1862.] on the Motion of Glaciers. 413 



the mass without dislocation. So far this property was as definite as 

 that of fluidity. In nature the cohesive power of plastic bodies was 

 generally comparatively small, but this was a less definite and less 

 important character than the other. With respect to ice, the question 

 before us was this : Is glacial ice a solid or a plastic substance, accord- 

 ing to the definitions which had just been given of those terms ? 

 Whether others might agree to those definitions or not was immaterial, 

 for they were here used to denote the exact properties which they had 

 been defined to express. Taking them in this sense, it would seem 

 to be impossible to say that ice was not distinctly solid. It breaks into 

 an indefinite number of fragments when crushed, and will bear only 

 a small elongation without fracture. But these were the properties 

 of a solid, and those which are directly antagonistic to the properties 

 which could alone justify our designating any proposed substance as 

 plastic, without a violation of all the strictness of scientific language. 



It would seem not improbable, Mr. Hopkins proceeded to state, 

 that the advocates of the Viscous Theory had reserved to themselves, 

 as it were, a more comprehensive, and consequently a more vague 

 and indeterminate meaning for the terms viscous and plastic. The 

 defect of that theory was, in fact, that it gave no definition of the term 

 by which it designated the property of ice on which it was founded. 

 When such definition was demanded, it was said that glacial ice must 

 be viscous, because a glacier was capable of adapting itself to the 

 inequalities of the valley containing it, as if it were viscous. This 

 was equivalent to the assumption that ice possessed no other property 

 by virtue of which a glacier might change its form, without losing its 

 continuity, and thus adapt itself to external conditions as if it were 

 viscous. Dr. Tyndall's experiments showed at once that ice did 

 possess such a property — that of regelation — by virtue of which a 

 glacier, after being crushed and broken by any unusual compression to 

 which it might be exposed, perfectly regained by pressure and contact 

 its continuity as a vitreous or crystalline, hard, and brittle mass. It 

 was this phenomenon, apparently so inexplicable, that formed till 

 recently the great stumbling-block in our glacial theories. It was to 

 meet this difficulty, that the author of the Viscous Theory introduced 

 the fundamental idea of that theory, the viscosity of ice. No attempt 

 was made to establish this property by experimental investigation, in 

 any sense in which it might be applied. It was only founded, as had 

 been already stated, on an induction, the fallacy of which was imme- 

 diately proved by Dr. Tyndall's beautiful and decisive experiment. 

 By that experiment the theory of glacial motion was made to rest on 

 a property of ice, established by clear and determinate experimental 

 evidence, instead of resting on a property which was never proved, and 

 which, if it meant anything definite, appeared to be in opposition to 

 the direct evidence of our senses. 



Still it might be said that there must be substances possessing pro- 

 perties similar to those which had been designated by the terms solid 

 and plastic, but intermediate to them. This was doubtless the case ; 



