the hundred-millionth of a 

 " centimetre, and we may consider it as highly probable that 

 " the distance from any point in a molecule of copper or zinc 

 " to the nearest corresponding point of a neighbouring 

 *' molecule is less than one one-hundred-millionth, and greater 

 " than one one-thousand-millionth of a centimetre " ; and 

 also to confirm amply the following definite statement which 

 I had given in my ' Nature' article (1870) already referred 

 to : — " Plates of zinc and copper of a three-hundred-millionth 

 " of a centimetre thick, placed close together alternately, form 

 4 ' a near approximation to a chemical combination, if indeed 

 " such thin plates could be made without splitting atoms/' 



§ 27. In that same article thermodynamic considerations 

 in stretching a fluid film against surface tension led to the 

 following result : — " The conclusion is unavoidable, that a 

 *' water-film falls off greatly in its contractile force before it 

 " is reduced to a thickness of a two hundred-millionth of a 

 " centimetre. It is scarcely possible, upon any conceivable 

 " molecular theory, that there can be any considerable falling 

 " off in the contractile force as long as there are several 

 " molecules in the thickness. It is therefore probable that 

 " there are not several molecules in a thickness of a two- 

 " hundred-millionth of a centimetre of water." More detailed 

 consideration of the work done in stretching a water-film 

 led me in my Royal Institution Lecture of 1883 to substitute 

 one one-hundred-millionth of a centimetre for one two- 

 hundred-millionth in this statement. On the other hand a 

 consideration of the large black spots which we now all know 

 in a soap-bubble or soap-film before it bursts, and which were 

 described in a most interesting manner by Newton J, gave 



* Republished as Appendix (F) in Thomson and Tait's i Natural 

 Philosophy, part ii. second edition. 



f Republished in ' Popular Lectures and Addresses,' vol. i. 



j Newton's ' Optics,' pp. 187, 191, Edition 1721, Second Book, Part i. : 

 quoted in my Royal Institution Lecture, ' Pop. Lectures and Addresses,' 

 vol. i. p. 175. 



