124 • THE ROYAL SOCIETY OF CANADA 



number of the substance of which the plate is made. The following 

 figures illustrate the increase: Aluminium, 9 .7; copper, 70; tin, 100; 

 lead, 266. A high coefificient of scattering means that the /3-particle 

 is subjected to many more collisions and consequently its path inside 

 the absorber is composed of many short zigzag paths. The total 

 path or sum of all these separate short paths within the absorber is 

 what is meant by the actual range, where as the efïective range is 

 the perpendicular distance from one face to the other. If the actual 

 range could be accurately estimated on the basis of the coefificient 

 of scattering, it seems certain that it would be found to increase as the 

 atomic number increases. Indeed, by means of a special experiment 

 W. H. Bragg, (Phil. Mag. 1910) has shown that this is the case. 



The writer desires to express her thanks to Dr. J. A. Gray for his 

 continuous help and valuable suggestions. 



Summary 



1. Experimental evidence is given to prove that when /3-rays are 

 scattered through large angles the loss of energy observed is not more 

 than about 10 per cent. 



2. Reasons are given for believing that the actual loss of energy 

 is so much less than 10 per cent, that to a first approximation it may 

 be said that there is no loss of energy due to scattering. 



3. The effective ranges of /3-rays in carbon, aluminium, copper 

 tin, lead and mixed foil are given. 



4. The distinction is drawn' between "effective" and "actual" 

 range and evidence is given to support the statement that whereas 

 the effective range decreases with increase of atomic number, the 

 actual range increases with increase of atomic number. 



