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



successive experimental numbers. The value is 3*3 x 10 -10 

 gm.-mol. per sq. cm., or *20 mgm. per sq. metre in a normal 

 solution, which is nearly ten times as much as the corre- 

 sponding defect in salt solutions; and it alters by less than 

 15 per cent, even when the concentration is increased eight-fold. 

 At concentrations lower than normal the curve becomes 

 uncertain ; but it is evident that, as the solution becomes more 

 dilute, the ratio of the excess to the concentration must ulti- 

 mately become very large. 



It is interesting to observe that very dilute solutions of 

 acetic acid will give a lasting foam, but that strong solutions 

 will hardly foam at all. We may infer that the question is 

 one of the relation of the surface excess to the concentration, 

 and the consequent difference in its rate of formation by 

 diffusion of the solute into the surface in strong and weak 

 solutions. In strong solutions, on the surface excess being- 

 pulled away from any portion of a film by a local inequality 

 of the forces, it may be formed again so quickly that the 

 surface tension remains practically unchanged, and the film 

 thins rapidly and collapses as in a pure liquid. In dilute 

 solutions the excess must take an appreciable time to form, 

 and the life of the film is correspondingly increased. 



Sodium Oleate Solutions. — Here it is more difficult to deter- 

 mine the surface excess from the tension curve. Marangoni 

 has shown that the surface tension, while very much less than 

 that of pure water, is practically independent of the concen- 

 tration, and indeed based on that fact an argument for 

 the existence of the pellicle. For the calculation oE the 

 excess a knowledge is required of the rate of diminution of 

 the tension with the concentration. I consequently made a 

 series of measurements, and continued them to as low con- 

 centrations as possible, to see if there were any portion of the 



curve at which ~ might be obtained. The solutions were 



contained in a U-tube, having one large and one capillary 

 limb, the difference in the heights of the liquid in the two 

 limbs being measured with a cathetometer. By forcing a 

 little air into the wide limb, the liquid could be expelled over 

 the top of the capillary and a perfectly fresh surface formed 

 at any instant. It was found that the surface tension, after 

 a fresh surface was formed in this way, always varied with 

 the time, falling rapidly at first and afterwards more slowly 

 until it reached a final value independent of the time. Fig. 2 

 shows two specimen curves of the variation in solutions of 

 different concentrations. In moderately strong solutions the 

 fall was initially very rapid, the tension reaching within 



