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the part already formed undergoes permanent change of form, such 

 as would be due to any plastic or ductile yielding. I think we must 

 suppose the molecules in the interior of one crystal to be so locked 

 into one another, by the forces of crystalline cohesion, that any 

 one of them, or set of them, would experience a difficulty in making 

 a beginning of the change of state from solid to liquid. I have not 

 succeeded even in forming any clear conception of continuous cry- 

 stalline structure admitting of what may be called ductile or malleable 

 bending (that is, bending beyond limits of elasticity such as occurs 

 in lead, copper, tin, and many other metals), and still remaining of 

 the nature of one continuous crystal. What in soft or malleable 

 crystals of copper or other metals, deposited in the electrotype process, 

 may be the nature of the change of molecular arrangement induced 

 by bending them, I cannot say ; but I suppose that, in their yielding, 

 their crystalline structure is materially altered, and rendered discon- 

 tinuous where, before, it was continuous. 



In a mass of plastic ice, I incline to think that the internal melting, 

 to which I attribute the plasticity, must occur at the surfaces of 

 junction of separate crystals or fragments of crystals ; though probably 

 pores formed by melting, by pressures, or by stresses, may penetrate 

 crystals by entering them from their moistened surfaces or their 

 junctions with other crystals. It now becomes clear, I think, that 

 the influence of stresses affecting the ice, and tending to make it melt 

 without there being necessarily any consequent pressure applied to 

 the water in contact with the ice, must come to be taken into account 

 in any theory of the plasticity of ice approaching to completeness. 

 This view does not, however, I think, supersede the theory of the 

 plasticity of ice sketched out by myself in former papers, but rather 

 constitutes an amendment, and further development of it. Any 

 complete theory of the plasticity of ice, and of the nature of glacier 

 motion, must comprise the conditions as to fluid pressure and 

 structural arrangement of the water and air included in the ice, and 

 must so explain the lamination of the glacier, seen as blue and white 

 veins. My brother, Professor William Thomson, in papers in the 

 ' Proceedings of the Royal Society ' for February 25 and April 22, 

 1858, endeavoured to follow up my previously published views on 

 the plasticity of ice with an explanation of the laminated structure, 

 based on the same principles. The explanation he then offered, I 



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