The Radio-activity of Metals and their Salts. 287 



11. It would be interesting to inquire whether the examples 

 of the same element when exercising different valencies would 

 exhibit the same activities. The case of mercury shown in the 

 table yields no certain result, for the activity is too small for any 

 definite statements to be made about it. Further investigation 

 into this question is in hand. 



The question also arises naturally whether any relation can be 

 traced between the activity of a metal and its other properties, 

 for instance its position in the periodic arrangement of the 

 elements. No such correlation is apparent at present, and indeed 

 the results obtained seem difficult to reconcile with any connec- 

 tion of the kind. The number of metals examined at present is 

 insufficient to enable any definite statements to be made ; but 

 progress will be easier now that it has been established that the 

 activity is an atomic property. Many elements, which it is very 

 desirable to establish from this point of view, such as the alkali 

 metals and the halogens, could not be tested before, because it 

 was impossible to use them in the elemental state. Now they 

 can be examined in the state of compounds. It should be pointed 

 out that the activity of a slab of a substance (p in § 2) does 

 not directly measure the activity of the substance itself: it is 

 a product of this activity by the penetration of the rays given 

 out. This consideration will doubtless complicate the investi- 

 gation. 



The expression of my indebtedness to Professor Thomson for 

 his advice and encouragement is not the less sincere because it is 

 repeated so often. 



