588 SCIENCE PROGRESS 



true, and even where it is observed seems to be restricted to 

 comparatively short intervals. From data obtained from 

 published observations Porter found that fluidity (/) and vapour 

 pressure (p) are connected by the formula 



\ogf = A\ogp + & 



which after differentiation in regard to temperature (t) gives 



dlogf ^ A dlogp 

 dt dt 



By means of this formula Arrhenius has recalculated the 

 values of A for water and mercury and finds that so far 

 from being constant, they decrease in the ratio of i to 0*48 in 

 the case of water over the interval of 1 50 C. ; and from 1 to 

 0*63 in the case of mercury in an interval of 180 (40°-220°) 

 and thereafter increase. Non-associated liquids show the 

 same peculiarity : A has a minimum value at a certain tem- 

 perature, which in the case of these liquids seems to be below 

 the boiling-point, with a variable range from about 45 ° in the 

 case of benzene to about 105 in that of octane. The value 

 of A has been calculated for a number of liquids from the 

 data given in the memoirs already cited. 1 Although Porter's 

 equation is pretty nearly fulfilled in some cases it is not in 

 others, and is especially inexact in the two instances he adduces 

 as evidence of its validity. 



In the case of associated liquids like the aliphatic alcohols 

 and certain of the acids, the value of A, although fairly uniform, 

 shows change with temperature and, as with normal liquids, 

 gives indication of a minimum, depending probably on the 

 complexity of the molecule. In all strongly associated liquids, 

 e.g. water, A steadily decreases with t below the boiling-point. 

 No simple relation between the value of A and molecular weight 

 is apparent in the different homologous series. In the case of 

 the alcohols it seems to increase as we ascend the series. In the 

 case of the aliphatic acids, so far as these have been observed 

 the reverse occurs : formic acid has the largest value of A, 

 propionic acid the smallest, butyric and tsobutyric acids have 

 the same value. 



Confining himself to the data furnished by the two memoirs 



1 Thorpe and Rodger, loc. cit. 



