476 Bar us — Diffusion of Vapor into Nucleated Air. 



changes of pressure decrement on exhaustion. Experiments 

 bears this out. Even for fixed air pressure differences (£/>'), 

 the condensation must progress with a sweep from the bottom 

 upward, and if the very small particles last formed evaporate 

 fast enough, an upper demarcation of the fog-bank will again 

 show itself which would easity be mistaken as a true case of the 

 diffusion of nuclei. In this way the diffusion of about semi- 

 saturation (jp='5) into benzol vapor initially about \ saturated 

 would fully account for the apparent diffusion of nuclei into 

 benzol vapor shown in the memoir cited. 



6. Special experiments must therefore be made to decide 

 whether when nuclei are added at the bottom of a homogeneous 

 column of nearly saturated vapor, the observed diffusion is that 

 of nuclei through the vapor, or of a greater concentration of 

 vapor through homogeneous nueleation. For this purpose it 

 is sufficient to add the nuclei in successive experiments at the 

 top and at the bottom of the receiver, A, figure 1. The nuclei 

 in such a case must diffuse alternately downward and upward, 

 while the vapor diffuses upward only. These experiments, 

 since made with care, showed that the addition of nuclei above 

 or below the column of vapor is without effect on the observed 

 diffusion. Hence it follows not only that the diffusion of the 

 vapor and not of the nuclei has been observed, but that the 

 nuclei must diffuse much more rapidly than the vapor. Indeed 

 in the time in which the nuclei travel from top to bottom of 

 the tall vessel nearly 1 meter high, the vapor has scarcely risen 

 and the fog-bank seen on exhaustion lies close to the surface 

 of the liquid. 



An attempt to measure this rapid diffusion of the nucleus in 

 benzol vapor by the present direct method failed, chiefly 

 because all attempts to rigorously saturate the air in the 

 receiver with the heavy vapor in a reasonable time were seri- 

 ously hampered by convection. The results merely showed 

 that the velocity of the nucleus in benzol vapor must be quite 

 of the same order as in water vapor, but sharp data could not 

 be obtained. 



Brown University, Providence, R. I. 



