854 Dr. A. F. Kovarik on the Absorption and 



this was in turn attached to a sheet of mica or aluminium o£ 

 a thickness slightly greater than would be necessary to stop 

 all the a-rays which might be emitted by the active material. 

 This sheet was then placed inside the ionization vessel, active 

 side downward and over the central opening of the cover, to 

 which it was held by means of springs. The absorbing sheets 

 were also placed on the inside and were also held in position 

 by springs. The change of capacity produced by the slight 

 elevation of a large number of absorbing sheets was found to 

 be negligible in the large vessel. The capacity of the instru- 

 ment was about 10 E.S. units. Care was taken to obtain 

 saturation in all the experiments. 



With this arrangement the /3-particles have the radius of 

 the vessel for their path, and all the scattered rays become 

 effective. Since, however, the /3-particles after passing through 

 matter have their velocity slightly decreased*, then, if the 

 ionization changes considerably with the velocity, the oblique 

 rays will be affected more than the normal rays. In addition, 

 with plane absorbing sheets, the oblique rays are more 

 absorbed than the normal rays, and this should result in a 

 slight drop in the initial portion of the absorption curve. 

 Experiments were tried with more or less normal rays, but 

 the absorption curves were nearly identical with those obtained 

 when radiations in all directions were used. 



In some of the experiments, the interior of the vessel was 

 lined w r ith a thick cardboard covered w r ith a conducting 

 paper. The result was a decrease of about 20 per cent, of 

 the ionization due to the fact that multiple reflexion of the 

 /3-particles is less from cardboard than from copper. 



Effect of reflected /3-particles on the absorption coefficient. 



The present form of the apparatus was well adapted for 

 the investigation of the amount of reflexion * of the /3-particles 

 by any substance, and also for the investigation of the effect 

 of the reflected /3-particles f on the absorption coefficient. 

 When air was underneath the thin aluminium foil on which 

 the active matter was deposited, the coefficient of absorption 

 was always found to be smaller than when a reflecting sub- 

 stance was placed under, and it increased with the atomic 



* W. Wilson, loc. cit. 



t The terms " reflexion" and tl reflected /3-particles or rays " are used 

 for convenience only. By " reflected /3-particles " is meant the /3-particles 

 which entering the substance, conveniently called the " reflector," are 

 deflected in their course by collision with the atoms of the substance so 

 that they emerge again as diffusely scattered /3-particles. 



