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XXXIX. On the a. Particles of Radium, and their Loss of 

 Range in passing through various Atoms and Molecules. By 

 W. H. Bragg, M.A., Elder Professor of Mathematics and 

 Physics in the University of Adelaide, and R. Kleeman, 

 P.Sc, Demonstrator *. 



IX previous papers which we have contributed to this 

 Magazine f we have shown that the a particle moves 

 always in a rectilinear course, spending its energy as it 

 traverses atoms of matter, until its velocity becomes so small 

 that it cannot ionize and there is in consequence no further 

 evidence of its motion. Each a. particle possesses therefore 

 a definite range in a given medium, the length of which 

 depends on the initial velocity of the particle and the nature 

 of the medium. Moreover, the a particles of radium which is 

 in radioactive equilibrium can be divided into four groups, 

 each group being produced by one of the first four radio- 

 active changes in which a particles are emitted. All the 

 particles of any one group have the same range and the same 

 initial velocity. 



The present paper may be regarded as a continuation of 

 the papers cited. Its contents are arranged under the 

 following heads : — 



1. Improvements in the apparatus used for measuring the 

 ranges and relative strength of the four groups of rays. 



2. Results of experiments with the new apparatus giving 

 the following values of the ranges in air at a pressure of 

 76 cm. and a temperature of 20° 0. 



Radium 3*50 cm. 



Emanation or") ,,no 



Radium A. J 



Radium A or | . .., 



Emanation. j 



Radium C 7-06 



These are correct, we believe, to '05 cm. The pressure 

 and temperature are stated, since a change of 1 cm. in the 

 pressure or 5 C C. in the temperature would produce an effect 

 which could be observed. We have not yet determined 

 which of the two middle groups belongs to the Emanation 

 and which to Ra A ; but we have constructed a special piece 

 of apparatus which will, we hope, settle the point. 



* Communicated by the Authors. Read before the Royal Society of 

 South Australia, June 6, 1904. 

 f Phil. Mag. Dec. 1904. 



