DISTANCE EFFECTS. 85 



slower and in case of the inclosed bulb (lead box with window) faster 

 than the first power of distance. 



In fig. 40 I have inserted data incidentally obtained in Chapter II, 

 table 1 5 (upper curve) , together with data of the same kind in the earlier 

 reports (lower curve). The former are the steepest curves obtained and 

 the latter the least so. Here, as elsewhere, it is difficult to conjecture 

 a reason for this apparently erratic difference of behavior, unless it be 

 referred to the facility with which secondary radiation is evoked, and to 

 the degree in which the fog chamber is pervious to it or generates it. 

 Since dn/dt = a-bn 2 = o, where a is the number of ions produced per 

 second, a must vary as the square of the number of ions, n, observed. 



58. The same, continued. Small wood fog chamber. The data of the 

 last paragraph with the glass fog chamber suggest a comparison with 

 the wood fog chamber. The latter is much the more pervious and in 

 the earlier work showed a much smaller distance effect. Table 33 con- 

 tains eight series of results. In series I, fig. 41, the march is not unlike 

 the case for the glass chamber; but in series II the insignificant differ- 

 ence between a bulb distance of 50 and 100 cm. from the fog chamber 

 (curves III and IV) are similar to I, and often betray the incidental 

 weakening of the X-ray bulb. In the series VI to VIII, change of the 

 drop of pressure (dp) was introduced, but the inherent difficulty of coping 

 with the bulb variations is seen in the details in fig. 43. 



It is probable that the ordinates of the curves V to VIII (figs. 42 and 

 43) are proportional to each other; but a discussion is beyond my present 

 purpose. The slow order of change with distance should, however, be 

 noticed. 



59. The same, continued. Large wood fog chamber. It is with this 

 apparatus that the coronas of almost the same aperture were obtained 

 in the earlier work, while the X-ray bulb was moved from 1 to 6 meters 

 from the fog chamber. Table 34 and fig. 44, however, show that this 

 result must have been due to other conditions, for there are changes of 

 nucleation here registered amounting in case of the distance specified 

 (1 to 6 meters) to w / /?i = 145/104 at dp = 22; 77/41 at dp = ig. For 

 greater pressure differences, dp = 25 and 29, the occurrence of terminal 

 coronas would interfere with the comparisons. At higher exhaustions 

 still, the efficiency of air nuclei would be gradually restored, so that the 

 observed nucleation may be greater with the bulb at 6 meters than at 

 1 meter from the fog chamber (see Chapter II, figs. 24 and 25). At the 

 very low exhaustion dp = 17 the coronas are too small to be serviceable 

 for comparison. 



