114 [May 22, 



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 

 always been in the entire want of any accurate definition of that term. 

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

 be viscous, because a glacier adapted. itself to the inequalities of its 

 valley as a viscous mass would do. This was equivalent to saying 

 that the mass was viscous because it moved in a particular manner, 

 instead of asserting that the mass moved in that particular manner 

 because it was viscous. Now this kind of inversion of the direct 

 enunciation of the proposition, is only admissible when there is no 

 other physical cause than the 'one assigned, to which it is conceivable 

 that the observed phenomena should be ascribed. Thus we may 

 assert with perfect conviction, that gravity exists as a property of 



