172 



Prof. E. Rutherford on some 



The following table shows the maximum ranges of the 

 u particle for the various products of radium, experimentally 

 observed by Bragg. The corresponding initial velocity of 

 projection is also shown. 



Product. 



Maximum range of 

 a particles in air. 



Velocity of 

 a particles. 



Radium 



3 cms. 



3-8 or 4-4 cms. 

 4*4 or 3 - 8 cms. 

 6"7 cms. 



•82 Vo 



•87 or -90 V 

 •90 or -87 Vo 

 1-00 V 



Emanation 



Radium A 



Radium C 



■ 



It is difficult to determine from the experiments whether 

 the range 3'8 cms. belongs to the rays from the emanation 

 or radium A. The mean velocity of the four sets of a par- 

 ticles is *90 V . The velocity of the a particles from 

 the different products thus does not vary more than 10 per 

 cent, from the mean value. 



The velocity of the a particles from the products of the 

 other radio-elements can be determined in a similar way when 

 their range in air is known. 



Range of Ionization and Photographic Action in Air. 



The abrupt falling off of the photographic impression on 

 the plate, after the rays had passed through 1 2 layers of 

 aluminium-foil, suggested that it might be directly connected 

 with the corresponding abrupt falling off of the ionization in 

 air, so clearly brought out by Bragg. This was found to be 

 the case. It will be shown in the next section that the 

 amount of absorption of the a rays in each thickness of foil 

 corresponded to 0'51 cm. of air. Twelve layers of foil thus 

 corresponded to 6*5 cms. of air. Now Bragg has shown that 

 the a rays from radium C ionize the air for a distance 

 6*7 cms., and that the ionization then falls off' very rapidly. 

 We may then conclude that the a rays cease to produce an 

 effect on the photographic plate at. the same velocity as that 

 at which they cease to ionize the gas. This is a very im- 

 portant result, and, as we shall see later, suggests that the 

 action of the photographic plate is due to the ionization of 

 the photographic salts. 



Assuming that the absorption of the a rays in aluminium 



