578 RICE ART. L 



increase of <j or of the potential of the solute. Supposing now 

 that <7a is the value of the surface tension and /X2a the value of the 

 potential of the solute at a concentration below this transforma- 

 tion point, and ah and )U25 values above it, then {<ja — cb)/ 

 (m26 — /X2a) would be intermediate in value between the surface 

 concentration of the gaseous jSlm and that of the liquid film. 

 The essential point is that it would be less than the actual con- 

 centration in the liquid film. This is a somewhat enlarged 

 version of an explanation suggested by Rideal in his book, on 

 pages 51 and 52, to account for the fact that V observed is 

 nearly always greater than r calculated. 



28. The Recent Experiments of McBain and Humphreys on 

 Slicing Off a Thin Layer at a Surface 



Note-. Just as this manuscript is going to press the writer has 

 read in the Journal of Physical Chemistry, 36, 300 (1932), a 

 preliminary account by McBain and Humphreys of some fresh 

 experiments in progress on the determination of the absolute 

 amount of adsorption at surfaces of solutions, and if subsequent 

 results follow the indications given by these then it may be 

 said that very dependable evidence for the truth of Gibbs' law 

 by du-ect measurement is at last available. The apparatus is 

 extremely ingenious, and is novel in that for the first time a 

 static surface is involved and not one which is in motion, as in 

 the experiments with bubbles; the criticisms levelled against the 

 latter have been referred to above. 



Briefly, the solution is at rest in a shallow trough of silver 

 surrounded by a saturated atmosphere. The ends of the 

 trough are paraflSned, so that the solution is made to bulge up 

 above them without overflowing. A uniform layer 0.05 to 0.1 

 mm. thick is cut off from a known area of the surface by a small 

 microtome blade travelling at a speed about 35 feet per second. 

 This layer is collected in a small silver-lined cylinder, on which 

 the blade is mounted, and is weighed, its concentration being 

 then compared with that of the bulk solution by means of a 

 Lewis interferometer. From the observed difference of con- 

 centration the adsorption can be calculated. Extraordinary 



