Section III., 1908. [ IS ] Trans. R. S. C. 



Ill- — On the Radioactivity of Potassium and other Alhali Metals. 

 By Professor J. C. McLennan, and Mr. W, T. Kennedy, B.A. 



(Read May 26, 1908.) 



I. — Introduction. 



In the course of some experiments made by them on the radio- 

 activity of a series of salts which had hitherto been considered inactive, 

 Messrs. Campbell and Wood, (Proc. Camb. Phil. Soc. Vol. XIV, Pt. i, 

 p. 15, 1907), found that potassium salts exhibited a radioactivity 

 greater than that of any other substance previously examined which 

 did not contain any of the so-called radioactive elements. 



In seeking for the source of this activity these experimenters 

 found it impossible to separate out any active impurity from the salts 

 examined, and they were led by the results of their investigation 

 which included measurements on the activities of a limited number 

 of compounds of potassium, to conclude that the activity originated 

 with the potassium itself and was an atomic property of that metal. 



In a later paper, (Proc. Camb. Phil. Soc. Vol. XIV, Part ii, 1907), 

 Campbell described some additional experiments dealing with the 

 character of the radiation emitted by the potassium salts, and in con- 

 cluding expressed the opinion that the radiation consisted of ft rays 

 possessing an average velocity less than that of the (3 rays of uranium. 



In the following paper an account is given of some experiments 

 which involved a minute examination of the radioactivity of a large 

 number of potassium and other salts, and while the results of this 

 examination confirm the discovery made by Campbell and Wood that 

 potassium salts generally possess an exceptionally high activity, and 

 emit a radiation possessing considerable penetrating power, they do 

 not support the conclusion that the activity of these salts is a normal 

 atomic property of potassium and that it is always directly propor- 

 tional to the amount of that metal present in the salt. 



In measuring and comparing the activities of the different salts 

 these were each spread out in turn in uniform layers on a shallow tray 

 which was placed on the bottom of the ionising chamber shewn in 

 Fig. 1, which was 40 cm. long, 25 cm. wide, and 28 cms. deep. The 

 saturation currents through the air in the chamber were measured 

 with a sensitive quadrant electrometer and were taken as measures 

 of the activities of the different salts. 



