188 BELL SYSTEM TECHNICAL JOURNAL 



found that, below 400° C, the solubility of oxygen in two samples of 

 silver with different ratios of surface to volume was the same. If an 

 appreciable part of the gas had been adsorbed on the surface of the 

 metal instead of being in solution, a difference in apparent solubility 

 should have been observed. They suggested, then, that the minimum 

 point might indicate a change in the silver from one allotropic form 

 to another. This explanation was found unsatisfactory later, because 

 experiments showed that the solubility of hydrogen in silver passes 

 through no minimum as does that of oxygen. 



Mechanism of Solution of Oxygen in Silver 

 Following Langmuir's conception of the mechanism of adsorption 

 Steacie and Johnson ^o proposed an explanation of the mechanism 

 of solution of oxygen in silver in which the solubility minimum is 

 attributed to a change in the form of the oxygen in solution. While 

 a detailed criticism of this explanation would require more space than 

 is available here, it seems unsatisfactory to the present authors, who 

 wish to suggest the following alternative explanation. 



Suppose that dissolved gas is held within the interior of the metal, 

 and that it is in equilibrium with that adsorbed on the surface. Now 

 the concentration in the interior which will be in equilibrium with a 

 given surface concentration increases with increasing temperature. 

 The surface concentration, however, which is due to adsorption, will 

 itself decrease with increasing temperature. Hence the final equi- 

 librium depends on two factors which vary with temperature in 

 opposite directions. Below 400° C. surface concentration may be 

 the controlling factor. Thus, as the temperature rises towards 400° C, 

 the surface concentration decreases faster than the dissolved gas in 

 equilibrium with it increases. Hence the solubility decreases with 

 increasing temperature. Above 400° C, the amount of dissolved gas 

 in equilibrium with the adsorbed surface gas increases faster with 

 increasing temperature than the surface concentration decreases, and 

 the amount of gas dissolved increases with increasing temperature. 

 Theoretically, this explanation applies equally well to other gas-metal 

 systems and would lead one to expect solubility minima in them. 

 These minima, however, in some systems may be at temperatures 

 below the range subject to investigation. 



The Rate of Solution of Oxygen in Silver 

 In addition to their determination of solubility of oxygen in silver, 

 Steacie and Johnson made very careful measurements of its rate of 

 20 Steacie and Johnson, Proc. Roy. Soc. London, 117, 662 (1928). 



