OF NATURAL PHILOSOPHY. 237 



to know much of it. By some recent delicate ex- 

 periments on the dimensions of wires violently 

 strained, it has been shown that they are to a 

 certain small extent capable of being dilated by 

 tension, as they are also of being compressed by 

 pressure, but within limits even narrower than those 

 of liquids. Usually, when strained too far, they 

 break, and refuse to re-unite ; or, if compressed too 

 forcibly, take a permanent contraction of dimension. 

 Thus, wood may be indented by a blow, and metals 

 rendered denser and heavier by hammering or 

 rolling. There is a certain degree of confusion 

 prevalent in ordinary language about the hardness, 

 elasticity, and other similar qualities, of solids, which 

 it may be well to remove. Hardness is that dis- 

 position of a solid which renders it difficult to 

 displace its parts among themselves. Thus, steel 

 is harder than iron ; and diamond almost infinitely 

 harder than any other substance in nature : but the 

 compressibility of steel, or the extent to which it 

 will yield to a given pressure and recover itself, is 

 not much less than that of soft iron, and that of 

 ice is very nearly the same with that of water. 



(258.) Again, we call Indian rubber a very elastic 

 body, and so it is ; but in a different sense from 

 steel. Its parts admit of great mutual displace- 

 ment without permanent dislocation ; however dis- 

 torted, it recovers its figure readily, but with a 

 small force. Yet, if Indian rubber were to be en- 

 closed in a space that it just filled, so as not to 

 permit its parts to yield laterally, doubtless it would 

 resist actual compression with great violence. Here, 



