42 . The Realm of Nature CHAP. 



turn, thus setting up complete circulation throughout the 

 mass. This transmission of heat by the translation of 

 heated portions is called convection, and in consequence of 

 it the temperature of a liquid heated from beneath becomes 

 much more rapidly uniform than that of a solid. The 

 conduction ( 59) of heat in liquids is very slow, and when 

 the upper layer is heated the vibrations of its particles are 

 passed on by conduction to the mass below very slowly 

 indeed ( 229), as the expanded upper layer tends to remain 

 in its position. 



68. States of Matter. If the particles of any kind of 

 matter were absolutely at rest, that is to say if they possessed 

 no kinetic energy, it is usually assumed that the body would 

 be absolutely cold, or at the absolute zero of temperature. 

 This total absence of heat has never been actually observed. 

 The difference between the same substance in the solid, 

 liquid, and gaseous states is due to the rate of motion of the 

 particles alone, and the work of moving the particles may 

 be readily expressed in terms of heat. Thus in solids which 

 contain relatively little heat the particles move so slowly 

 that cohesion confines them to excessively minute paths, and 

 the substance possesses rigidity ( 35). In liquids there is 

 much more internal movement or heat, and the particles 

 having a longer path and greater rapidity of motion partly 

 overcome cohesion and show the property of fluidity. Gases 

 contain so much heat that their particles are in very rapid 

 motion through comparatively long paths and the power of 

 cohesion is quite overcome. When the pressure remains 

 the same, every additional degree of temperature makes the 

 particles of a gas move more quickly through a longer path, 

 and the volume occupied by the gas is increased by ~-^ 

 (_L g. for each centigrade degree). A fall of i reduces the 

 volume by -^\ 1S . Hence a fall of 490 of temperature in 

 a gas at o should reduce its volume to nothing, which 

 is impossible ; hence it is believed that no gas or liquid 

 can exist at 490 F. In other words the particles of 

 solid matter would be motionless, that is, absolutely without 

 heat or at the Absolute Zero of temperature. 



69. Action of Heat on Ice. We may follow the action 



