136 THE STRtJCTl'R?: oK THE NUCLEUS. 



subsidence ami tiiay lie tlismissetl fiuin consideration. On the otbei' hand, pai- 

 tieles whose radius is smaller than the abscissa corresponding to .s, will grow so that 

 (f.s is the stable radius of the nucleus obtained by shaking the given dilute solution. 



If the solution is weaker, the droplet shaken out of it will have to evaporate 

 further to reach the critical density of the stable nuclear state, and the increment 

 of vapor pressure due to surface tension will also be larger or the maxiiuuni, 911', 

 will be higher. The curve hm' «' now represents the conditions and is to be similarly 

 interpreted. 



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 giveu pressure 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 saturatetl, the chances for eva[)or;itioii will be 

 enhanced. The line ah will be correspondingly lowered, but equilibrium may 

 result for a smaller size of nucleus, until eventually tlie 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 sui'face tension and vapoi- density') will increase under 

 mean atmospheric conditions as they are suspended at higher distances above the 

 earth's sui'face, until the levels of perpetual saturation are invaded. 



There is one outstanding question relating to the time losses which must now 

 be considered. These coefficients, (/»/r7/, are much smaller in concentrated than in 

 weak solutions. This observation was referred to the diffusion of the nucleus and 

 its absorption at the walls of the vessel with different velocities, L The diagram, 

 figure 20, shows, however, that near the points 6' there must be retarded evaporation 

 for all particle.s, because of the small differences of vapor pressure remaining. 

 Hence the persistence of nuclei shaken out of solutions might be ascribed to this 

 effect. True, no reason is evident why strong solutions should differ from weak 

 .solutions in their relative time losses, d log 7i/dt. See § 49. 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 ainoiuit. If this is large enough to be conqiatible with the data for k 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 chapter are con- 

 trived 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 corroboi-ated. 



48. The critical deimiy. — In this place I may again call attention to tiie fact, 

 that if retarded evaporation were effective in giving the nucleus permanence, if the 

 observed dissipation of the nuclei of solutions were in any way dejieudent on 

 evapoi'ation and not on the motion of nuclei, then those nuclei which are produced 

 by shaking solutions of hygroscopic solutes like CaCl,, HjSO^, etc., which can not 



' Note that temperature and elevation produce opposed effects on T'and ff. T increases at the 

 lower temperatures ; a decreases from preponderating pressure. 



