On Gibbs's Theory of Surface- Concentration. 467 



It was shown how this expression might be applied to the 

 phenomenon o£ adsorption, which is considered to be more 

 or less a surface condensation. The determinations referred 

 to consisted in measurements of the various quantities 

 occurring in the above expression, and in comparing the 

 values obtained on the two sides of the equation. The 

 general results showed that the actually determined values 

 for r far exceeded the calculated values — those found being 

 of the order 10 -6 grms./cm. 2 , while the calculated ranged 

 from 10~ 7 to 10~ 8 grms./cm. 2 This discrepancy was ob- 

 served in the case of all the substances mentioned in the 

 previous paper. The adsorption coefficient (T) was measured 

 experimentally under two quite different conditions — namely, 

 at an approximately plane surface ; and secondly at a very 

 curved surface, with concordant results in both cases. 



With the view of throwing some light on the cause of this 

 discrepancy, further determinations of a similar character 

 were carried out and are recorded in the present paper. 



It may be pointed out that the substances mentioned are 

 •electrolytes (sodium salts of organic acids) and would there- 

 fore suffer considerable dissociation in aqueous solution. 

 To account for the discrepancy referred to one possibility 

 suggested itself, namely, the presence of some kind of 

 " double layer " electrical effect. Of course this may or 

 may not be the cause, and it will be shown that for these 

 bodies this is probably not the cause, but evidently the first 

 step was to examine other electrolytes — ordinary inorganic 

 salts of simple constitution and comparatively low molecular 

 weight, and compare the effects obtained in these cases with 

 that obtained for sodium glycocholate and the dye-stuffs. 



Finally, to carry out adsorption determinations if possible 

 with typical non-electrolytes and compare the results obtained 

 in this case also. 



The Interfacial Tension between Oil and Water 

 measured by the drop-pipette. 

 The pipette was identical with that shown in fig. 1 (pre- 

 vious paper). An expression giving the tension directly in 

 absolute units may be derived as follows : — 



Consider the equilibrium of a drop of oil formed in water 

 and just about to break away from the nozzle of the pipette. 

 Let a — the interfacial tension ; 

 a — the radius of the orifice ; 

 r = the mean radius of the drop ; 

 p n . = the density of water ; 

 p = the density of oil. 



2K2 



