I PHYSICAL PROPERTIES AND STATES OF MATTER 5 



reduced in volume, that is, the particles can be forced nearer 

 together. 



Thus we learn that compressibility is not only a consequence of 

 porosity but actually a proof of its existence. 



Elasticity. Imagine a gas to have been made to assume 

 one-half its size by compressing it. What would happen if the 

 pressure, which is the cause of the diminution, were suddenly 

 removed ? .The gas would resume its original size or volume, 

 and it would, so far as appearances are concerned, seem to have 

 undergone no change. The gas is said to be perfectly elastic 

 and the property which enabled it to go back to its original 

 state is called elasticity. Similar results follow with liquids ; 

 they also are perfectly elastic. 



Some differences arise when solids come to be examined. 

 Though the property can be developed in solids in at least four 

 wavs by pressure, by pulling, by bending, and by tunsting 

 we need only in this connection consider the first, as it is the 

 elasticity which is developed by pressure which is most marked 

 in all forms of matter. Ivory, marble, and glass are examples of 

 elastic solids ; while putty, clays, fats, and even lead are 

 instances of solids with scarcely any elasticity. In a scientific 

 sense, glass is more perfectly elastic than india-rubber, because 

 it returns to its original shape after it has been forced out of 

 that shape, whereas india-rubber does not exactly return to its 

 original shape. 



A solid will only resume its former dimensions when the 

 pressure is removed, provided that the pressure is within a 

 certain limit. If the pressure be more than this minimum 

 amount, or if it exceeds the limit of elasticity, as it is called, 

 the solid will not return to the initial size ; it will undergo a 

 permanent change. As the student will see later from Expt. 10, 

 this limit of elasticity is only exceeded in the case of india- 

 rubber when the pressure applied is very great. 



EXPT. 4. Procure a slab of polished marble or some similar 

 material and smear it with oil. Drop a billiard ball or a large 

 glass marble from a considerable height on to the slab. Catch 

 it as it rebounds. Notice that a blot of oil is found where the 

 ball came into contact with the slab. Compare the size of the 

 blot with the spot which is formed when the marble is placed 

 in contact with the slab. 



Evidently the ball underwent a compression as the result of 



