102 VAPOR NUCLEI AND IONS. 



conditions are fixed, shows that groups of nuclei counted by millions per 

 cubic centimeter must be available for condensation long before the 

 molecular sizes are approached. It is convenient to refer to the molec- 

 ular aggregates in question by the term "colloidal nuclei." They are 

 necessarily much smaller than ions. The number of such nuclei increases 

 as the supersaturation is greater and the size needed therefore smaller 

 until condensation is actually spontaneous on the molecules themselves. 



It is in this stage that one would naturally expect the fog particles of 

 all sizes to produce a medium opaque to transmitted light, as is actually 

 the case provided the fog particles are themselves large in comparison with 

 the wave-length of light. It does not by any means follow, however, that 

 the turbulent phenomenon is complete at the lower edge of the opaque 

 zone. It is much more probable, seeing how gradually the violet drops 

 into the opaque, as the supersaturations increase with increase of steam 

 pressure at a given temperature of the inflowing air, that the number of 

 nuclei continually increases. As this goes on (i. e., in a vertical march 

 through the opaque zone) the time must arrive when the fog particles 

 become small in comparison with the wave-length of light; for although 

 the influx of steam has increased, the air influx increases in the same 

 proportion. Under these circumstances, even though fog particles of 

 widely different sizes are the rule, the Rayleigh effect of scattering is to 

 be looked for. 



In fact, the yellows of the first order emerge from the opaque as mag- 

 nificently saturated orange-browns and thereafter gradually become 

 yellower. Deep crimson at the edge of the opaque does not appear. To 

 prove that the passage from blue to yellow through opaque is due to an 

 increase of nucleation, it is merely necessary to add phosphorus nuclei, 

 when axial violet appears in the steam tube at a sufficiently high tem- 

 perature. Without further change the bluish tone at once passes to 

 yellow. Again, the intense oranges above the opaque may be obtained 

 at once by introducing the fumes of the intensely nucleating sulphur 

 flame into the steam tube. Finally, the probability of the Rayleigh effect 

 is increased by the fact that colors of smaller wave-length than orange 

 and yellow never occur above the opaque zone. 



66. Summary. The present chapter has made use of the method of 

 testing the presence and number of relatively large nuclei by the 

 depression produced in the terminal asymptote obtained when nucleation 

 varies with the drop in pressure, or of the diminution in aperture of the 

 terminal corona corresponding in a given apparatus to dust-free non- 

 energized air. It was thus easily possible to trace the growth in number 



