CHEMICAL THEORY OF SOLUTIONS. PAKT I. 19 



where q^, q-i, represent the molecular heat of evaporation. 



The limitations to which the application of the equations are 

 subject are too well known to be mentioned in this place, qi, 



q2, diminish with rising temperatures, but the exact form 



of the function is not known. Within a short range of tem- 

 perature, say thirty to fifty degrees, and particularly in the 



region where the vapour pressure is not very large, q^, q-i, 



may be considered constant without causing much error. '^ The 

 equations can then be integrated into 



P, = e"' '^, P., = e" '^, etc (A) 



where ai, ao, are constants. In cases where the differences 



between the boiling points are not great the equation (9) can be 

 written 



_ 9l _ Î2 



Ci/' '^^+C,e'' ^'^+ = ^ (10) 



Tkouton's law simplifies the matter considerably. According 

 to this law the ratio between the molecular heat of evaporation 

 and the absolute temperature of the boiling point is constant for 

 various substances. This has been verified for many unassociated 

 liquids under ordinary atmospheric pressure. But there is no 

 doubt that the law is also applicable to cases in which the pressure 

 is considerably smaller. Only the ratio must increase with 

 decreasing pressure. Thus under ordinary atmospheric pressure, 



1) Otherwise we have tlie usual equation with more than two constants in the form 



In P=A—-jp + Cln T+DT+ But then the constants can no longer be determined 



with necessary definiteness from empirical data, because slight experimental errore cause 

 considerable shifting between the values of the constants. Hence the approximate equation 

 is employed advisedly, not only to lessen mathematical complicacy, but also to make the 

 deductions more definite. 



