108 C. Barns — Rate of Decay of Nuclei. 



time, t, between the instant the X-radiation is cut off and the 

 instant of the exhaustion made to catch the remaining ions, 

 together with the number of nuclei, n, per cu. cm. caught, as 

 estimated from the apertures of the corresponding coronas. 

 The column r is an estimate of the size of nuclei, obtained 

 from the drop of pressure &p, and b is the rate of decay. In 

 the table, the nucleation discovered in dust-free wet air at any 

 drop of pressure 8p/p, in the absence of X-radiation is always 

 relatively small, even at the highest exhaustions made. Never- 

 theless, the persistence of these nuclei, surviving as much as 

 even ten minutes after exposure, is remarkably large, as if all 

 nuclei (vapor nuclei and ions) were eventually caught together. 

 The results, in fact, often point to an almost indefinite persist- 

 ence. On the other hand, at the lower exhaustion, the number 

 of nuclei soon vanishes. The rates of decay are determined by 

 the usual equation 



A — = bM or dn I dt = — bn 2 

 n ' 



Briefly, therefore, at a sufficiently high drop in pressure, 

 Bp/p = "35, the nuclei produced in the presence of ions in dust- 

 free moist air by the (moderate) X-rays, decay at phenomenally 

 small rates, or are almost indefinitely persistent, whereas larger 

 nuclei decay faster in proportion to their size. 



Similar facts were brought out by all the other tests with 

 coronas. As hp/p decreases, or r the radius of the nucleus 

 increases, b increases at a rapidly accelerated rate, from very 

 small values b = 10" 7 to enormous values. The small coeffi- 

 cient b is less than 1/10 the normal value obtained for ions 

 with the electrometer (of the order of 10~ 6 ), while the large 

 values are nearly fifty times the normal value. 



The interpretation of these results is made difficult by the 

 variability of the X-ray bulb. It is safe to assert, however, 

 that the large ions or nuclei produced by the X-rays in dust- 

 free wet air vanish with relatively enormous rapidity, whereas 

 the very small nuclei are almost indefinitely persistent, and 

 that there is a definite relation between the rate of decay and 

 the size of nucleus. If, therefore, we regard these nuclei as 

 water droplets of different sizes, evaporation is rapid until a 

 limiting diameter depending on the intensity of radiation is 

 reached, after which evaporation nearly ceases. It is also 

 probable, that the limiting diameter increases with the intens- 

 ity of radiation, so that with strong X-rays almost no super- 

 saturation is required. If, therefore, the X-rays produce any 

 chemical body which may go into solution, as has been sup- 

 posed, the greater or less abundance of this body, supplied by 

 greater or less intensity of X-radiation, would account for 

 larger or smaller persistent nuclei. 



