of the a Particle from Radioactive Substances. 283 



with stand atmospheric pressure. After some trials, Mr. 

 Baumbach succeeded in blowing such fine tubes very uniform 

 in thickness. The thickness of the wall of the tube employed 

 in most of the experiments was less than T J mm., and was 

 equivalent in stopping power of the a particle to about 

 2 cms. of air. Since the ranges of the a particles from the 

 emanation and its products radium A and radium are 4*3, 

 4*8^ and 7 cms. respectively, it is seen that the great 

 majority* of the a particles expelled by the active matter 

 escape through the walls of the tube. The ranges of the 

 a particles after passing through the glass were determined 

 with the aid of a zinc-sulphide screen. Immediately after 

 the introduction of the emanation the phosphorescence showed 

 brilliantly when the screen was close to the tube, but practi- 

 cally disappeared at a distance of 3 cms. After an hour, 

 bright phosphorescence was observable at a distance of 

 5 cms. Such a result is to be expected. The phosphorescence 

 initially observed was due mainly to the a. particles of the 

 emanation and its product radium A (period 3 mins.). In 

 the course of time the amount of radium C, initially zero, 

 gradually increased^ and the a. radiations from it of range 

 7 cms. were able to cause phosphorescence at a greater 

 distance. 



The glass tube A was surrounded by a cylindrical glass 

 tube T, 7*5 cms. long and 1*5 cms. diameter, by means of a 

 ground-glass joint C. A small vacuum-tube V was attached 

 to the upper end of T. The outer glass tube T was exhausted 

 by a pump through the stopcock D, and the exhaustion 

 completed with the aid of the charcoal tube F cooled by 

 liquid air. By means of a mercury column H attached to a 

 reservoir, mercury was forced into the tube T until it reached 

 the bottom of the tube A. 



Part of the ol particles which escaped through the walls of 

 the line tube were stopped by the outer glass tube and part 

 by the mercury surface. If the a. particle is a helium atom, 

 lielium should gradually diffuse from the glass and mercury 

 into the exhausted space, and its presence could then be 

 detected spectroscopically by raising the mercury and com- 

 pressing the gases into the vacuum-tube. 



In order to avoid any possible contamination of the 

 apparatus with helium, freshly distilled mercury and entirely 

 new glass apparatus were used. Before introducing the 

 emanation into A, the absence of helium was confirmed 



* The a particles fired at a very oblique angle to the tube would be 

 stopped iu the glass. The frnction'stopped iu this way would be small 

 under the experimental conditions. 



