441 



ferent philosophers to assign the true physical principle of action, 

 but also by the great differences between the views which they have 

 taken. 



Two pieces of thawing ice, if put together, adhere and become 

 one ; at a place where liquefaction was proceeding, congelation sud- 

 denly occurs. The effect will take place in air, or in water, or in 

 vacuo. It will occur at every point where the two pieces of ice 

 touch ; but not with ice below the freezing-point, i. e. with dry ice, 

 or ice so cold as to be everywhere in the solid state. 



Three different views are taken of the nature of this phenomenon. 

 When first observed in 1850, I explained it by supposing that a 

 particle of water, which could retain the liquid state whilst touching 

 ice only on one side, could not retain the liquid state if it were 

 touched by ice on both sides ; but became solid, the general tem- 

 perature remaining the same*. Professor J. Thomson, who dis- 

 covered that pressure lowered the freezing-point of water f, attributed 

 the regelation to the fact that two pieces of ice could not be made 

 to bear on each other without pressure ; and that the pressure, how- 

 ever slight, would cause fusion at the place where the particles 

 touched, accompanied by relief of the pressure and resolidification of 

 the water at the place of contact, in the manner that he has fully 

 explained in a recent communication to the Royal Society^. Profes- 

 sor Forbes assents to neither of these views ; but admitting Person's 

 idea of the gradual liquefaction of ice, and assuming that ice is 

 essentially colder than ice-cold water, i. e. the water in contact with 

 it, he concludes that two wet pieces of ice will have the water be- 

 tween them frozen at the place where they come into contact. 



Though some might think that Professor Thomson, in his last 

 communication, was trusting to changes of pressure and tempera- 

 ture so inappreciably small as to be not merely imperceptible, but 

 also ineffectual, still he carried his conditions with him into all the 

 cases he referred to, even though some of his assumed pressures 

 were due to capillary attraction, or to the consequent pressure of the 



* Researches in Chemistry and Physics, 8vo. pp. 373, 378. 

 t Mousson says that a pressure of 13,000 atmospheres lowers the temperature 

 of freezing from to 18 Cent. 



J Royal Society Proceedings, vol. x. p. 152. 



Proceedings of the Royal Society of Edinburgh, April 19, 1858. 



