ii The Substance of Nature 21 



for the water occupies only the cavities between the sponge 

 fibres. The particles of many kinds of matter are packed 

 loosely together so that vacant spaces or pores occur. 

 Porous bodies, like unglazed earthenware, sandstone, and 

 charcoal, apparently allow air or water to pass through 

 them ; really, however, the fluid passes through the other- 

 wise empty pores. The exclusiveness of the space-occupa- 

 tion thus holds good for the smallest particles of matter 

 only ( 48). The term impenetrability is often used for 

 this property. 



35. Stresses and Strains. When the form or volume 

 of a body is altered the body is said to be strained, and 

 the set of forces which produce a strain is called a stress. 

 Stresses act always in two opposite directions, either as a 

 push or a pull. Rigidity is the resistance that a solid body 

 offers to shearing stress. Extremely rigid substances, such 

 as steel, require the action of powerful stresses in order to 

 change their form ; while less rigid substances may be 

 readily deformed or strained, as a rod of lead is bent or 

 a piece of sandstone pounded into dust. When uniform 

 pressure is applied all solid substances, and still more all 

 liquids and gases, are reduced in volume, the matter in them 

 being compressed into smaller space and the density being 

 of course increased. The amount of compression which the 

 same pressure effects is called Compressibility and it differs 

 in various kinds of matter, being greatest of all in gases 

 ( 72, 148). The tendency of a body to recover from 

 strain and return to its previous form and volume when the 

 stress ceases to act is termed Elasticity. A steel watch- 

 spring is said to be elastic, because afte-r being coiled up 

 tight it returns to its former size and shape. Air is said to 

 be elastic, because when it has been compressed and the 

 pressure is removed it returns at once to its previous 

 volume. 



36. Gravitation. Every portion of Matter attracts or 

 tends to approach every other portion of Matter in the 

 Universe ivith a force proportional to the masses and 

 inversely as the square of the distance. This is Newton's 

 Law of Universal Gravitation, and is established beyond 



