(J, Barus — Yelocity and Structure of the Nucleus. 22^ 



If the solvent is pure or contains only a trace of solute, the 

 nucleus will vanish completely, or else the particle left may be 

 too small to serve as a condensation nucleus for a given pres- 

 sure decrement of exhaustion. The size after the lapse of 

 time depends on the fixed quantity of salt originally entrapped. 



If the vapor is not quite saturated, the chances for evapora- 

 tion will be enhanced. The line ah will be correspondingly 

 lowered, but equilibrium may result for a smaller size of 

 nucleus, until eventually the solid saline residue of the nucleus 

 alone is left. In so far as these concentration nuclei occur in 

 the atmosphere, one is justified in concluding that their size 

 (apart from the effects of temperature and barometric pressure 

 on surface tension and vapor density*) will increase under 

 mean atmospheric conditions as they are suspended at higher 

 distances above the earth's surface, until the levels of perpetual 

 saturation are invaded. 



6. There is one outstanding question relating to the time 

 losses which must now be considered. These coefficients, 

 cln/dt, are much smaller for concentrated than for weak solu- 

 tions. This observation was referred to the diffusion of the 

 nucleus and its absorption at the walls of the vessel with dif- 

 ferent velocities, A*. The diagram shows, however, that near 

 the points s there must be retarded evaporation for all parti- 

 cles, because of the small differences of vapor pressure remain- 

 ing. Hence the persistence of nuclei shaken out of solutions 

 might be ascribed to this effect. True, no reason is evident why 

 stronger solutions should differ from weak solutions in their rela- 

 tive time losses, cl log n/dt. Keferring to Table 1 again, solu- 

 tions of CaCJj, H^SO^, which can not dry in a saturated atmos- 

 phere, are seen to show nothing exceptional in their behavior. 

 It may be computed that the stable nucleus is, in this case, not 

 even very concentrated. Special experiments are nevertheless 

 needed to clear up the matter, and they must be so devised as 

 to give direct evidence of the occurrence of diffusion or motion 

 of nuclei, and the value of its amount. If this is large enough 

 to be compatible with the data for Ic in this chapter, then the 

 hypothesis of retarded evaporation may be dismissed. 



It is with this end in view that the experiments of the next 

 table are contrived and the results show that the motion of the 

 phosphorus nucleus, as actually observed, is considerably faster 

 than the average case computed for the nuclei in the present 

 chapter, and consequently the interpretation here accepted is 

 corroborated. 



7. Motion of nicclei directly observed. — Table II shows the 

 velocity of the nuclei as found directly in a tower-like receiver 

 1 meter high, into the bottom of which the nuclei have been 

 introduced. The height of the fog bank, k,\ seen on exhaustion 



* Note tliat temperature and elevation produce opposed efEects. 



