SIZE OF FLEETING NUCLEI. 67 



curves, i. c., the increment of nucleation per centimeter of pressure 

 difference , increases very rapidly. It makes little difference how 

 the ionization is produced, whether by instantaneous exposure to the 

 X-rays or to other radiation. In fact, the curves become nearly ver- 

 tical in relative steepness. Additional experiments are nevertheless 

 needed, and table 39 contains such results. In the first part radium 

 (io,oooX) in the thin aluminum tube is the energizer; in the second 

 part, the X-rays perform a similar function more strongly. The results 

 may be summarized in curves 63, 64, 65, 66, and as follows: 



Both series are in general agreement and indicate the extremely slow 

 depression of the fog limit (indicating the nuclei of maximum size) 

 with the increase of number, 8Nj8(8p) (fig. 65). In fact, the method 

 for measuring 8p is rather too crude to bring out the fog limits prop- 

 erly. They may be found from two consecutive measurements of A'" 

 for two values of 8p close together and near the fog limit. Figures 

 65 and 66 contain a comparison between the fog limits, &p , and 

 8NjS(&p), as well as N for 8p = 2i cm., below the fog limit of dust-free 

 air. They show the accentuated variation of the nucleations ; but 

 they announce also the decided variation of the fog limits for the case 

 of fleeting nuclei, i. c., that the maximum size of nucleus is a variable 

 quantity among the gradations. 



The last part of the table shows data for the persistent nuclei 

 (curves 67). These were produced here by placing the bulb above 

 one end of the cylindrical fog chamber, for the glass at the bottom 

 was too thick to admit a sufficiently intense radiation. The inevitable 

 difficulty in the investigation of these results is the weakening of the 

 X-ray bulb in continued use. In a measure, this is a proof that elec- 

 trical resonance or any more direct induction is not operative, for 

 these effects would not vary with the past history of the bulb. Never- 

 theless, to avoid the possibility of such disturbances an earthed sheet of 

 aluminum was adjusted to cover the top of the fog chamber. Experi- 

 ments showed that all these precautions are unessential. 



The curves (67) show the rapid decrease of the number of persistent 

 nuclei with the height of the bulb (measured from the outside) above 

 the cylindrical glass chamber (walls 0.3 cm. thick). They also show 

 the extremely rapid increase of the nucleation with the time of expos- 

 ure, suggesting radio-activity on the part of the nuclei as stated in 

 section 56. 



In table 41 the endeavor was made to determine the fog limit from 

 two observations of A^ lying slightly above it. The work was very 

 carefully done, but the data nevertheless fail to mark out definite loci. 

 Beginning with the coronas for nouenergized air* (radium at infinity), 



* The curve for air in case of high pressure differences is very difficult to deter- 

 mine, and will again be treated elsewhere. 



