MATTER, SPACE, AND TIME 211 



smaller than an atom were discovered by physical 

 means, and are but parts of the structures which 

 take part In ordinary chemical action. The atom 

 is, moreover, defined as the unit of the chemical 

 elements. An atom of a compound is a meaning- 

 less term ; the atoms of water, for instance, would 

 be, not water, but hydrogen and oxygen. 



As to the outward chemical nature of atoms, 

 physics has had till lately little to say ; but 

 molecules, on the other hand, from the first had 

 to be regarded either In a chemical or in a physical 

 aspect. Chemically they are the ultimate units 

 of the compound, the smallest parts of that 

 compound which can exist and still retain the 

 properties of the compound. Any further sub- 

 division would result in the liberation of the 

 elements. Physically, on the other hand, mole- 

 cules are the smallest particles of matter which 

 act as wholes in the incessant irregular move- 

 ments which the particles of matter are always 

 undergoing. The energy of these molecular 

 movements Is the energy of heat ; and. In the 

 most striking case, that of a gas, the impact of 

 the molecules on the walls of the containing 

 vessel gives the physical explanation of the 

 pressure which the gas exerts. It is evident 

 that the physical molecule may contain one or 

 more chemical atoms. Clear evidence shows 

 that in well-known gases such as oxygen and 

 hydrogen, the molecule consists of two atoms, 

 while some gases like argon and the vapours 

 of some metals, mercury, for example, possess 

 monatomic molecules. 



Thus the relations between atoms and mole- 

 cules are ascertained, and further inquiry must 



