ATOMS AND MOLECULES 51 



Dalton's definition of a solid body is of interest. 

 He defines it thus : " A solid body is one the 

 particles of which are in a state of equilibrium 

 betwixt two great powers, attraction and repulsion, 

 but in such a manner that no change can be made 

 in their distances without considerable force. If an 

 approximation of the particles is attempted, then 

 the heat resists it ; if a separation, then the attrac- 

 tion resists it." 



It is a little difficult to believe that the molecules 

 of hard substances are in constant motion, but 

 even in the hardest substances the motion takes 

 place. Sir Roberts Austen placed pieces of lead 

 and gold in contact at atmospheric temperatures, 

 and found after four years that the gold had 

 travelled into the lead to such an extent that 

 appreciable quantities of the gold were detected 

 at more than five millimetres from the common 

 surface. 



If the molecules are far apart we have a gas ; if 

 nearer together, a liquid ; if still nearer together, a 

 solid. By means of heat we can cause a separation 

 of molecules, and thus can convert a solid into a 

 liquid, a liquid into a gas. By means of cold, again, 

 we can convert a gas into a liquid, a liquid into a 

 solid ; and the contraction of the molecules may 

 be assisted by pressure. By means of combined 

 cold and pressure, Sir James Dewar succeeded in 



