238 Prof. J. A. McClelland on 



thick ; another set of observations was taken with the end 

 of the tnbe T covered with a sheet of aluminium '3 mm. 

 thick in addition to the tinfoil. No difference in the order 

 of the substances took place, but if we again express the 

 secondary radiation from lead as 100, the corresponding 

 number for each substance is less than before when the rays 

 passed through tinfoil only; the relative diminution is different 

 for different substances, and is particularly great for ebonite, 

 cardboard, and paraffin. In other words, of all the substances 

 examined the secondary rays from lead are the most pene- 

 trating, those from paraffin, cardboard, and ebonite the 

 least penetrating. The difference in penetrating power 

 of the rays from the different substances is not very 

 marked. 



A rather rough experiment was tried to see if the tempera- 

 ture of the substance had any effect on its power of giving 

 off this secondary radiation. A plate of copper was used, 

 and its temperature increased by a strong bunsen-flame 

 playing on the back of the plate ; no change in the amount 

 of secondary radiation was noticed. It should also be stated 

 that no particular care was taken with the preparation of the 

 surface of the different plates used ; the surface was always 

 well-cleaned, but the degree of polish was not attended to 

 and did not seem to be of any importance. 



As all numbers in this paper are given in arbitrary units, it 

 may be useful to state a result specifically to give an idea of 

 the absolute magnitude of the radiation we are dealing with ; 

 50 milligrams of radium bromide are placed 26 cms. from a 

 lead plate as in fig. 1, the rays from the radium passing 

 through a hole 1*2 cms. in diameter in a lead screen ; the 

 lead plate is 9 cm. from the ionization-tube T ; the electro- 

 meter gives a deflexion of 2000 scale-divisions per volt, and 

 the total capacity of the system charged is '001 microfarad ; 

 the secondary radiation then produces a deflexion of 115 

 scale-divisions per minute. 



Relative Importance of Secondary ft rays. 

 Penetrating Power of Secondary ft rays. 



In making some experiments on the penetrating power of 

 the secondary ft rays, and comparing it with that of the 

 primary rays, some results were obtained which show that 

 very different values may be deduced for the absorption of 

 ft rays according to the apparatus used, and which also show 

 very forcibly the importance of secondary effects when 

 dealing with this type of radiation. Suppose we place the 

 radium so that a pencil of ft (and y) rays falls directly on 



