•100 Royal Society : — 



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 

 Phil. Mag. for October and December 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 think, cannot fail to assist in 

 suggesting the direction in which the true solution is ultimately to 

 be sought for ; yet I feel confident that no full and true solution has 

 as yet been found*. 



In the foregoing part of the present paper, I have shown reason 

 why stresses applied to crystals when in contact with the liquid from 

 which they have been produced, should be expected to cause them 

 to melt or dissolve away. The following line of reasoning to show 

 that stresses applied to a crystal will cause a resistance to the de- 

 position of additions to it from the liquid, or, in other words, a re- 

 sistance to its growth, will, I think, prove to be correct. "When a 

 crystal grows, the additions, it seems to me, must lay themselves 

 do^Yn in a state of molecular fitting, or regular interlocking with the 

 parts on which they apply themselves ; or, in other words, they must 

 lay themselves down so as to form one continuous crystalline structure 

 Avith the parts already crystallized. It thus seems to me that, if a 

 crystal grows when under a stress, the new crystalline matter must 

 deposit itself in the same state of stress as the part is in on which it 

 lays itself. If, then, we consider a spiculum of ice growing in water, 

 and if we apply any stress, a pull for instance, to it while it is thin, and 



* I have my brother's authority for slating that, although he believes the 

 physical principles suggested in his papers here referred to to be capable of 

 being developed into a true explanation of the phenomena, yet he considers 

 further investigation necessary, and does not feel confident as to the correct- 

 ness of that part of the explanation he offered, in which the mutual action of 

 two vesicles in a line oblique to that of maximum pressure is considered. 



