50 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



TABLE 29. Fog limit and effect of pressure difference for combined filter and wet- 

 sponge tube. Dust-free air, not energized. 



The data for aperture, s, reach a limit as &p continually increases; 

 but beyond the highest pressure difference applied, s will probably 

 again decrease, ^therefore must continually increase within the 

 limits observed, but beyond them it will also eventually decrease 

 (curves 45 and 46). 



Incidental observations show the relative effect of a cotton filter 

 without the sponge tube (), figs. 45 and 46) and of the Pasteur filter, P. 

 The former is apparently more efficient in its filtrations than the Pas- 

 teur filter, which has the additional disadvantage of being too slow 

 for purposes of the present kind (period of influx prolonged over 10 

 minutes). 



Curiously the fog limit of dust-free air is reduced by the sponge 

 tube, lying in the given apparatus below 8p = 23 crn. for rain and 

 below 8^=24 for cloud. Such precipitations therefore cease when 

 the volume increase on expansion in the two cases is respectively 

 below 1.45 or just below 1.48, data which are naturally larger than 

 C.T. R.Wilson's (Phil. Trans. Royal Soc. 1897, vol. 189, p. 265), in view 

 of the differences of the apparatus employed. The reduced fog limit 

 with the wet-sponge tube may be referable to increased supersatura- 

 tion, but there is probably some specific cause yet to be investigated.* 

 The highest nucleations observed lie within 7V== 25,000, which is very 

 far below the conditions at which axial colors occur. 



The following chart (figs. 45-51) illustrates tables 29, 32, 33, 34, 

 35, 36. "Exp." refers to time of exposure to radiation; "8p," to 

 lapse of time after radiation ceases. D is the distance of the ionizer 

 (X-ray bulb or radium tube) from the nearer end of the fog chamber. 



*It will be shown elsewhere that the (very sfau} rate of filtration is here essentially 

 in question. 



