54 A CONTINUOUS RECORD OF ATMOSPHERIC NUCLEATION. 



in question, the diffusion of vapor into partially saturated vapor may be com- 

 puted, as has already been done since 



and the initial saturation is ^o = i at t = o. The results are constructed in 

 figure 3, and from them the parabolas showing the rise of the levels of succes- 

 sively increasing saturation may be derived. 



An inspection of figure 3 shows that if the exhaustion were carried some- 

 what further than corresponds to the lower limit p = I, the fog -banks would 

 be capped at a definite height, and that the latter would be enormously influ- 

 enced by slight changes of pressure decrement on exhaustion. Experiment 

 bears this out. Even for fixed pressure differences {6p) 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 easily be mistaken as a true case of 

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

 (^ = .5) into benzol vapor initially about j^ saturated would fully account for 

 the apparent diffusion of nuclei into benzol vapor shown in the memoir cited. 



6. Crucial experiment and conclusion. — Special experiments must thei-efore 

 be made to decide whether, when nuclei are added at the bottom of a homo- 

 geneous column of nearly saturated vapor, the observed diffusion is that of 

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

 geneous nucleation. For this purpose it is sufficient to add the nticlei in suc- 

 cessive experiments at the top and at the bottom of the receiver, A, figure i. 

 The nuclei in such a case must diffuse alternately downward and upward, while 

 the vapor diffuses upward only. Such experiments since made with care 

 showed that the addition of nuclei above or below the column of vapor is without 

 effect on the obsei"v^ed diffusion. Hence it follows not only that the diffusion of 

 the vapor and not of the nuclei has been observed, but that the nticlei 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 neai'ly i 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 difftision 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 

 seriously hampered by convection. The results merely showed that the ve- 

 locity of the nucleus in benzol vapor must be quite of the same order as in water 

 vapor, but sharp data could not be obtained. 



A curious obser^^ation, obtained particularly in the case of coronas from 

 alcoholic fog particles, deserves mention. Here the tendency to irregular 

 coronas decreases as the number of nuclei becomes smaller. The final coronas 

 are generally regiilar, though small. It follows from this that the diminished 



