

Molecular Attraction. 633 



trouble is caused by the numerator factor of the force as 

 defined in statement 2 above, and not by the denominator. 

 Now undoubtedly the molecular attraction is a mutual 

 property of the molecules, but it is not necessary to suppose 

 that the attraction of one molecule can be indefinitely 

 multiplied by the introduction of new molecules into the 

 surrounding space. If we assume that the amount of the 

 molecular attraction is a constant, and does not vary with 

 the total mass of the surrounding molecules, all of the above 

 facts can be reconciled at once. From this point of view the 

 total attractive force of each molecule is independent of the 

 number of molecules and we can write for the law of the force 



as exerted between two molecules, force = s . But 



s 2 



in order to deduce the experimentally true equation (and for 

 other reasons) it is convenient to consider the force as being a 

 function of the mass of the individual molecule and to write 

 for the law governing the attractive force of any molecule, 



LL771 



force = £-V i where a is a constant and m is the mass of the 

 s 2 ' ^ 



molecule. Now if all of the attractive force is utilized by 



being concentrated upon another molecule we would have 



for the energy necessary to pull the molecules apart from 



distance s 1 to s 2 , 



E^fV^mg-I). ... (2) 



For a mass of liquid M containing n molecules, and of 

 molecular weight m, we have, if v is the volume of the liquid 

 and Y the volume of the vapour, 



,-■ 3 /v 3 /V nm Tr nm 



nm=M, fi-V-, * 8 =Vn' U= ^' V== TP 



and equation 2 becomes 



E=\ /tm ^=^/ r i 7= _ r J 7W \ = ^(^-^/D). (3) 



v m 



h) 



This equation gives the energy necessary to pull two 

 molecules from each other during the given expansion if all 

 of the attractive force of one molecule be regarded as con- 

 centrated upon the other. The energy necessary to pull n 



