24 REPORT — 1859. 



an explanation of an experiment by Prof. James D. Forbes which had been advanced 

 as in opposition to the author's theory. 



He referred at the outset to the fact pointed out by Mr. Faraday in 1850 *, that 

 two pieces of moist ice, when placed together in contact, will unite together, even 

 when the surrounding temperature is such as to keep them in a thawing state. 

 Mr. Faraday had attributed this phenomenon to a property which he supposed ice to 

 possess, of tending to solidify water in contact with it, and of tending more strongly 

 to solidify a film or a particle of water when the water has ice in contact with it on 

 both sides, than when it has ice on only one side. 



Dr. Tyndall had subsequently adopted this fact as the basis of a theory by which 

 he proposed to explain the viscidity or plasticity of ice, or its capability of under- 

 going change of form, which had previously been known to be the property in gla- 

 ciers in virtue of which their motion down their valleys is produced by gravitation. 

 Designating Mr. Faraday's fact under the term "vegetation," Dr. Tyndall, in the 

 theory referred to, described the capability of glacier ice to undergo changes of form, 

 as being not true viscosity, but as being the result of vast numbers of successively 

 occurring minute fractures, changes of position of the fractured parts, and vegeta- 

 tions of those parts in their new positions. The terms fracture and regelation had 

 then come to be the brief expression of Dr. Tyndall's idea of the plasticity of ice. 



The author, Professor James Thomson, considered, on the contrary, that if, in a 

 material having no inherent property of plasticity independent of fracture, any steady 

 force applied (such as the force of gravity acting on a glacier) be sufficient to cause 

 fracture, the substance must go down suddenly until a position of repose is attained, 

 and that the addition of a principle of reunion (such as "regelation") cannot have 

 a tendency to reiterate the fractures after such position of repose has been attained. 



His own theory, he stated, might 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 its 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 recongealed must in their turn, through the yielding of other 

 parts, receive pressures from the applied forces, thereby to be liquefied, and then to 

 go through successive processes as before. He thus considered that the plasticity 

 consists not of fracture and regelation, but essentially of melting by pressure and 

 recongelation on relief from pressure. 



Professor James D. Forbes f had adopted the view, that the dissolution 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 soft- 

 ness or viscosity. He thought that ice must be essentially colder than water in 

 contact with it ; and that, between the ice and the water, there is a film having its 

 temperature varying from side to side, which may be called plastic ice, or viscid 

 water ; and that through this film heat must be constantly passing from the water 

 to the ice, and the ice must be wasting away, though the water be what is called ice-* 

 cold. Professor Forbes had stated afterwards, as a modification of this supposition, 

 that if a small quantity of water be enclosed in a cavity in ice, it will undergo a gra- 

 dual "regelation," or that the ice will in this case be increased instead of wasted. 

 In reference to this, Professor J. Thomson put forward the case of a medium quan- 

 tity of water, in contact with a medium quantity of ice, without addition or abstrac- 

 tion of heat ; and stated that, were the idea of Professor Forbes on this subject cor- 

 rect, the result in this case ought to be that the water and ice should ultimately pass 

 into the state of uniform viscidity ; for Professor Forbes's own words distinctly deny 

 the permanence of the water and ice in contact in their two separate states, as he 

 says, "bodies of different temperatures cannot continue so without interaction. The 

 water must give off heat to the ice, but it spends it in an insignificant thaw at the 

 surface." Thus then it would follow from the admission of Professor Forbes's views, 



* See Faraday's Researches in Chemistry and Physics, 1859. 



t See Forbes ' On the Recent Progress and Present Aspect of the Theory of Glaciers,' 

 forming the introduction to a volume of Occasional Papers on the Theory of Glaciers, 1859. 



