32 VAPOR NUCLEI AND IONS. 



SUMMARY OF THE RESULTS OF THE CHAPTER. 



30. Distribution of ions within the fog chamber. Within the fog 

 chamber, the coronas due to the ions produced either by radium or 

 by the X-rays acting axially from long distances, as a rule vary in 

 aperture from one end of the long vessel to the other. Hence the 

 distribution of nuclei is not uniform in the direction of the axis of the 

 fog chamber. The reason for this is to be ascribed to secondary radia- 

 tion. As interpreted from the nucleation produced within the fog 

 chamber containing dust-free moist air, dense bodies (brass cap) produce 

 more secondary radiation than rare bodies (glass end) under like con- 

 ditions of primary radiation. 



If the primary radiation is relatively intense, rare bodies like glass 

 may produce more secondary radiation than dense bodies (like the 

 metals) under less intense primary radiation ; or at least the decrement 

 of primary radiation with the distance from the source predominates 

 within the fog chamber. 



The secondary radiation is rapidly absorbed by the air, as the dimin- 

 ishing coronas testify. 



31. Minimum of efficient nucleation. When ions and colloidal nuclei 

 are in presence of each other (or in general any two groups of nuclei of 

 different average size), the number of efficient nuclei for the case of a 

 drop of pressure not too small passes through a well-defined minimum, 

 if the number of ions continually increases from zero. This occurs, for 

 instance, when a radium tube is moved from a long distance quite up to 

 the fog chamber. The ions abstract more and more of the available 

 moisture, until the colloidal nuclei are practically inactive (minimum), 

 after which the nucleation increases again with the number of ions. 



32. Persistent nuclei. For the moderately intense X-ray bulb (5- 

 inch spark) and a glass fog chamber several millimeters thick, persistent 

 nuclei may be demonstrably produced when the distance from bulb to 

 chamber increases to about 50 cm. They may even be produced through 

 thin tin plate. The decrease with distance of the number generated 

 within a given time is very rapid, and the number producible if the 

 anticathode could touch the fog chamber would be enormous. In like 

 manner the number increases at an accelerated rate with time of expos- 

 ure as if the nuclei were themselves active. 



If we regard the ionized state of a gas as characterized by a kind of 

 kinetic ionization pressure, we may further conceive the ionized gas to 



