134 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



of exhaustion, approaching but always below the fog limit of non- 

 energized air. They are usually instantaneously generated (within a 

 second) by the radiation, so that their number is definite independent of 

 the time of exposure. They decay in a few seconds after the radiation 

 ceases, z. e. , roughly, to one-half their number in 2 seconds to one-fifth 

 in 20 seconds, in the usual way. (Cf. Chapter III, figs. 42, 43.) I 

 fancy that these nuclei are what most physicists would call ions ; but 

 nevertheless the particles are not of a size, the dimensions depending 

 on the intensity of the penetrating radiation to which they are usually 

 due, and they pass continuously into the persistent nuclei, as shown in 

 the next paragraph, where decay of ionization and of nucleation are 

 very different things. They are abundantly produced by the y-rays, 

 which, though weak ionizers, become from this point of view strong 

 nucleators (section 16). Finally (section 6) they are stable on solution. 

 The case seems rather to be one in which the rate of decay exceeds 

 the rate of production. The following is an example of data bearing 

 on this case, N being the number of nuclei caught per cubic centimeter 

 of the dust-free air at normal pressure. The anticathode is at a dis- 

 tance from the fog chamber and the exhaustion carried to the verge 

 of the fog limit of dust-free air. 



Time of exposure (rays on) ........ o 5 15 30 60 120 sees. 



JV_X io- 3 ....................... *i.6 74 74 74 



Time after exposure (rays off), . . . o 5 15 30 60 120 sees. 



, ....................... 92 30 23 18 io 4 



The two series refer respectively to generation and to decay. f 

 If ./Vis expressed in thousands of nuclei per cubic centimeter, and 

 time, /, reckoned in seconds, and if i\N=-a-\-bt, so the dN\dt bN z , 

 the datum =0.002 reproduces the results satisfactorily, while a simple 

 exponential law will not do so. The decay is of the kind characteriz- 

 ing mutual action between ions. In figures 42, 44, Chapter III, com- 

 puted data are distinguished by crosses. 



92. Fog limits Of fleeting nuclei. The mean increment of nucleation, 

 8 N, per centimeter of increment of pressure difference, 8/>, varies 

 very rapidly as compared with the fog limit, 8p, so long as the refer- 

 ence is to the interval of large variation. It makes no difference by 

 what radiation the nucleation is produced (X-rays or other radiation), 

 8 NJ8 (8/>) is rapidly larger as the intensity of ionization is greater, 

 while 8p Q becomse slowly smaller within the interval in question 

 (Chapter III, figs. 49, 63 to 68). Below the fog limit of air and 



* Fog limit of dust-free air just exceeded. 



t Including loss by diffusion or other time loss. 



