Prof, J. Thomson on Regelation. 41 1 



and the tension in the extended part cannot be of small intensity, 

 being sufficient to break that side of the isthmus. In the experi- 

 ments which gave flexible adhesion seemingly under tension, it is not 

 to be admitted that tension was really the condition under which the 

 ice existed at the places where the union was occurring. To apply 

 a simple disruptive force to the whole isthmus of ice, it would be 

 necessary to take very special precautions in order to arrange that 

 the line of application of the disruptive forces should pass through 

 the point of contact of the two pieces. If that were done, and the 

 forces were gradually increased till the cohesive strength of the isth- 

 mus were overcome, it is clear that the two pieces of ice would sepa- 

 rate altogether, and there would be no flexible adhesion ; but the 

 flexible adhesion, when it occurs, is essentially dependent on the 

 existence of an intense pressure at the side of the isthmus remote 

 from the line of the externally applied disruptive forces, or of the 

 single force applied in some of the experiments to one only of the 

 pieces, and resisted by the inertia of the other. 



It is further to be observed that tremors and slight agitations to 

 which the two pieces of ice united at their point of contact, may be 

 subject, arising from undulations imparted to the water in which the 

 ice is immersed, by manipulation of the experimenter, — from the 

 tread of people on adjacent floors, — from the passage of vehicles on 

 neighbouring streets, — from convective movements of the water, — 

 and from other causes, — will be sources of power or energy operative 

 in bringing about an increase of adhesion with time ; that is to say, 

 in changing gradually the flexible into the rigid adhesion. 



It will now of course be obvious that the conditions involved in 

 the explanation just offered of Prof. Faraday's experiments must 

 also usually be present in the experiment of Principal Forbes. Their 

 incidental occurrence, however, as additional causes increasing the 

 rapidity of the union of the two slabs of ice, does not overthrow the 

 particular explanation of Principal Forbes' s experiment which I had 

 offered as a perfect answer to the objection raised by that experiment 

 against my general view of the plasticity of ice ; and as indicating 

 clearly and certainly the occurrence of all the conditions required for 

 the union of the two pieces of ice under my theory. The contingent 

 occurrence of the additional conditions now specially brought for- 

 ward, was indeed from the first somewhat familiar to my mind, but 

 was left out of the explanation as being unessential and not perhaps 

 quite so clearly apparent. Their occurrence has, however, now be- 

 come essential to the explanation of Prof. Faraday's new experi- 

 ments : — and by it I consider these are shown not to militate against 

 my general theory of the plasticity of ice, but to corroborate it 

 strongly, and to confirm its application to the various observed 

 cases of the union of two pieces of moist ice when placed in 

 contact. 



