3tf Dr. A. Ferguson on the Variation of 



made (for unassociated liquids) by observations of the tem- 

 perature-variation of surface-tension over a comparatively 

 limited range. 



Some little time ago I pointed out that the equation 



T = T o (l-60)* (iii.) 



represented the surface-tension of benzene with considerable 

 accuracy between 0° and the critical temperature*. Such a 

 formula, if of wide application, would be very valuable, as, 

 in addition to its use in obtaining the surface-tension of a 

 liquid at any temperature over a wide range, it gives the 

 critical temperature at once from the equation 



0c=\> (IT.) 



and so enables us to make a comparison of surface-tensions 

 at any desired corresponding temperatures. 



There are very few formulae which represent the effect of 

 temperature on surface-tension over any wide range, but I 

 have recently noticed that Van der Waalst has put forward 

 a binomial expression 



T ro =A^J(l- m )= (v.) 



where m is the " reduced" temperature, and A and B are 

 constants for all bodies following Eotvos' law. This equation 

 represents the experimental facts very accurately, but, 

 demanding as it does a knowledge of the critical data, it is 

 hardly so suitable for our purposes as (iii.), which is put in 

 a form which enables one to calculate the critical tempera- 

 ture from observations of the surface-tension alone. More- 

 over, as will be seen later, (iii.) fits the experimental data 

 more accurately than (v.). 



The manner in which it was shown that (iii.) represents 

 the experimental results may be of interest. Differentiating 

 (iii.) with respect to 0, we have 



and therefore 



g = -?mT (l-^)»-S .... (vi.) 



T 3T : hT ( vll -> 



* Science Progress, Jan. 1915, p. 445. 



t Zeit.fiir Phys. Chem. xiii. p. 710 (1894). 



