12 ROYAL SOCIETY OF CANADA 



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 particles is thus .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 knoT\Ti. 



Range of Ionization and Photograpliic Action in Air. 



The abrupt falling off of the photographic impression on the plate, 

 after the rays had passed through 12 layers of aluminium foil, suggested 

 that it m.ight be directly connected with the corresponding abrupt falling 

 off of the ionization in air, so clearly brought out by Bragg. This was 

 found to };e the case. It will be shown in the next section that the 

 amount of absorption of the « rays in each thickness of foil corre- 

 sponded to 0.54 cms. of air. Twelve layers of foil thus corresponded 

 to 6.5 cms. of air. i^ow, Bragg has shoAvn that the «' 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 <x 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 important 

 result, and, as W3 shall S3e later, suggests that the action of the photo- 

 graphic plate is due to the ionization of the photographic salts. 



Assuming that the absorption of the o: rays in aluminium is 

 directly pro])ortional to its density compared with air, 13 layers of 

 aluminium, each of thickness of .00031 cm., corresponds to 8 cms. of 

 air. The equivalent distance in air found by experiment is 6.5 cms., 

 so that the aljsorption in aluminium is somewhat less than the density 

 law would lead us to expect. 



Range of Phosphorescent Action in Air. 



Some experiments were also made to see whether the action of the 

 a rays in produciri'^; luminosity in substances like zinc sulphide, barium 

 platinocyanide, and willemite, ceased at the same aista.nce as the 

 ionization property. 



A very active wire was taken and placed on a movable plate, the 

 distance of which, from a fixed screen of phosphorescent substance, 

 could be quickly varied. The distance at which the phosphorescent 

 action ceased could be fairly accurately determined. Different thick- 

 nesses of aluminium foil were then placed over the active wire, 'and 



