298 Barus — Groups of Efficient Nuclei in Dust-Free Air. 



the end (bottom) toward the tube nearly 1 cm. thick, 7- rays 

 only will penetrate into the inside apart from the secondary 

 radiation there produced. In figure SF is the cylindrical fog- 

 chamber, R the radium tube at an axial distance D from the 

 nearer end. In addition to this the radium was also tested at 

 T (top) in the figure, where it is nearest the body of dust-free 

 air under experiment. 



Within the fog-chamber the coronas are everywhere normal 

 and of the same size, in spite of the axial length of 45 cm. 

 available. This is a singular result when contrasted with the 

 marked positional effect observed for the case of radium placed 

 at the different distances D outside of the chamber. The 

 data investigated are shown in the curve (fig. 2), where the 

 abscissas are the distances D and the ordinates the number of 

 efficient nuclei per cm 3 . 





It follows from the graph that as the radium is brought in 

 an axial direction from 00 to the end of the fog-chamber, the 

 number of efficient nuclei in the dust-free air contained is 

 gradually but enormously reduced to a minimum for D = 25 cm. 

 (about), after which the number again increases to the maxi- 

 mum at I) = 0. Curiously enough, if the radium is further 

 approached to the body of the air by being placed at T, the 

 number of nuclei does not increase ; in some observations it 

 even diminishes. 



If the radium is enclosed in a long thick lead tube (60 cm. long, 

 walls # 5 cm. thick), the nucleation is but moderately reduced 

 (see fig. 2, crosses), showing that 7-rays are in question. 



4. Cause of the minimum. — This is easity explained since 

 the ions are relatively large bodies and relatively few in num- 

 ber as compared with the nuclei of dust-free air for the same 

 8p. Hence the ions virtually capture the moisture more and 

 more fully as their number, with diminishing distance D, 

 becomes greater. At D — 25 cm. probably the whole of the 



