PROPERTIES OF MATTER 15 



Some substances, though the spaces between their 

 molecules cannot be seen, include in their structure 

 many larger spaces that can be seen. Such substances 

 are usually made of many fibers or cells loosely put 

 together, and the spaces between these small parts are 

 sometimes called pores also. A sponge or a piece of 

 blotting paper clearly shows this structure. A body 

 of this sort is generally said to be porous. 



22. Compressibility. Given a bundle of loose cot- 

 ton, we know that it could be crowded into a much 

 smaller bundle ; but in such a case its fibers would 

 be much nearer together. In a similar manner the mol- 

 ecules of some substances may be crowded nearer to- 

 gether, the pores becoming smaller and the whole body 

 losing some of its size. The property whereby a body 

 may thus be crowded into a smaller space is called 

 compressibility. As a general rule, solids and liquids 

 are not very compressible ; great force is required to 

 crowd their molecules nearer together. Gases, however, 

 have great compressibility; their molecules under ordi- 

 nary pressure are widely separated, and when great 

 force is used they may be driven very much nearer 

 together. 



23. Elasticity Elasticity is that property by which 



a body goes back to its former size and shape after the 

 force which changed it has been removed. It is very 

 important to notice that a body is elastic not because it 

 may be bent or stretched, but because it goes back to 

 its former state as soon as the force ceases to act. Ivory 

 is very elastic, as is glass; rubber is not so nearly 



