PLASTICITY. 3 



Viscid liquids, e.g., treacle. Requiring a containing vessel, but only flow- 

 ing sluggishly when poured. 



Soft plastic solids, e.g., wax, putty, modelling clay, and lead. Sufficiently 

 stiff to retain form without a containing vessel, but readily moulded 

 into any required shape by moderate pressure. 



Tough plastic solids, e.g., copper, brass, wrought iron, silver, gold, &c. 

 Completely stiff under ordinary conditions, but exhibiting plasticity, 

 ductility, and malleability when subjected to a sufficient degree of 

 sompelling force. In general, substances of this class become softer 

 and more plastic on heating ; the property of wrought iron to be 

 ' ' forged " whilst red hot depends on this circumstance. Many of 

 them exhibit "tenacity" to a very high extent, being, in fact, the 

 most tough and tenacious substances known. 



Elastic solids, e.g., steel watch-springs, whalebone, thin filaments of glass. 

 When strained or bent out of shape, solids of this kind immediately 

 return to their original shape on being released. 



Brittle solids. When strained, compressed, or otherwise treated in such 

 a fashion as to tend to alter the shape, such substances snap and 

 break asunder easily. Brittleness and elasticity are often exhibited 

 simultaneously by the same substance, e.g., glass. 



Hard or rigid solids, e.g., flintstones. Most kinds of building stone and 

 rock, and gems, especially the diamond. Bodies exhibiting great 

 power of resistance to forces tending to alter their shape. Great hard- 

 ness is often combined with a considerable degree of brittleness, a 

 sudden blow often sufficing to shiver an intensely hard substance. 



Plasticity. 



One of the very earliest of the arts practised by semi-civilised 

 man appears to have been the moulding of vessels for various 

 purposes out of natural clay, this material possessing the 

 property of being highly plastic whilst moist, but becoming 

 tolerably rigid and coherent when dry. Bricks simply dried in 

 the sun, and similar dried clay building materials, are still used in 

 hot climates where a high degree of permanence in building is 

 not indispensable ; but when such bricks or tiles are strongly 

 heated or fired, they become far more lasting, and (saving for a 

 certain degree of brittleness) almost as indestructible as stone. 



As already mentioned, the property of plasticity under con- 

 siderable pressure is exhibited by most of the metals in common 

 use. In order to impress the devices visible on the two sides of 

 a coin, such as a penny piece, a shilling, or a sovereign, the copper, 

 silver, or gold employed must be squeezed very hard indeed 

 between moulds (termed dies) by means of machinery ; but the 

 principles involved in the art of embossing devices by pressure 

 upon flat pieces of metal in a powerful press, so as to convert them 

 into coins or medals, are identical with those concerned in the 

 stamping of a device upon a piece of softened sealing-wax by 

 means of an ordinary seal (Expt. 2), the main difference being that, 



