412 Mr, W. Hophins [Jan. 31, 



for different modes of displacement. Thus if the body, or any of its 

 elements, were compressed, the elasticity developed might not be the 

 same as if it were extended, or as if it were twisted or contorted 

 without a change of volume. Thus hard bodies, like wood, metal, 

 stone, &c., would usually exhibit nearly the same amount of elasticity 

 whether they were compressed or extended, provided the compression 

 or extension should not exceed certain small limits. On the contrary, 

 semifluid masses generally exert a large force of restitution when 

 under compression, but a very small one when extended, i. e. in the 

 former case the elasticity developed is very large, in the latter it is 

 very small. And again, if a semifluid mass, such as soft paste for 

 instance, or any portion of it, were angularly distorted without altera- 

 tion of volume, it would exhibit but a small amount of elasticity, and 

 little or no tendency to regain its original form and position. More- 

 over, in the class of substances first mentioned, such as wood, metal, 

 &c., the cohesive power was very great, while it was manifestly small 

 in the others. 



With the preceding preliminary observations, it was easy to define 

 with the required degree of exactness the terms solid and plastic, or 

 viscous. When a substance admitted of only a small extension without 

 being dislocated, and of only a small compression without being 

 crushed, and having its structure destroyed ; when its cohesion as 

 opposing direct tension or torsion was comparatively great, as well as 

 its power of resistance to direct pressure ; and also, when the elasticity 

 developed by each of the three displacements or changes of form 

 above described — then might such a substance be emphatically called 

 solid. To define the term plastic, it was most simple to conceive the 

 form of the mass, or of its elements to be changed by angular distor- 

 tion alone without any change of volume. In such case, if the cohesive 

 power resisting this change should be small, and the elasticity developed 

 by it should become sensibly = zero ; and if, moreover, the substance 

 should be able to bear any number of repetitions of this change of 

 form, and therefore an indefinite elongation without rupture and dis- 

 location — then might it be emphatically termed plastic. In all such 

 substances in nature, the force of displacement for the change of form 

 here described would always be small. Viscosity might be regarded, 

 so far as we were directly concerned with these properties, as a higher 

 degree of plasticity. A viscous body would differ from a plastic one 

 only so far as the required force of displacement should be smaller. 

 Tar, soft paste, and fluid lava afforded obvious examples of plastic or 

 viscous substances. When the relative positions of their component 

 particles were disturbed they exhibited no sensible elasticity, and 

 made no effort to regain the position from which they had been dis- 

 turbed. They admitted of any degree of elongation without change of 

 volume or discontinuity in their mass. 



The most important character of plasticity was the absence of all 

 elasticity when the particles of a body were displaced without change 

 of volume, and the consequent capability of an indefinite extension of 



