474 Barus — Diffusion of Vapor into Nucleated Air. 



The table also contains a second series of data, in which the 

 diffusion takes place into a vapor -J- saturated, to which ref- 

 erence will be made below. The results of Table I may be 

 constructed graphically, showing respectively the advance of 

 diffusion at a given height or at a given time. The latter are 

 exhibited in connection with figure 1. From either set of 

 curves the parabolas which show the rise of a given vapor 

 pressure in the lapse of time may be obtained by graphic 

 interpolation. 



If the exhaustion chosen is such as to reduce the vapor pres- 

 sure to -J- the intersection of the vertical line of the figure 

 with the successive curves will show the heights of the fog- 

 banks on condensation. Thus after 20, 40, or 60 minutes the 

 fog-banks having attained heights of roughly 23, 33, and 40 

 centims., will be in good position for observation. 



For any other vapor than water, the times will increase 

 inversely as the coefficients of diffusion. Thus for benzol § 4, 

 the time intervals should be increased about 2-J- times ; etc. 



4. The feature of these curves is the extreme slowness of 

 diffusion even for water vapor. At but 20 cm above the liquid 

 surface it takes half an hour to reach semi-saturation. The 

 case is accentuated for other liquids where the coefficients are 

 smaller, as for instance for the following liquids at about 20° ; 



Vapor, CS 2 C 6 H G H,0 CH 4 C 2 H 6 C 3 H 8 

 h— -1 '09 -23 -16 -]2 -07 



If therefore the fog particles are relatively large and subside 

 rapidly, the air will soon become highly desaturated. In other 

 words, if the air in the receiver A is cleaned of nuclei by con- 

 densation, there is no vapor available to replace the liquid lost. 



In case of water vapor the fog particles are small and subside 

 slowly while the vapor is lighter than air. Hence the latter is 

 liable to be reheated from the rapid radiation of gases assisted 

 by convection as stated, before much desaturation takes place, 

 unless the vessel is very long and the sides dry. The opposite 

 is the case for the hydrocarbon vapors in spite of their vola- 

 tility, since the fog particles are larger and fall rapidly and the 

 vapors are heavier than air. After successive precipitation at 

 a given pressure difference, the vapor may be so far desaturated 

 that it nearly ceases to condense even if nuclei are present. It 

 cannot quite cease to respond, for some vapor must return to 

 the air after condensation almost instantaneously ; but it is not 

 improbable that a vapor exhausted to a slightly higher pressure 

 difference will fail to condense thereafter at the original pres- 

 sure difference. Thus there is considerable chance for error 

 in judgment and what is taken for the diffusion of nuclei 



