1862.] 113 



may call a body emphatically a solid body when it possesses the 

 following properties : (I) small extensibility and compressibility, 

 (2) great power of resistance and great cohesive power, both normal 

 and tangential, and (3) great elasticity. It will thus require a com- 

 paratively great force to produce any sensible relative displacement 

 among the constituent molecules of the body ; if we conceive the 

 force required to become infinitely great, we arrive at absolute rigidity 

 as the limit of solidity. Again, we shall best, perhaps, define 

 plasticity or viscosity, if we suppose the forces of displacement to be 

 such as to produce only a small transverse or tangential displacement 

 of the constituent particles, i. e. a superficial, not a cubical, extension 

 or compression. Then if the force of restitution bear only an inap- 

 preciable ratio to the corresponding force of displacement, f . e. if the 

 tangential elasticity be not of sensible magnitude, the mass may be 

 emphatically said to be plastic. This is the essential condition of 

 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 



VOL, XII. I 



