26 



VAPOR NUCLEI AND IONS. 



19. The same, continued. The other characteristic, which was 

 entirely unforeseen, is the relative absence in what has been called the 

 ionized region (placed in the former curves, fig. 15, between about dp = 2 1 

 and dp = 26). The contrast is clearly shown in fig. 19. The only ex- 

 planation which suggests itself to me is this, that what were supposed 

 to be ions were really water nuclei due to the somewhat slower exhaus- 

 tion of the preceding experiment. In other words, particles caught at 

 the end of the somewhat less rapid expansion evaporated into the larger 

 particles, leaving water nuclei behind. But apart from this, the definite 

 trend with which the curve reaches the abscissa is characteristic, and, so 

 far as coronas go, there seems to be an absence of nuclei below dp = 2 4. 

 In other words, the fog limit has been raised, in spite of the general lower- 

 ing of the necessary supersaturations throughout the curve. The new 

 curve is almost wholly coronal. 



+ .52 



wPcor 



18 ZO ZZ ZA Z6 28 30 32 31 36 38 40 42 



Fig. 19. Improved fog chamber. Apertures of coronas (s) seen in dust-free air, 

 energized or not, as stated, by radium or X-rays, at different exhaustions (dp). 

 Table 10. 



Usually the same results are obtained in a pressure-increasing and in 

 a pressure-decreasing series. There is an equally striking uniformity 

 in the lapse of time. 



