102 Dr. S. R. Milner on Surface Concentration, 



dr 

 These numbers show no trace of any finite value for -=- 



down to a concentration of *002 normal, and would in them- 

 selves seem to negative the idea, of a surface excess. On the 

 other hand, the very large negative, and probably infinite 



dr 

 value which - ,- must possess in the neighbourhood of zero 

 etc 



concentration, renders it probable that an excess of consi- 

 derable magnitude exists even in the dilutest solutions. This 

 conclusion is supported by the way in which in the different 

 solutions the tension \aries with the time. The time-curves 

 suggest that the surface excess is so large, that in dilute 

 solutions the diffusion of sufficient oleate into the surface to 

 form it takes an appreciable time, and that the fall in the 

 tension is proportional to the amount of oleate which has 

 diffused into the surface in the interval *. 



The theoretical method for the determination of the surface 

 excess thus fails to give a result; but I have verified its 

 existence experimentally, and made a rough estimate of its 

 value, in the following way. A current of air, saturated by 

 previously bubbling through water, was passed through a 

 number of fine holes into a beaker about two-thirds full 

 of a solution of sodium oleate. The foam rising to the top of 

 the beaker was periodically removed, and the resistance of 

 the solution was measured in situ at intervals during the 

 experiment. In dilute solutions the resistance was always 

 found to perceptibly increase after bubbling for about an 

 hour's time — indicating that the oleate is removed from the 

 interior by the continual re-formation of the surface. By 

 measuring the rate of increase of the volume of the foam and 

 the average size of the bubbles, and by assuming that each 

 bubble as it forms removes a portion of the surface equal to 

 its sectional area, the total area of the surface removed in a 

 given time can be readily, if only roughly, estimated. Also, 

 from the alterations in the concentration determined from the 

 electrical measurements, the amount of oleate removed from 

 the interior of the solution by the foam is known. Thus the 

 surface excess, which is the amount removed per sq. cm. 

 of fresh surface formed, is simply calculated. The following- 

 table shows the results obtained : — 



* The curves roughly agree with those calculated on this supposition, 

 but the effects are so much complicated by the alterations of the surface 

 as the liquid moves in the tube, that I have not been able to make an 

 accurate calculation. 



