Bar us — Diffusion of Vapor into Nucleated Air. 473 



above the surface of the liquid in the receiver, A. 

 from well known principles it may be shown that 



P 



2 



V 



r/ 



x /zVkt 



dq 



Then 



0) 



where Jc is the coefficient of pressure diffusion. 



If the exhaustion at the time t is made adiabatically from 

 air pressure p\ to^/ corresponding to the temperatures # and 

 #', the relation is approximately &'/# = (p' /p' ) {7 ~ 1)/y where 

 a correction for precipitated liquid, etc., is needed. 



The vapor pressure corresponding to the reduced tempera- 

 ture #' so obtained after division by the saturation pressure at 

 # , is then the value of p in equation (1), which therefore like 

 x and t is known so that Jc may be computed. 



3. In order to have an example for use in the discussion 

 below, I computed the case for water vapor, which though 

 unsuitable from its lightness for experiment, is convenient for 

 comparison with other vapors, almost all of which are heavier 

 than air. The well known expansion of (1) judiciously manipu- 

 lated is sufficient for the purpose, though 1 afterwards availed 

 myself of the tables in Dienger's Method of Least Squares in 

 the absence of larger tables. 



Table I. — Pressure diffusion of water vapor into air. Initial (t = 0) satura- 

 tion, p = 0. k= -23 at 20°. 

 Values of p when height of fog-bank x = 



t 



Jem 



5cm 



10 CI " 



20cm 



40 cm 



60" 



l m 



•85 



•34 



•06 



__ 



._ 





5 



•93 



•68 



•40 



•10 



.. 





10 



•94 



•77 



•56 



•24 



•02 



.. 



20 



•97 



•83 



•68 



•40 



•10 



•01 



40 



•98 



•88 



•77 



•56 



•24 



•08 



60 



•98 



•91 



•81 



•63 



•34 



•15 



100 



•99 



•92 



•85 



•71 



•45 



•26 





The 



same for the initial saturation, p = 



•33. 





i ra 



•90 



•56 



•37 







.. 



5 



•95 



•79 



•60 



•39 



.. 





10 



•97 



•84 



•71 



•49 



•34 



._ 



20 



•98 



•89 



•79 



•60 



•39 



•34 



40 



•98 



•92 



•84 



•71 



•49 



•38 



60 



•99 



•94 



•87 



•75 



•55 



•43 



100 



•99 



•95 



•90 



•81 



•53 



•50 



The results for water vapor are given in Table I, where the 

 time, t, is in minutes and the height of the fog-bank, x, is in 

 centimeters. 



