226 Royal Society : — 



surface and not. the volume of the element. It may be called super- 

 ficial extension or compression. These changes of volume and form 

 in any element must be produced by the forces acting on it. Thus 

 we may conceive linear extension alone produced at any interior point 

 of the mass by two equal and opposite tensions acting on two ele- 

 mentary component particles there in the direction of the line joining 

 their centres of gravity, while compression alone would result if those 

 tensions were changed into pressures. In such cases extension or 

 compression would be the result of forces which may be called direct 

 or normal forces. In the case above mentioned, in which the volume 

 and density of every element of the mass remain unaltered, there 

 can be no such direct normal action as that just mentioned. It must 

 be perpendicular to the normal action, and therefore a transversal or 

 tangential action. There would be no tendency to make the con- 

 tiguous particles approach to or recede from each other, but to cause 

 the one to slide tangentially past the other. 



If the body have a structure like that of any hard, vitreous or 

 crystalline mass, pressure at any point will tend to break or crush 

 the body, and thus to destroy the continuity of its structure. This 

 tendency will be opposed by the resisting power of the substance. 

 The tendency of the direct or normal tension is to separate the con- 

 tiguous particles, and thus produce a finite fissure, or a discontinuity 

 in the mass. It is resisted by the normal cohesive power ; and in 

 like manner the transverse or tangential action is resisted by the 

 tangential cohesion, or that which prevents the component particles 

 from sliding past each other. Again, when the component particles 

 at any point of a body are relatively displaced, they have always a 

 certain tendency to regain their originally undisturbed position, and 

 the force thus excited, considered with reference to the force of 

 displacement at that point, affords a measure of what is called the 

 elasticity of the body. Since the force of restitution may vary from 

 zero to the corresponding force of displacement, the elasticity, when 

 measured by their ratio, may vary from zero to unity. 



2. We may now define such terms as solid, plastic, viscous, and 

 the like, with all the accuracy which their definitions admit of. We 

 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, i. 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 



