Noti-plasticity of Glacier Ice, 587 



whole of British geologists, I have little doubt of yet seeing 

 the same fate meet the plasticity theory. In saying this, I 

 beg not to be understood as slighting the undoubted merits 

 of Prof. Forbes, who has been the first to demonstrate by 

 well devised measures and experiments the actual motion of 

 the glacier, but who has not, that I can perceive, thrown any 

 fresh light upon the causes of these motions, by the assertion 

 of an ill-defined plasticity in the mass of ice, and its illustra- 

 tion, in common with Prof. Gordon and others, by small ex- 

 periments upon pitch* and other admittedly plastic bodies, 

 whose molecular properties are totally distinct from those of 

 ice in any known state. 



My present intention is merely to call the attention of those 

 interested in these questions to certain ascertained facts as to 

 the molecular constitution of bodies, which appear to me to 

 preclude the possibility of such a property as plasticity exist- 

 ing in ice, and thence to show that the power of moulding its 

 mighty mass to the sinuous, or unequal, or occasionally nar- 

 rowed atid wire-drawn portions of the glacier's bed is due, 

 not to any such imaginary property, but simply to the break- 

 ing up of the rigid mass, and its continual subdivision by the 

 various forces to which it is exposed. 



And first, by plasticity is meant, I presume, " the intermo- 

 bility of particles, without discontinuity, of a body beyond the 

 range of its elasticity," the extreme case of which is fluidity. 

 This property may be accompanied with great elasticity and 

 elastic range, or these additional properties may be wholly 

 wanting. Indian rubber is an example of the first, tempered 

 clay of the second. 



Now I assert that no known case can be quoted of a cry- 

 stallized body possessing these properties. The very essen- 

 tial idea of crystallization is mutual, rigid fixation of particles, 

 except within the elastic range, upon which the external form 

 of any individual crystal depends, and which, break it up as 

 you will, causes any fragment to be of the form of the origi- 

 nal or derived from it. A single crystal or a crystalline mass 

 may, by the application of external force, be bent or twisted 

 or compressed within certain limits due to its particular elas- 

 tic modulus ; but increase the force beyond those limits, and 

 rupture or fracture instantly takes place. This is as true of 

 the most imperfectly crystallized bodies as of those most per- 

 fectly so. Thus a plume of amianthus or a plate of mica or 

 selenite may be bent almost double, or a plate of elastic 

 marble may suffer a large amount of flexure, but increase this 

 beyond a given point and fracture results. So in the metals 

 * See present volume, p. 206. — Edit. 



