SURFACES OF DISCONTINUITY 581 



tend to weaken the field of force to which it is due and so 

 produce a diminution in the surface energy. 



30. The Experiments of Iredale 



We shall first briefly review the results obtained in Donnan's 

 Laboratory by Iredale {Phil. Mag., 45, 1088 (1923); 48, 177 

 (1924); 49, 603 (1925)). He deals principally with the adsorp- 

 tion of vapors of organic substances at the surface of mercury; 

 these have the property of lowering the surface tension of mer- 

 cury. The drop weight method of determining surface tension 

 was used and its accuracy is carefully discussed. The vapors 

 were generated by passing a very slow current of dry air at con- 

 stant pressure through the organic liquids. The adsorption of 

 the vapor at the surface of the drops appeared to be a fairly 

 rapid process; for "the period of drop formation was never less 

 than 3| minutes and with longer periods the weights of the drops 

 were not found to decrease appreciably" thus indicating that a 

 steady condition of surface tension had been reached. The re- 

 sults with methyl acetate vapor showed a fall from 470 dyne per 

 cm. to about 430 for a partial pressure of 40 mm. in the vapor; 

 thereafter the fall was much slower, reaching a value about 412 

 dynes as saturation of the vapor at about 225 mm. was ap- 

 proached. At this point there was a sudden fall of the surface 

 tension to about 370 dynes which is the value of the surface 

 tension of mercury in liquid methyl acetate. Taking the slope 

 of the graph, which gives da/dp at 62 mm. pressure, where the 

 conditions of maximum adsorption are being approached 

 although the vapor pressure is still well away from saturation, 

 and multiplying it by y for the vapor there, a value about 

 4.5 X 10~* gram of methyl acetate per sq. cm. is obtained. 

 This corresponds to about 0.37 X lO^^ methyl acetate molecules 

 per sq. cm. of mercury surface. This figure is near the values 

 given by Langmuir {J. Am. Chem. Soc., 38, 2288, (1916)) for 

 unimolecular layers of carbon dioxide, nitrogen, etc. "More- 

 over the space taken up by each molecule (27 X 10^^^ sq. cm.) 

 is near that required for molecules of esters and fatty acids on 

 the surface of water, namely, 23 X 10"^ sq. cm., and it is possible 

 that the same type of orientation obtains on the mercury surface. 



