146 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



If measurement be made from trie line of sight (10 cm. from the ends 

 of the fog chamber), the nucleation decreases less rapidly than the first 

 power of distance. Hence, whereas the distance effect in case of 

 X-rays is small, it is very large in case of radium. On the other hand, 

 rays from radium show remarkable nucleating power after penetrating 

 many centimeters of lead, whereas the nucleating power of the X-rays 

 after such penetration is relatively negligible. (Figs. 69 and 78, 89 

 and 90, Chapter III.) 



105. Distribution of nucleation within the fog chamber Radium. 



Finally, when the rays have once entered the fog chamber, the nuclea- 

 tion along the axis seems nearly uniform. Measurements are diffi- 

 cult ; but while the nucleation decreases nearly to one-fourth when 

 the radium is placed on the outside of the fog chamber, 40 cm. axially 

 from the end, the coronas along 40 cm. within the fog chamber are 

 nearly of the same aperture for any given position of the radium tube. 



106. Distribution of nucleation within the fog chamber X-rays.- 



Obviously when the X-ray bulb is at a distance from the fog chamber 

 and the nuclei fleeting, they will be uniformly distributed within the 

 chamber, being everywhere at saturation density for the given inten- 

 sity of radiation. 



The conditions are far different, however, when the bulb, as in figure 

 i , Chapter I, is near the chamber and the nuclei persistent. In such a 

 case, if we distinguish between the A and the B sides of the fog chamber 

 (where A is nearer the bulb), and if we use a pressure difference, Sp, 

 decidedly below the fog limit, 8p , of dust-free air, the nuclei within the 

 given range of condensation are for short times of exposure found on 

 the A side only. The coronas are relatively small in size, roundish, 

 decreasing in aperture to a vanishing angular radius from the bulb end 

 of the chamber toward the middle. Beyond this, on the right, nuclei 

 are too small to respond to the given pressure difference, Sp, and the B 

 side remains clear on exhaustion . As the time of exposure to the X-radia- 

 tion is increased from i to 10 minutes, the nucleation of the A side 

 becomes denser, coarser, and nonuniform in distribution, vertically as 

 well as horizontally, while the efficient nuclei are found in continually 

 increasing numbers, and at greater distances on the B side, until they 

 eventually occur throughout the chamber. The growth of the coronas 

 seen on the first exhaustion after successively increasing times of 

 exposure show a characteristic sequence of types (figs. 2-6, Chapter I), 

 as they pass (when seen through plate-glass apparatus) from roundish 

 to oval, spindle-shaped, gourd-shaped with a long serpentine neck, 



