Royal Society. 407 



ROYAL SOCIETY. 



[Continued from p, 331.] 



May 2, 1861. — Major-General Sabine, R.A., Treasurer and Yice- 

 President, in the Chair. 



The following communication was read : — 



"Note on Professor Faraday's Recent Experiments on 'Regela- 

 tion.' " By Professor James Thomson, Queen's College, Belfast. 



Some time ago*, Principal James D. Forbes showed that 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 met 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 i — 

 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 

 be impelled to freeze by the joint action of this condition with the 

 cold given out in the melting by pressure of the ice at the points of 

 contact where the two slabs are urged against one another. 



To this explanation of Principal Forbes' s experiment I still adhere 

 as mainly correct, though admitting of some further development 

 and slight modification in reference to a point to which I shall have 

 to make further allusion in what follows, and which seems to me to 

 be as yet rather obscure : — the influence, namely, of the tension in 

 the ice due to its own weight, which makes it not be subject in- 



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

 Phil. Mag. vol. xvi. 1838, p. 544. 



+ Phil. Mag. S. 4. vol. xiy. p. 548, and British Association Keports, 1857. 

 X Phil. Mag. S. 4. vol. xix. p. 396. 



