December 14, 1917] 



SCIENCE 



591 



tube shoiild be earth connected. When the 

 bulb or spark gap is 2 or 3 meters from the 

 electroscope and the air velocity is diminished, 

 a considerable time will elapse before any of 

 the ions can reach the electroscope and these 

 will be but a small percentage of the number 

 originally present. 



As the first ions arriving are swept into the 

 chamber of the electroscope the leaf begins to 

 move and its rate of fall increases and finally 

 reaches a constant value which is maintained 

 until a short time after the X-rays (or spark) 

 is stopped, following which the rate of leak 

 slowly reduces to zero. The apparent slow- 

 ness of the leaf in starting and stopping is 

 largely due to the effect of friction between 

 the air and the inner surface of the tube. 

 This appreciably diminishes the velocity of 

 the air in that region, so that on starting, ions 

 passing through the central portion of the 

 tube arrive first. After the rays are stopped, 

 ions near the surface trail along behind, grad- 

 ually decreasing in number as recombination 

 and diffusion proceed. The efl^ect will of 

 course vary with the length, diameter and 

 material of the tube and the velocity of the 

 air. It will later be shown that this irregular 

 distribution of ions in the tube may affect 

 the value obtained for the recombination con- 

 stant. For high velocity and a short length 

 of tube the leaf starts at once with a uniform 

 rate of deflection and stops abruptly. Using 

 a spark gap 2 meters from the electroscope 

 and a slow air current, a relatively large rate 

 of leak was observed after 35 seconds had 

 elapsed between the stoppage of the spark and 

 the arrival of the first ions in the chamber. 



The rapidity with which gaseous ions dif- 

 fuse may be- well illustrated by inserting a 

 compact bundle of tiny, thin-walled metal 

 tubes inside the tube near the slit. These 

 should be soldered together and make good 

 contact with the inner surface of the tube. 

 Diffusion takes place so rapidly, as the ions 

 pass through the tubes, that with the same air 

 velocity and ionizing source, the number of 

 ions reaching the electroscope is enormously 

 diminished. 



The effect of water vapor or dust particles 



in increasing the ionization, where otherwise 

 the conditions of experiment remain un- 

 changed, is easily demonstrated. 



PART n 



Experimental Proof of the Law of Becom- 

 hination 

 Rutherford has shown that the rate of re- 

 combination, at a given instant, of the iona 

 produced in gases exposed to X-rays^ and the 

 radiation from uranium^ is proportional to 

 the square of the number present at that in- 

 stant, from which it follows that 



1 1 

 n N 

 where N and n are the number of ions present 

 in the gas at the beginning and end of time 

 tj respectively. This law has also been verified 

 for gases exposed to X-rays by McClung^ also 

 by McClelland^ using arcs and flames as the 

 ionizing agents. 



The method most generally employed when 

 large quantities of the gas are available has 

 been to pass the ionized gas through an 

 earthed metal tube with constant velocity and 

 measure the saturation currents at different 

 points along the tube by means of an electrom- 

 eter. A gas meter was used to measure the 

 velocity through the tube as already intimated. 



The deflection of the electrometer indicates 

 the number of ions in a certain portion of 

 the tube at a given instant. The fall of the 

 gold leaf of an electroscope is, however, an in- 

 tegrating process lilte that of the gas meter 

 and continues over a considerable time for 

 each reading. 



If the ionizing agent or the velocity of the 

 ions themselves should undergo slight changes, 

 the rate of fall of the gold leaf would give a 

 good indication of the average number of ions 

 passing at a given time. The sensibility of 

 the electroscope will also remain fairly con- 

 stant over long intervals and is readily tested. 



In the course of some work involving the 

 use of X-rays and 7-rays from radium salt, it 



1 Kutherf ord, Phil. Mag., V., 44, p. 422, 1897. 



2 Rutherford, Phil. Mag., V., 47, p. 142, 1899. 

 s McClung, Phil. Mag., VI., 3, p. 283, 1902. 



« McClelland, Phil. Mag., V., 46, p. 29, 1898. 



