446 Mr. J. H. Shaxby on Vapour Pressures 



Similarly, ^d* 



A 2 = 2RTd 5 



from either of which equations it immediately follows that 



log 



A = 2RT 



d 2 



dy-d 2 > 



as before. 



Before we can proceed further it becomes necessary to 

 consider the meaning of the quantities A and b. 



A may be termed the Cohesive constant of a substance, 

 and is the quotient of the work per unit mass against the 

 forces of cohesion divided by the density 



a _ Al £^2 



d 1 ~ d 2 ' 



If A 1 and A 2 depended simply on the density, we should 

 have A the same for all temperatures, which is not the case. 

 A diminishes with rise of temperature, indicating, what one 

 might expect, a decrease in the forces of cohesion as the 

 temperature is raised. 



The quantity (di~\-d 2 ) which plays such a fundamental 

 part in the equations deduced above, may be regarded as a 

 measure of the closeness of molecular packing which would 

 result from the cohesive forces alone in the absence of 

 molecular motions ; for, if there were no " free " space, 

 v = b. Thus the density would be {d 1 + d 2 ), an d we mav 

 regard " free "■ space, in the sense of the equations of the 

 kinetic theory, as a result of the molecular movements, 

 which prevent the molecules from being as closely packed 

 as they would be under their mutual attractions if they did 

 not possess kinetic energy. If this be so, we can regard the 

 quantity (d x -)- d 2 j, which we will term the Cohesive Density, 

 as a measure of the intensity of the forces of cohesion. We 

 should thus expect (d 1 +d 2 ) to depend in some simple 

 manner upon A. The two quantities, as a matter of fact, 

 prove to be almost exactly proportional to one another. 

 That is to say, the product Ab is nearly constant, as is 

 shown in column 8 of Table I. There is a clear tendency 

 of the product to increase slightly with rise of temperature 

 to a maximum value, and then to decrease again as the 

 critical temperature is approached in all non-associated 

 substances. This lack of exact constancy in Ab may be 



