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in a paper communicated to the Royal Society, a theory which had 

 occurred to me mainly in or about the year 1848, or perhaps 1850 ; 

 but which, up till the date of the paper referred to, had only been 

 described to a few friends verbally. That theory of mine may be 

 sketched in outline as follows : If to a mass of ice at its melting- 

 point, pressures tending to change its form be applied, there will be 

 a continual succession of pressures applied to particular parts 

 liquefaction occurring in those parts through the lowering of the 

 melting-point by pressure evolution of the cold by which the so 

 melted portions had been held in the frozen state dispersion of the 

 water so produced in such directions as will afford relief to the 

 pressure and recongelation, by the cold previously evolved, of the 

 water on its being relieved from this pressure: and the cycle of 

 operations will then begin again ; for the parts re-congealed, after 

 having been melted, must in their turn, through the yielding of other 

 parts, receive pressures from the applied forces, thereby to be again 

 liquefied and to proceed through successive operations as before. 



Professor Tyndall, in papers and lectures subsequent to the publi- 

 cation of this theory, appears to adopt it to some extent, and to endea- 

 vour to make its principles cooperate with the views he had previously 

 founded on Mr. Faraday's fact of so called " regelation"*. 



Professor James D. Forbes adopts Person's view, that the dissolu- 

 tion of ice is a gradual, not a sudden process, and so far resembles 

 the tardy liquefaction of fatty bodies or of the metals, which in 

 melting pass through intermediate stages of softness or viscosity. He 

 thinks that ice must essentially be colder than water in contact with 



in order to explain the observed plasticity or viscosity. That fractures both 

 large and exceedingly small both large at rare intervals, and small, momentarily 

 repeated do, under various circumstances, arise in the plastic yielding of masses 

 of ice, is, of course, an undoubted fact : but it is one which I regard not as the 

 cause, but as a consequence, of the plastic yielding of the mass in the manner 

 supposed in my own theory. It yields by its plasticity in some parts until other 

 parts are overstrained and snap asunder, or perhaps also sometimes slide suddenly 

 past one another. 



* I suppose the term regelation has been given by Prof. Tyndall as denoting the 

 second, or mending stage in his theory of "fracture and regelation" Congela- 

 tion would seem to me the more proper word to use after fracture, as regelation 

 implies previous melting. If my theory of melting by pressure and freezing again 

 on relief of pressure be admitted, then the term regelation will come to be quite 

 suitable for a part of the process of the union of the two pieces of ice, though not 

 for the whole, which then ought to be designated as the process of melting and 

 rcgelation. 



