48 CONDENSATION OF VAPOR AS INDUCED BY NUCLEI AND IONS. 



and may be due to turbulent motion immediately after the exhaustion. 

 At higher exhaustions the distribution is often more uniform and other 

 exceptional conditions supervene. For instance, with radium at d = 8o 

 cm. from the exhaustion end and dp/p = 0.43, nX io~ 3 = 120, 150, 140 at 

 D= 20, 50, 80 cm., respectively; with a tube containing six radium tube- 

 lets evenly distributed and extending from ^ = 65 to no cm., the corre- 

 sponding data at dp/p = o.^^, were Xio~ 3 =i55, 185, 240. In no case 

 was more than a single maximum encountered. 



Finally, the assumption has been made tacitly that the ions may be 

 removed faster during the exhaustion and displacement than they can 

 be reproduced by the presence of the radium, and that they may be 

 caught before they appreciably decay. To take the example of the above 

 cases: if the nucleation is equal to N = 500,000 per cubic centimeter, the 

 production would be a = b N 2 = io~ 6 X 25 X io 10 = 25 X io 4 or 2 50,000 nuclei 

 per second per cubic centimeter, nearly. If the interval of displacement 

 is between o.oi second and o.i second, 2,500 to 25,000 nuclei should be 

 produced to replace the 250,000 removed, which would at once imply a 

 striking difference in the size of the coronas near the radium, such as has 

 actually been observed. 



In conclusion, therefore, the feature of great interest in these experi- 

 ments is the definite proof contained that the ions may be displaced 

 during the exhaustion at a rate much faster than they can be reproduced 

 and that the maximum of ionization is therefore rarely found where it 

 was generated. 



MISCELLANEOUS EXPERIMENTS. 



38. Experiments to detect the region of positive ions. In table 16 

 a series of detailed experiments is recorded to determine the character of 

 the relation between the drop of pressure and the number of ions captured 

 in the fog chamber of the usual shape (No. i, section 27). It was partic- 

 ularly desirable to locate the rather sudden flexure of the curve of dis- 

 tribution between its oblique and its horizontal branches and to see if 

 any evidence could be found of a second region of flexure corresponding 

 to the positive ions, by aid of a fog chamber like the one in question. 

 The method of two independent sources was employed, the coronas being 

 put in contact, 5 denoting the chord on radius of ^ = 250 cm., or if 26 

 is the angular diameter of the coronas, 5=2 R tan 6. It was not thought 

 necessary to reduce 5 to the number of nuclei per cubic centimeter, as the 

 former is in some respects a more convenient datum. There are two 

 independent series (see fig. 16), in the first of which the sealed radium 

 tubelet is on the outside of the chamber, in the other on the inside in the 

 axial aluminum tube. In both cases the flexure of the curve lies at about 

 = o.2%$, notwithstanding the decidedly greater nucleation in series 2. 



