468 Mr. W. C. M. Lewis : Experimental Investigation 



The forces tending to draw the drop off are (1) gravita- 

 tional, due to the difference in densities of the oil and water 



and (2) the difference of pressure due to curvature, 



= 2™ 2 ~. 

 r 



The opposing force is due to the tension and = ^rraa. 

 Hence 



<t 4 



2ira<j— 2ira 2 - = 7j7rr z (p tv —p ). 



Applying this to the particular case of the oil in which 



Pw = 1*0, 

 P o = 0-899, 



a = 0*175 cm., 



r — 0-4125 cm., 



we obtain cr = 0*0467 gram per cm. 



or 45 '8 dynes per cm. 



Employing the empiric formula of Lohnstein * and Kohl- 

 rausch | the value 40 dynes per cm. is obtained. 



The above expression had been already obtained when it 

 w r as found that an almost identical one had been deduced by 

 Gr. Gruglielmo t for liquid-air surfaces. He has applied it 

 to the case of the water-air tension with the following 

 results : — 



Eadius of orifice of pipette. 



Tension in milligrams/mm. 



O'S mm. 

 215 „ 



7-78^1 



7-60 L mean 7'69 at 2 5° 0. 



7-69 J 



The value obtained by the capillary-tube method is 7*485. 

 at 25° C. The results are therefore in fair agreement. 



As in previous determinations, the drop-pipette was em- 

 ployed for the various cases of adsorption dealt with in the 

 present paper, the only difference being that the absolute 

 values for the tension are here calculated by means of tho 

 expression deduced above. 



* Lohnstein. See previous paper. 



t Kohlrausch. See previous paper. 



% G. Guglielmo, Accad. Lincei, Atti, xv. p. 287 (1906). 



