DISTANCE EFFECT. 9 



7. Distance effects of radium. To throw further light on present and 

 preceding questions at issue, I shall next anticipate some of the work 

 below and describe certain experiments specially adapted for the pur- 

 pose. By "dust-free air" I mean atmospheric air filtered with extreme 

 slowness (through large wide filters of packed cotton) and thereafter 

 left without interference for two or more hours. Such air shows a high 

 fog limit. In the fog chamber used the condensation began at an 

 observed pressure difference of about dp = 26 cm.; rain-like condensation 

 at dp 21 cm. 



In the present experiments all tests are made at dp = 41.5 cm., at 

 a pressure difference therefore much above the fog limit, and probably 

 approaching the condensing power of the apparatus. The number of 

 nuclei computed from the coronas observed is an estimate merely, as 

 the constants needed for the very large range of variation in question 

 are not available. Nevertheless if the same dp is used throughout, the 

 nucleations obtained are immediately comparable. With these reser- 

 vations the number of nuclei found in the dust-free air and at the dp 

 in question is about N = 380 X io 3 to 400 X io 3 , per cubic centi- 

 meter. It is obvious, moreover, that these nuclei are excessively small, 

 much smaller than ions, smaller even than those which would respond 

 to smaller dp's exceeding dp = 26 cm. 



Let the fog chamber (fig. 1) be subjected to the radiations from weak 

 radium (io,oooX, 10 mg.) contained in a thin hermetically sealed 

 aluminum tube. As the walls of the fog chamber are 0.1 to 0.3 cm. thick 

 and the end (bottom) toward the tube nearly 1 cm. thick, y-rays only 

 will penetrate into the inside, apart from the secondary radiation there 

 produced. As shown in fig. 1 (cylindrical fog chamber) the radium 

 tube R is 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. 



The data investigated are shown in table 5 and in the curve (fig. 6), 

 where the abscissas are the distances D and the ordinates the number 

 of efficient nuclei per cubic centimeter. 



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

 direction from 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 = 2$ cm. (about), after which the number again 

 increases to the maximum at D = 0. Curiously enough, when the radium 

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

 the mean number of nuclei did not increase.* 



*This result is to be further interpreted in special experiments below. 



