II 



Manchester Memoirs, Vol. Ixv. (1921), No. 5 



V. — Studies in Capillarity. 



On a Modification of the Capillary Tube Method for the 

 Measurement of Surface Tensions. 



By Allan Ferguson, M.A., D.Sc, and P. E. Dowson, M.A., 

 Department of Physics, Manchester College of Technology. 



{Read February 1 il/i, 1921, at a meeting held jointly with The Faraday 



Society.) 



The determination of surface tensions by the measurement of the rise 

 of a liquid in a capillary tube is more than ordinarily difficult. It is not 

 easy, in the first place, to find a tube of sufficiently uniform cross-section 

 to use with different liquids — some experimenters have spent as much as 

 four months in searching for a uniform piece of capillary tubing. 1 When 

 this is found, the calibrating, cleaning and keeping clean of such a piece 

 of tubing are no small matters. Nor is it easy to measure the height of 

 ascent of the liquid, and to estimate the temperature of the meniscus with 

 the accuracy demanded. And when we remember that, all the measure- 

 ments having been made with due care, the value obtained is not T, but 

 Tcos 6 it becomes increasingly clear that the convenience of this widely 

 used method is more apparent than real. 



The importance of a knowledge of the temperature-coefficient of a 

 surface tension requires - special emphasis. Not only does a knowledge of 

 this coefficient enable us to make comparisons between different liquids 

 under comparable temperature conditions, but it also enables us to calcu- 

 late the critical temperature of an unassociated liquid with considerable 

 accuracy. For the surface tension of such a liquid is expressed very 

 closely, from freezing point to critical temperature, by the equation 



T = T (i - b&f 

 where b is accurately the reciprocal of the critical temperature. 2 The 

 exactness with which this equation gives the critical temperature may be 

 judged from the figures given in Table 1 below. 





TABLE 



I. 





Substance. 



»<H> 



6 , Observed. 



Difference. 



Ether 



Benzene .... 



Methyl formate 

 Chloro-benzene 

 Methyl butyrate 

 Ethyl formate . 

 Propyl formate 



194-0° 

 288-0° 

 213-0° 

 35 8-o° 

 281-0° 

 235-0° 

 265-0° 



193-8° 

 288-5° 

 214*0° 

 359*2° 

 281-3° 



235-3° 

 2649° 



+ 0'2° 



-0-5° 



- i-o° 



- 1-2° 



-0-3° 

 -o-3° 

 + o-i° 



1 Harkins and Brown, Jour. Amer. Chem. Soc, 41, 503 (1919). 



2 Ferguson, Phil. Mag., Jan. 1916, p. 37. 



May iph, 1921. 



