EFFICIENCY OF FOG CHAMBER. 



21 



X-ray bulb, at a distance of about 12 cm. between the anticathode and 

 the glass wall (0.3 cm. thick) of the fog chamber, are in question. The 

 earthed iron plate was placed between bulb and chamber, resting on the 

 latter. A lapse of time of 1 minute was allowed after the radiation had 

 been cut off. This suffices to show that nuclei of the persistent kind 

 have been entrapped. Naturally the pressure difference used precip- 

 itated no appreciable fog in the absence of radiation. 



These results show conclusively that persistent nuclei may be pro- 

 duced through earthed tin plate, provided the thickness used is not 

 excessive. 



Table 9. Persistent nuclei through earthed tinned iron plate, 0.03 cm. Sp-=2scm., 

 practically below fog limit. Exposure to X-rays, 2 minutes; lapse, 1 minute; obser- 

 vation during exposure. 



>c)p = 24.5; corona too faint for measurement, but definitely present. 



16. Discussion. The simplest of the graphs are those of figure 18, 

 showing the decrease of persistent nuclei as the bulb is gradually removed 

 (D increasing and measured from the side of the chamber) from the 

 apparatus. This decrease is very rapid and implies that if the anti- 

 cathode were actually in contact with the glass walls the production 

 of persistent nuclei in 2 minutes would be enormous. The distortion 

 of the coronas resulting under these circumstances, as detailed in the 

 last memoir, is not, therefore, surprising. It should be remembered that 

 the rays have to penetrate more than 2 mm. of glass. On the whole 

 the action is very much like what C. T. R. Wilson describes for ultra- 

 violet light. 



As in the earlier case, there is secondary generation; i.e., the number of 

 nuclei is larger if the observations are made at 1 or 2 minutes or more 

 after the radiation has been cut off. 



With the given apparatus, persistent nuclei were still produced for a 

 distance of 0.5 meter between the X-ray bulb and fog chamber, though 

 the coronas in that case were but just discernible and vanished too 

 rapidly for measurement. The corresponding intensity of ionization 



