ULTRA-SHORT WAVE PROPAGATION 155 



the situation for pure water vapor. He showed that in his own experi- 

 ments the anomaUes which appeared at the lower temperatures were 

 probably due to adsorption and not to association, as Jona ^^ had 

 assumed, and he states that his results are consistent with those 

 measured by Jona at higher temperatures. 



For pure water vapor, we may use the following formula which has 

 been based on Zahn's data: 



.-l = 1800X10-|(l+?^). 



Even though the separate values for water and for air may be con- 

 sidered to be known with sufficient accuracy, it does not follow that a 

 mixture of the two will necessarily follow the usual additivity law for 

 mixtures of gases. According to this law the values of e — 1 for the 

 several components may be added to give the e — 1 for the mixture. 

 Delcelier, Guinchant and Hirsch " gave some data for moist air taken 

 as a preliminary to a more thorough study. They interpreted their 

 results as denying this law for a mixture of water and air. Their 

 experiments were carried out at 15° C. and at 25° C. It should be 

 noted that this is the temperature range in which Zahn found 

 anomalous behavior due to adsorption. It is therefore natural to 

 suppose that this same spurious effect may have been present in the 

 work of Delcelier, Guinchant and Hirsch. 



It seems, therefore, that the law of additivity has at least not been 

 disproved for this particular mixture and that we can do no better for 

 the present than to assume that it does hold. We shall therefore 

 proceed on this basis. 



In obtaining the derivative with respect to height de/dH, it must be 

 remembered that e is a function of the partial pressures of dry air and 

 water vapor, and of the temperature. All of these vary with H. 

 The values of significance are those occurring in the first kilometer or 

 so above the ground. The conditions actually observed are variable 

 and we have therefore chosen to use average values as given by 

 Humphreys,^* obtaining the rates of change from the values that he 

 gives for 0.0 and 0.5 km. above sea-level. 



The following table summarizes the results obtained: 



26 M. Jona, Phys. ZeiL, 20, 14 (1919). 



^'' L'Onde Electrique, May 1926, p. 211 et seq. 



28 "Physics of the Air," McGraw-Hill, 1929, p. 55 and p. 74. 



