December 8, 1905.] 



SCIENCE. 



763 



when the time after the removal of the ra- 

 dium, *. e., the interval of decay, increases 

 indefinitely. 



This minimum, moreover, is very sharp, 

 almost cusp-like, as if one law were passing 

 abruptly into another. Thus below the 

 minimum (t = 13 sec. about) the curve for 

 8p = Sl nearly coincides with the curve for 

 8p = 24, which is practically independent of 

 the colloidal nuclei of air. The decay may 

 be computed to be of the order of that of ions. 

 After a lapse of 13 seconds the effect of col- 

 loidal nuclei is marked for gp = 31 ; and even 

 after a lapse of 60 seconds, when the ions 



lesces" approximately with the other for lapses 

 of time less than i^ ^^ 13 sec. It has its own 

 minimum, however, and from the lower pres- 

 siire difference, necessarily its own asymptote 

 at n= 40,000, since only the coarser order of 

 air nuclei fall within the given limits of con- 

 densation in the apparatus used. For the 

 same reason the minimum is lower and later, 

 seeing that the ions are present throughout 

 in relatively greater numbers, as compared 

 with the efficient colloidal nuclei, than was 

 the case at 8p = 31. 



3. The curves as a whole have so close a 

 resemblance to the data investigated by Pro- 



(lower curve) have vanished to a few hundred, 

 the upper curve is only half way on its march 

 toward the asymptote. This shows the remark- 

 able sensitiveness of the method as a test for 

 the presence of ions or of any nuclei larger 

 than the colloidal sizes. Moreover, measure- 

 ment of the large coronas is relatively easy. 

 Finally, the curve, 8p = 31, if prolonged back- 

 wards, would seem to start nearly from the 

 origin; in such a case one would have to pic- 

 ture to oneself a single coagulated particle 

 breaking to pieces in the absence of radiation, 

 into fragments of continually decreasing size, 

 until the debris ultimately numbers 150,000 

 colloidal nuclei. 



The intermediate curve (8p = 28) also coa- 



fessor Barus for the effect of radium at dif- 

 ferent distances from the fog chamber that the 

 same cause must underlie both series of ob- 

 servations. In the former case (distance ef- 

 fects) any given intensity of ionization be- 

 tween the maximum and the vanishing values 

 may be maintained indefinitely by properly 

 placing the radium tube; in the latter case 

 (decay) all stages are passed through in two 

 or three minutes. Beginning with dust-free 

 non-energized air, the number of efficient 

 nuclei decreases as the number of ions in- 



^ Considered relatively to the wide divergence 

 after i = 13 see. is passed. The coalescence need 

 not be perfect. Small coronas fall out too rapidly 

 for close measurement. 



