'DIVERSITY 

 THE PROPERTIES 



(a.) When iron is heated, the molecules are pushed further apart, 

 the pores are enlarged, and we say that the iron has expanded. If 

 a piece of iron or lead be hammered, it will be made smaller, because 

 the molecules are forced nearer together, thus reducing the size of 

 the pores. Cavities or cells, like those of bread or sponge, are some- 

 times spoken of as "sensible pores," but these are not properly in- 

 cluded under this head. 



43. What is Compressibility? Compressibil- 

 ity is that property of matter by virtue of which 

 a body may be reduced in size. 



44. What is Expansibility? Expansibility is 

 that property of matter by virtue of which a body 

 may be increased in size. 



(a.) Compressibility and expansibility are the opposites of each 

 other, resulting alike from porosity. Illustrations have been given 

 under the head of porosity. Let each pupil prove by experiment 

 that air is compressible and expansible. 



45. What is Elasticity ? Elasticity is that 

 property of matter by virtue of which bodies resume 

 their original form or size when that form or 

 size has been changed by any external force. 



(a.) All bodies possess this property in some degree, because all 

 bodies, solid, liquid or aeriform, when subjected to pressure (within 

 limits varying with the substance), will resume their original size 

 upon the removal of the pressure. The amount of compression mat- 

 ters not except in the case of solids. It was formerly thought that 

 liquids were incompressible ; hence aeriform bodies were called 

 elastic fluids, while liquids were called non-elastic fluids. But the 

 compressibility and perfect elasticity of liquids having been shown, 

 the term "non-elastic fluid" involves a contradiction of terms and 

 would better be dropped. Fluids have no elasticity of form ; on 

 the other hand, all fluids have perfect elasticity of size. What 

 properties of matter are illustrated by the action of the common 

 pop-gun ? 



46. What are Cohesion and Adhesion? 



Cohesion is the force that holds together like mole- 



