98 Prof. W. E. Story on Partial 



d. The partial pressure of any component shall be if the 



proportion of that component is — that is, if the 

 component in question is absent from the mixture, 

 and only then. 



e. The rate at which the partial pressure of any component 



changes as the proportion of that or any other com- 

 ponent changes continuously shall be finite ; and the 

 rate at which the partial pressure of any component 

 increases from as that component is added gradually 

 to any mixture of the other components alone shall 

 not be 0. 



/. Any component by itself shall have a perfectly definite 

 pressure at the temperature in question. 



g. A possible association or dissociation of the molecules 

 of any component shall be regarded as producing a 

 corresponding change in the molar proportion of 

 that component. 



It is important to observe the distinction between the 

 partial pressure of a component of the mixture (to which 

 conditions c, d, and e refer) and the pressure of the same 

 substance in liquid form at the same temperature when 

 existing by itself (see condition /). In the latter case the 

 component by itself may be regarded as the whole mixture ; 

 its pressure is then the total pressure of the mixture. Other- 

 wise the total pressure of the mixture is the sum of the partial 

 pressures of its components ; it is in this sense that the 

 partial pressure of an absent component is (as in con- 

 dition d), because such a component contributes nothing to 

 the total pressure of the mixture. These two cases — that in 

 which the component in question is abs mt and that in which 

 it constitutes the whole mixture — are the two extremes, as 

 far as that component is concerned. 



2. The molar proportion of any component of the mixture 

 is the ratio of the number of " mols " of that component 

 present to the total number of mols of all the components 

 present. Let k be the n amber of components under con- 

 sideration (not necessarily all present in the mixture), 

 n i, n 2, n 3> • • • n K the. numbers of mols of the several com- 

 ponents in the mixture, x x , x 2 , x 3 , . . . x K their molar propor- 

 tions, and p\,p2>Pz, • • • P K their partial pressures, respectively. 

 I^et 



N = n 1 + w 2 + w 3 + • . • +n« ; .... (1) 



