i;2 POPULAR LECTURES AND ADDRESSES. 



Effect of Drawing out a Film of Liquid," published 

 in the Proceedings of the Royal Society for 

 April 1858, [Math, and Phys. Papers, vol. iii. 

 Art. XCV.], I 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 milli- 

 metre-milligrammes 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-milligrammes. The 

 heat equivalent to this is more than half a degree 

 Centigrade (0*57) of elevation of temperature of 

 the substance. The thickness to which the film 

 is reduced on this supposition is very approxi- 

 mately i/io,oooth of a millimetre. The com- 





