256 HELMHOLTZ 



it in three places, the total surface of which is a square 

 millimetre, will produce on these surfaces a pressure of an 

 atmosphere. Ice will therefore be formed more rapidly in 

 the surrounding water than it was in the flask, where the 

 side of the glass was interposed between the ice and the 

 water. Even with a much smaller weight the same result 

 will follow in the course of an hour. The broader the 

 bridges become, owing to the freshly formed ice, the greater 

 will be the surfaces over which the pressure exerted by 

 the upper piece of ice is distributed, and the feebler it will 

 become; so that with such feeble pressure the bridges can 

 only slowly increase, and therefore they will be readily 

 broken when we try to separate the pieces. 



It cannot, moreover, be doubted that in Faraday's ex- 

 periments, in which two perforated discs of ice were 

 placed in contact on a horizontal glass rod, so that grav- 

 ity exerted no pressure, capillary attraction is sufficient to 

 produce a pressure of some grammes between the plates, and 

 the preceding discussions show that such a pressure, if ade- 

 quate time be given, can form bridges between the plates. 



If, on the other hand, two of the above-described cylinders 

 of ice are powerfully pressed together by the hands, they 

 adhere in a few minutes so firmly that they can only be 

 detached by the exertion of a considerable force, for which 

 indeed that of the hands is sometimes inadequate. 



In my experiments I found that the force and rapidity 

 with which the pieces of ice united were so entirely pro- 

 portional to the pressure that I cannot but assign this as the 

 actual and sufficient cause of their union. 



In Faraday's explanation, according to which regelation 

 is due to a contact action of ice and water, I find a theo- 

 retical difficulty. By the water freezing, a considerable 

 quantity of latent heat must be set free, and it is not clear 

 what becomes of this. 



Finally, if ice in its change into water passes through 

 an intermediate viscous condition, a mixture of ice and water 

 which was kept for days at a temperature of o must ulti- 

 mately assume this condition in its entire mass, provided 

 its temperature was uniform throughout; this however is 

 never the case. 



