402 Prof. Tyndall on Ice and Glaciers. 



With a certain amount of skill, numerous interesting observa- 

 tions may be made on the regelation of floating fragments of ice. 



According to Professor James Thomson's theory, to produce 

 regelation the pieces of ice have to exercise pressure, to draw from 

 the surrounding ice the heat necessary for the liquefaction of the 

 compressed part ; and then this water must escape and be re- 

 frozen. All this requires time. In the foregoing experiments, 

 moreover, the water liquefied by the pressure issued into the 

 surrounding warm water, but notwithstanding this the float- 

 ing fragments regelated in a moment. It is not necessary 

 that the touching surfaces should be flat ; for in this case a film 

 of water might be supposed to exist between them of the tem- 

 perature 0° C. The surfaces in contact may be convex : they 

 may be virtual points that touch each other, clasped all round by 

 the warm liquid, which is rapidly dissolving them as they ap- 

 proach each other. Still they freeze immediately when they touch. 



There are two points urged by Helmholtz — one in favour of 

 the view he has adopted, and the other showing a difficulty 

 associated with the view of Faraday — on which a few words may 

 be said. " I found/' says Helmholtz, " the strength and rapi- 

 dity of the union of the pieces of ice in such complete correspond- 

 ence with the amount of pressure employed, that I cannot doubt 

 that the pressure is actually the sufficient cause of the union." 



But, according to Mr. Faraday's explanation, the strength and 

 quickness of the regelation must also go hand in hand with the 

 magnitude of the pressure employed. Helmholtz rightly dwells 

 upon the fact that the appressed surfaces are usually not per- 

 fectly congruent — that they really touch each other in a few points 

 only, the pressure being thus concentrated. Now the effect of 

 pressure exerted on two pieces of ice at a temperature of 0° C. is 

 not only to lessen the thickness of the liquid film between the 

 pieces, but also to flatten out the appressed points, and thus to 

 spread the film over a greater space. On both theories, there- 

 fore, the strength and quickness of the regelation ought to cor- 

 respond to the magnitude of the pressure. 



The difficulty referred to above is thus stated by Helmholtz : — 

 " In the explanation given by Faraday, according to which the 

 regelation is caused by a contact action of ice and water, I find a 

 theoretic difficulty. By the freezing of the water a very sensible 

 quantity of heat would be set free ; and it does not appear how 

 this is to be disposed of." 



On the part of those who accept Faraday's explanation, the 

 answer here must be that the free heat is diffused through the 

 adjacent ice. But against this it will doubtless be urged that ice 

 already at a temperature of 0° C. cannot take up more heat with- 

 out liquefaction. If this be true under all circumstances, Fara- 



