198 



III. "Note on Professor Faraday's Recent Experiments on 

 ' Regelation/" By Professor JAMES THOMSON, Queen's 

 College, Belfast. Communicated by Professor WILLIAM 

 THOMSON. Eeceived April 25, 1861. 



Some time ago*, Principal James D. Forbes sbowed tbat two 

 slabs of ice, having each a face ground tolerably flat, and being both 

 suspended in an atmosphere a little above the freezing-point upon a 

 horizontal rod of glass passed through two holes in the plates of ice, 

 so that the plates might hang vertically and in contact with one 

 another, would unite gradually so as to adhere strongly together. 

 This interesting experiment Principal Forbes adduced as being in 

 opposition to the theory offered by mef of the plasticity of ice, and 

 of the tendency of pieces of thawing ice to unite when placed in 

 contact. He thought it showed that pressure was not essential to 

 the union of the two pieces of ice. I pointed out, in reply J, that 

 the film of water between the two slabs, being held up against gravity 

 by the capillary tension or contractile force of its free upper surface, 

 and being distended besides against the atmospheric pressure, by the 

 contractile force of its free surface round its whole perimeter except 

 for a very small space at bottom, from which water trickles away, or 

 is on the point of trickling away, exists under a pressure which, 

 though increasing from above downwards, is everywhere, except at 

 that little space near the bottom, less than atmospheric pressure : 

 that hence the two slabs are urged against one another by the excess 

 of the external atmospheric pressure above the internal water press- 

 ure, and are thus pressed against one another by a force quite not- 

 able in amount ; that, further, the film of water existing as it does, 

 under less than atmospheric pressure, has its freezing-point raised in 

 virtue of the reduced pressure ; and would therefore freeze even at 

 the temperature of the surrounding ice, which I took to be the free- 

 zing-point for atmospheric pressure ; and would still more strongly 



* " On some Properties of Ice near its Melting-Point." By Prof. Forbes, 

 Proceedings of the Royal Society of Edinburgh, April 1858. 



t Proceedings of Royal Society, May 1857, and British Association Eeports, 

 1857. 



I Proceedings of Royal Society, Nov. 24, 1859, vol. x. p. 159. 



