EFFECT OF RADIUM. 85 



NUCLEATION DUE TO GAMMA RAYS. 



67. Lead-cased wooden fog: chamber Penetration. in order to in- 

 terpret the above data for X-rays, it will first be necessary to deter- 

 mine the facility with which nuclei are produced by very penetrating 

 radiation. The radiation of radium filtered through lead walls about 

 i cm. thick was therefore tested. Table 54 gives a series of results in 

 which the radium (io,oooX) was first tested when hermetically sealed 

 in a thin aluminum tube and placed 6 to 10 cm. from the line of sight 

 (curve 87). In this case the radiator nearly touched the free end of 

 the lead-cased fog chamber A (fig. 43). The aluminum tube was then 

 successively enveloped in one or more lead tubes, T, with wall 0.5 

 cm. in thickness. The length of the tubes exceeded the width of the 

 fog chamber, and they were placed with their axes parallel to the 

 plane of the end, so that any radiation entering would have to pass 

 through the lead ; or, passing out of the lead tube, enter the fog cham- 

 ber laterally under very unfavorable conditions. Leaving the latter 

 case (which is here negligible) for further experiment, table 54 gives 

 the coronal apertures s lt s 2 , s s , etc., and nucleations NI, N 2 , N- d , etc., 

 found in successive exhaustions under the conditions stated. The 

 figures show that periodicity is a frequent and unavoidable occurrence. 

 Many exhaustions were therefore made in each case and the means 

 taken in triads. These are given in detail in the summary at the end 

 of the table, and in the curve (87). Sometimes the particular adjust- 

 ment of the tube (as, for instance, the position of the radium in the 

 tube) seems to be of importance, for the results in any given position 

 are fairly uniform. An additional lead plate is ineffective. The 

 summary shows that of the radiation which escapes from the alumi- 

 num tube, 85 per cent passes through 0.5 cm. of lead and 70 per cent 

 through i cm. of lead, assuming that there is no secondary radiation. 

 In one case (tube capped by a lead plate) nothing at all seems to enter 

 the fog chamber. This suggested the following group of experiments, 

 which show that zero nucleation may occur periodically under any 

 conditions. 



68. Continuation. The new results (table 54, part IV) show a curious 

 irregularity, which is borne out by the behavior of radium when 

 placed in the fog chamber (Chapter II, section 31). In the present 

 case the tube was 60 cm. long (similar to P, fig. 44), parallel to the 

 plane of the end (about 20 cm. across) of the fog chamber and placed 

 close to it. The data for the open and closed tube are about the 

 same. In both cases the values of N at times descend to the low 

 nucleations of nonenergized air, though as a whole they lie pro- 



