

50 PHYSICAL PROPERTIES AND COMPOSITION 



An equally important application is to the case of phos- 

 phonium chloride PH 4 C1, whose critical temperature 5o-j, 

 lies only a few degrees above the melting point. Here 



g = ^3^o 1 5 =4; 



whilst 



P + 4H + C1 = 3-01 + 2-28 + 2-66 = 7-95. 



It is natural to suggest, as a reason for the abnormally 

 small critical coefficient, a dissociation into PH 3 and HC1. 

 From which again it may be concluded, with some prob- 

 ability, that the melting of phosphonium chloride is not 

 a fusion in the strict sense, but a transition into a liquid 

 mixture of the components. 



B. Boiling Point, 

 i. Additive Relations. 



The additive relations found in the critical coefficient, 

 are to be expected also in the quotient of temperature by 

 pressure for corresponding temperatures ; for then the 

 temperatures are equal fractions of the critical 



T=aT k , 

 and the pressures equal fractions of the critical pressures 



P = 



so that 



T aT k 



The relations observed in the critical coefficients conse- 



T 



quently appear also in the quotients -p for corresponding 



temperatures. Remembering, then, that according to p. 36 

 the boiling points are approximately corresponding tem- 

 peratures, and that here P is unity, additive relations are 

 to be expected in the boiling points, as Kopp, in particular, 

 pointed out. These additive relations may be referred to 

 those, also observed by Kopp, in the molecular volume at 



