162 HEAT. 



energy, will behave to each other as gas molecules, colliding, and 

 escaping again from each other's spheres of action, cannonading the 

 sides of the vessel, and so exerting pressure. Through their collisions 

 with each other and with the sides, many of the molecules will have 

 their direction of motion reversed, and will return to the liquid. Some 

 of these will become entangled, and again become liquid molecules. The 

 greater the number of molecules in the space, the greater is the number 

 thus returning to the liquid, and as evaporation continues a point is at 

 last reached when the number returning is equal to the number escaping. 

 This corresponds to the "maximum vapour-pressure," the steady state 

 being due not to a cessation of evaporation- but to a balance between 

 evaporation and condensation. If, through diminution of volume, the 

 pressure tends to exceed the maximum vapour-pressure, the condensation 

 exceeds the rarefaction until the steady state is again arrived at. 



We may note that the molecules escaping are the most energetic in 

 the liquid ; their escape therefore lessens the average energy of those 

 remaining, and this is the meaning of the fall of temperature in a liquid, 

 produced by evaporation from its surface. 



If the temperature of a liquid rises, the average energy of the mole- 

 cules increases, and the number of molecules with velocity sufficient to 



escape also increases. Hence evapora- 



! ^ tion goes on more rapidly. The number 



of molecules in the space required to 



*. .". . ' . ". . . . .*. produce a balancing condensation must, 



..'...'..'. .'..'..'..'..'. .'..'..'..'..'..'.. v therefore, also be greater in other 



..*.... words, the maximum vapour-pressure 



increases. 



| ' ' The rapidly rising rate of increase 



' may, perhaps, be explained, or at least 



FIG. 87. illustrated, as follows : Let us plot on a 



diagram the energy possessed by each 



molecule of a given mass of liquid by putting a point at a distance 

 from OO' (Fig. 87) proportional to its energy. We may suppose that 

 the points are chiefly crowded about the line AV, whose distance from 

 OO' represents the average energy. But there will be numbers of 

 molecules possessing both more and less than the average, though the 

 further the distance from the average, the less the number of points. 



Let the energy which a molecule must possess in order to escape, be re- 

 presented by the distance between ES and OO'. All molecules, therefore, 

 represented by points above ES will escape if they have the opportunity. 



As the temperature rises, the average energy rises, so that the whole 

 diagram of points may be supposed to be stretched upwards and the 

 number above ES increases. But the Specific Heat being nearly 

 constant, the total energy, and therefore the average energy, increases by 

 nearly the same amount for each rise of temperature of 1, that is, AV 

 approaches ES by nearly equal steps. But as the points are crowded 

 more and more, the nearer we approach to AV, each successive degree- 

 rise brings a greater and greater number above ES. The number able 

 to escape and the vapour-tension, therefore, increase much more than in 

 proportion to the rise of temperature. 



So far we have supposed the space above the liquid to contain 

 only the vapour of the liquid. But if some other gas say air is 



