1883.] 071 the Size of Atoms. 193 



the colours were emerged at the top, there grew in the centre of 

 the rings a small round black spot like that in the first observation, 

 which continually dilated itself till it became sometimes more than 

 one-half or three-quarters of an inch in breadth before the bubble 

 broke. At first I thought there had been no light reflected from the 

 water in that place, but observing it more curiously I saw within it 

 several smaller round spots, which appeared much blacker and darker 

 than the rest, whereby I knew that there was some reflection at the 

 other places which were not so dark as those spots. And by farther 

 trial I found that I could see the images of some things (as of a 

 candle or the sun) very faintly reflected, not only from the great 

 black spot, but also from the little darker spots which were within it. 



" Obs. 18. — If the water was not very tenacious, the black spots 

 would break forth in the white without any sensible intervention of 

 the blue. And sometimes they would break forth within the pre- 

 cedent yellow, or red, or perhaps within the blue of the second order, 

 before the intermediate colours had time to disj)lay themselves.'* 



Now I have a reason, an irrefragable reason, for saying that the 

 film cannot keep up its tensile strength to 1-100, 000,000th of a 

 centimetre, and that is, that the work which would be required to 

 stretch the film a little more than that would be enough to drive it 

 into vapour. 



The theory of capillary attraction shows that when a bubble — a 

 soap-bubble, for instance — is blown larger and larger, work is done 

 by the stretching of a film which resists extension as if it were an 

 elastic membrane with a constant contractile force. This contractile 

 force is to be reckoned as a certain number of units of force per unit 

 of breadth. Observation of the ascent of water in capillary tubes 

 shows that the contractile force of a thin film of water is about 16 

 milligrammes weight per millimetre of breadth. Hence the work 

 done in stretching a water film to any degree of thinness, reckoned 

 in millimetre-milligrammes, is equal to sixteen times the number of 

 square millimetres by which the area is augmented, provided the film 

 is not made so thin that there is any sensible diminution of its con- 

 tractile force. In an article " On the Thermal Effect of Drawing out 

 a Film of Liquid," published in the ' Proceedings ' of the Royal Society 

 for April 1858, 1 have proved from the second law of thermodynamics 

 that about half as much more energy, in the shape of heat, must be 

 given to the film, to prevent it from sinking in temperature while it is 

 being drawn out. Hence the intrinsic energy of a mass of water in 

 the shape of a film kept at constant temperature increases by 24 

 milligramme-millimetres for every square millimetre added to its 

 area. 



Suppose, then, a film to be given with the thickness of a millimetre, 

 and suppose its area to be augmented ten thousand and one fold : the 

 work done per square millimetre of the original film, that is to say 

 per milligramme of the mass, would be 240,000 millimetre-milli- 

 grammes. The heat equivalent to this is more than half a degree 



O 2 



