562 



SCIENCE. 



[N. S. Vol. XXI. Xo. 537. 



size, their, dimeusions depending on the in- 

 tensity of the penetrating radiation to 

 which they are usually due ; and they pass 

 continuously into the persistent nuclei as 

 sho^vn in the next paragraph, where decay 

 of ionization and of nueleation are very 

 different things. Finally (§3) 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 

 c\;bic centimeter. The anticathode is at a 

 distance from the fog-chamber and the ex- 

 haustion carried to the verge of the fog- 

 limit of dust-free air. 



Time of exposure : 



(rays on) 5 15 30 60 120 sees. 



JV'XIO-' *1.6 74 74 — 74 — 



Time after exposure : 



(rays off) 5 15 30 60 120 " 



A^X10-= 92 30 23 IS 10 4 



The two series refer respectively to genera- 

 tion and to decay, f 



2. Persistent Nuclei.— It the X-ray bulb 

 is approached nearer the fog-chamber or if 

 a more efficient bulb is used so that the 

 density of the ionization within the fog- 

 chamber is sufficiently increased, the rate 

 of production of nuclei will eventually ex- 

 ceed the rate of decay. The nuclei are now 

 persistent for hours after the radiation 

 ceases. The number N per cubic centi- 

 meter increases in marl^ed degree and at 

 an accelerated rate with the time of ex- 

 posure to the radiation, certainly for ten 

 minutes or more, barring the invariable loss 

 of efficiency of the X-ray bulb. These 

 nuclei are large, requiring very little super- 

 saturation for condensation and are much 

 like any ordinary nuclei. They are pro- 

 nouncedly of all sizes and the initial co- 

 ronas are apt to be distorted and stratified 

 beyond recognition. Whirling rains and 

 fog accompany the first condensation. 

 While small nuclei occur throughout the 



* Fog limit of dust-free air just exceeded. 



t Including loss by diflfusion or other time-loss. 



chamber, the end near the bulb is at first 

 the seat of growth which gradually extends 

 to the other end, as I have shown else- 

 where.* The following two series of data 

 showing the generation and decay of nuclei 

 in question may be cited as illustrations. 

 The pressure difference Sp = 20 cm., much 

 below the fog-limit for dust-free air, in the 

 given apparatus. 



Time of exposure 5 10 20 60 120 180 sees. 



^VxIO-^ 2 11 10 20 t(lOO) t(500) 



Time after exposure .. 36 85 240 minutes. 

 A'XIO-' t(lOO) 36 20 vanishing. 



Hence there is a decay of one half in ten 

 minutes, and to one fifth in eighty minutes, 

 or the degree of persistence is 200-300 

 times larger than in the first paragraph. 

 The data indicate, moreover, that both of 

 these extreme types of nuclei and all inter- 

 mediate types now occur together, as may 

 be tested by changing the pressure differ- 

 ence. Bp, on exhaustion. Intermediate 

 rates of generation and decay may be ob- 

 tained by moving the bulb nearer to or 

 farther from the end of the fog chamber. 

 Finally the rates at which the nuclei and 

 the ionization severally decay, between 

 which it would be difficult to distinguish 

 in the case of the very fleeting nuclei, stand 

 in sharp contrast with the persistence of 

 the nuclei of the present paragraph. 



3. Fleeting Nuclei Become Persistent on 

 Solution. Origin of i?a?n.— Let the fog- 

 chamber be exposed to radiation for a few 

 seconds and thereafter exhausted (8p = 25) 

 as usual. Closing the exhaustion cock and 

 allowing only time enough to measure the 

 first corona, let the influx cock be opened 

 and the fog-chamber be refilled with dust- 

 free air. The (primary) corona observed 

 is thus dispelled before much subsidence of 

 fog-particles can take place, though the 

 rain will naturally drop out. If the fog- 

 chamber is now left without interference 



* American Journ. Sci, XIX., 175. 

 t Computed from the second exhaustion, after 

 subsidence of the dense fogs of the first. 



