680 
ME. HOPKINS ON THE THEOET OP THE MOTION OP OLACIEES. 
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 semifluidity are terms which only express similar proper- 
ties of bodies, but indicating that still smaller forces only are required to produce a 
given displacement in viscous or semifluid bodies than in plastic ones. The limiting 
case is that of jyerfect 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 designated as elastic masses. 
In other words, the normal elasticity, with reference to pressure, may be of any magni- 
tude, while the tangential elasticity equals zero. 
It will be observed that I have here spoken of a body as held in a state of constraint 
by external forces, but without any kind of dislocation which should destroy its conti- 
nuity or injure its structure. If, however, the external forces should be sufiiciently 
increased, the structure of a vitreous or crystalline mass, or that of any mass possessing 
hardness and brittleness, will be destroyed by a pressure greater than its power of resist- 
ance 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 cohesion. The normal tension would then 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 elas- 
ticity 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 therefore 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 I have given 
of these terms. Solidity and plasticity with respect to numerous cases in nature thus 
become determinate properties of those aggregates of material particles which we call 
bodies. Ice, a vitreous or crystalhne and brittle mass, which will neither bear any but 
the smallest extension without breaking, nor more than the smallest compression udth- 
out being crushed, must be sohd, 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 glacial ice. But the 
defect of that theory has always been in the entire want of any accurate definition of 
that term. When such a definition was demanded, it was said that glacial ice must 
be viscous, because a glacier adapted itself to the inequalities of its valley as a viscous 
