Mr. W. Hopkins on the Theory of the Motion of Glaciers. 227 



what may with strict propriety be termed plasticity ; it might also 

 be added that, as bodies are constituted in nature, the force required 

 to produce the original displacement in plastic bodies will be small 

 as compared with that required in solid bodies. Viscosity and semi- 

 fluidity are terms which only express similar properties of bodies, but 

 usually indicating that still smaller forces only are required to produce 

 a given displacement in such bodies than in plastic ones. The limit- 

 ing case is that of perfect fluidity, in which both the forces of original 

 displacement and those of restitution are indefinitely small. In these 

 latter cases the tangential cohesion is necessarily small, and such also 

 (as bodies are usually constituted) will be the normal cohesion. At 

 the same time the power of resisting compression of volume may be 

 very great, as in fact it is in nearly all masses not technically desig- 

 nated as elastic masses. In other words, the normal elasticity, with 

 reference to pressure, may be of any magnitude, while the tangential 

 elasticity equals zero. 



It will be observed that a body is here spoken of as held in a state 

 of constraint by internal forces, but without any kind of dislocation 

 which should destroy its continuity or injure its structure. If, how- 

 ever, the external forces should be sufficiently increased, the structure 

 of a vitreous or crystalline mass, or that of any mass possessing hard- 

 ness and brittleness, will be destroyed by a pressure greater than its 

 power of resistance can withstand ; or the continuity of its mass will 

 be destroyed by any normal tension greater than the normal cohesion ; 

 or, again, by any tangential tension greater than the tangential cohe- 

 sion. The normal tension would thus produce an open fissure ; and 

 the tangential tension would cause one particle of the mass to slide 

 past another, but without producing any open discontinuity. On the 

 contrary, in a properly plastic or viscous mass there is no definite 

 structure for excessive pressure to destroy ; there is no question as 

 to the formation of open fissures ; and the characteristic absence of 

 tangential elasticity allows of any amount of change in the relative 

 positions of the constituent particles of the mass without breach of 

 its continuity. 



It would of course be impossible to draw an exact and determinate 

 line of demarcation between solidity and plasticity, but it is not there- 

 fore the less certain that there are bodies which do unequivocally 

 possess the property of solidity, and others which do as unequivocally 

 possess the property of plasticity, according to the definitions here 

 given of these terms. Solidity and plasticity with respect to nume- 

 rous cases in nature thus become determinate properties of those 

 aggregates of material particles which we call bodies. Ice, a vitreous 

 or crystalline and brittle mass, which will neither bear any but the 

 smallest extension without breaking, nor more than the smallest com- 

 pression without being crushed, must be solid, and cannot be plastic, 

 if we are to use those terms as significant of determinate properties 

 of bodies. 



3. The advocates of the viscous theory would not probably admit 

 the necessity of the above rigorous definition of the term viscous in 

 its application to glacier ice. But the defect of that theory has 



Q2 



