38 ENERGY OF SUBSTANCE IN SOLUTION 



and therefore the attractive forces are predominant. If a greater 

 kinetic energy be given to the molecules by means of heat, for 

 instance, their path will be increased at a rate corresponding to 

 the coefficient of expansion. At a point, the repellent force will 



1 v 2 

 exactly balance the attractive force, that is, ~^ = ^- The sub ' 



stance then assumes the liquid state, gravity alone determining 

 the arrangement of the molecules. 



If the temperature of the liquid be raised, some of the mole- 

 cules will have sufficient velocity to burst through the surface 

 layer and become free gas molecules. If these gas molecules 

 move away unhindered, other molecules from the liquid will 

 take their place, and the liquid will evaporate. If, however, 

 the liquid is kept in a closed space, the gas molecules which 

 leave its surface will be able to proceed no farther than the walls 

 of this space, and rebounding from these must eventually return 

 in the direction of the liquid. Some will strike the surface of 

 the liquid and will be retained by it. But the molecules still 

 continue as before to leave the surface of the liquid, so that, at 

 one and the same time, there are molecules entering and leaving 

 the liquid. When the pressure of the molecules leaving the 

 surface of the liquid balances the gaseous pressure above it, a 

 stationary state will be reached, i.e. the same number of molecules 

 will be freed from the liquid as are being absorbed by it. That 

 pressure is the Vapour Pressure of the substance. (Cf. tension 

 of gas in solution.) 



In addition to the Kinetic Theory of gases, one must assume 

 the statement generally known as the Hypothesis of Avogadro : 

 " Equal volumes of different gases, at the same temperature and 

 pressure, contain the same number of molecules." This proposition 

 has been adopted as a working hypothesis, and as such has stood 

 the test of time. It is, in fact, a necessary supplement to the 

 Atomic Theory. 



The laws governing the physical behaviour of gases are simple 

 statements correlating pressure, volume and temperature. 



(1) Boyle 's Law. The volume of a given mass of gas varies 

 inversely as the pressure on it, if the temperature of the gas 

 remains constant. 



(2) Charles' or Gay Lussac's Law. The volume of a given mass 



