2 SCIENTIFIC AMUSEMENTS. 



pressure, they are called plastic solids. Some solids require but 

 little force to alter their shape ; well-tempered stiff clay, wax, and 

 lead are examples of bodies of this kind, which are said to be 

 soft ; others require much more powerful pressure to mould them 

 into different form, but can be readily shaped when the force 

 employed is sufficiently great. Copper, iron, and most of the 

 ordinary metals are examples of this kind of solid, and owe much 

 of their usefulness in ordinary life to the property of being 

 sufficiently pliable to be capable of being wrought into shape by 

 appropriate tools, whilst sufficiently hard and tough to bear rough 

 handling and resist wear and tear. Such bodies are said to be 

 malleable, i.e., they can be flattened out and extended by blows 

 from a hammer, or by powerful squeezing machines and rolls ; 

 when they cohere or stick together sufficiently to enable them to 

 be drawn into wire, they are said to be ductile ; the stronger the 

 wire, the greater is said to be the tenacity of the body, i.e., the 

 greater is its power of sticking together under the influence of 

 forces tending to pull it asunder. Some solids are almost entirely 

 destitute of malleability and ductility, although possessing con- 

 siderable hardness and rigidity. When only moderate degrees of 

 pressure are applied they resist completely ; but when subjected 

 to greater force they splinter and break up. Such substances are 

 said to be brittle when easily broken in this way, e.g., most kinds 

 of glass. When the effect of pressure is to alter the shape as 

 lonj as the pressure is applied but no longer, the body springing 

 back to its original shape when released, they are termed elastic. 

 As regards the quality of elasticity, both liquids and gases possess 

 this property usually to a greater extent than solids; thus, if 

 water be strongly compressed by a powerful pump, it shrinks in 

 volume as a piece of india-rubber similarly treated would do, though 

 not to the same relative extent; on releasing the pressure the 

 water instantly springs back to its original bulk. Exactly the 

 same thing occurs with air and all other gases, this class of 

 substances being, however, far more readily compressible and 

 expansible than water and other liquids. 



The following table will serve as illustration of the different 

 degrees of texture and compactness exhibited by different classes 

 of ordinarily occurring substances : 



Gases, e.g., the air. Bulk for bulk much lighter than most liquids and 

 solids ; excessively easily set in motion, and eminently mobile. 

 Keadily compressible and highly elastic. 



Mobile liquids, e.g., water and ether. Much heavier bulk for bulk than 

 gases, but considerably lighter than most ordinarily occurring metals, 

 such as lead, silver, copper, &c. Compressible and elastic, but requir- 

 ing far more force to compress them than gases. 



