Secondary Radiation. 237 



for example, produces less radiation than lead, and tin 

 more than copper. 



It gives a more interesting result to compare the secondary 

 radiation with the atomic weight of the substance: the atomic 

 weight and the ratio of the secondary radiation to the atomic 

 weight i> given in the table. It will be seen that throughout 

 the table the greater the atomic weight the greater is the 

 secondary radiation. The ratio is not constant, but it will be 

 noticed that the substances tall into groups with respect to 

 this ratio. There is a well-marked group tor which the 

 ratio is unity, and another for which the ratio is one-half. 

 Of course, the absolute value of the ratio is merely a question 

 of the scale in which the secondary radiation is expressed. 



It is intended to largely extend the list of substances 

 examined, to see if the dependence of the radiation on the 

 atomic weight holds throughout, and to follow out the apparent 

 grouping. The radioactive substances, radium, thorium, and 

 uranium, are all substances of high atomic weight, and the 

 results now before us indicate that the greater the atomic 

 weight, the greater is the secondary radiation under the action 

 of /3 rays. 



The secondary radiation seems therefore to be closely allied 

 to the spontaneous radiation from radioactive substances. 

 The impact of the ft rays produces a disturbance sufficient to 

 cause the disintegration of the atom in substances which in 

 the normal state are in stable equilibrium. 



The dependence of the secondary radiation on the atomic 

 weight and the close relation between it and radioactivity, 

 suggests that the activity of radium might be increased by 

 the action of its own radiations ; and Yoller {Pliysihalische 

 Zeitschrift, Dec. 1, 1904) has obtained indications of such an 

 action. We must remember, however, that the secondary 

 radiation we are considering is one of ft particles, so that in 

 extending our table to radium we should consider a trans- 

 formation product of radium which emits ft particles. 



It is interesting to compare the table given above with the 

 results of observers who have studied the apparent small 

 radioactivity of ordinary substances, when there is no arti- 

 ficially applied exciting cause. McLennan found that lead 

 was about twice as active as zinc, while tin gave an inter- 

 mediate value ; this is the same order as in our table of 

 substances. A comparison with numbers given by Strutt for 

 the activity of a number of substances does not, however, 

 show much agreement. 



In the above comparison of different substances, the 

 secondary rays passed through a -ingle sheet of tinfoil, *013 mm. 



