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BELL SYSTEM TECHNICAL JOURNAL 



of an underlying volume concentration. Langmuir i° has shown by 

 the use of the Gibbs adsorption isotherm that in the case of solutions 

 where the solute greatly lowers the surface tension, there is a layer of 

 solute at the surface which is approximately a monomolecular film 

 under a wide range of concentrations in the body of the solution. 

 The same type of relationship may well obtain in the system under 

 discussion, and the concept of a surface film merges into that of a 

 limiting concentration at the surface of the matrix. 



Measurements of spectral response were taken with the light 



8000 

 WAVELENGTH IN A 



Fig. 8 — Relative spectral response to equal energy for cell A, position 1. 



incident upon four different positions on the cathode surface. The 

 relative positions are the same as shown in Fig. 7. The effects were 

 quite similar for positions 1 and 3 and again for positions 2 and 4, 

 so curves are shown only for positions 1 and 2 for cells A and B to 

 illustrate the effects at opposite ends of the cathode. In cells C and 

 D the results were so similar for all four positions that curves are 

 shown only for position 1. 



In Figs. 8 and 9 are shown the effects upon cell A of the admission 

 of oxygen equivalent to half an atomic layer of free caesium. Position 



1° Irving Langmuir, Proc. Nat. Acad. Sci., 3, 251 (1917). 



