Manchester Memoirs, Vol. xlviii. ( 1 904), No. 8. 2 [ 



thorium emanation, and in about four days in the case of 

 the radium emanation. 



A relation perhaps more generally useful than the 

 radio-active constant is its reciprocal i/X, which has a 

 very interesting physical significance. It represents the 

 average life of the metabolon in seconds, and affords a 

 more concrete mental picture than the rate of change. 

 Thus for the thorium emanation the average life of the 

 metabolon is 87 seconds, for the radium emanation 5'8 

 days. The average life of a metabolon may be compared 

 with the atomic weight in the case of a stable atom as a 

 •constant well suited for its experimental identification. 

 It may be pointed out that the actual life of the 

 different atoms of the same unstable element has all 

 values between zero and infinity. Some break up during 

 the first second of existence, and since only a fraction of 

 the total changes per second, the quantity is, theoretically, 

 never reduced to zero, and some persist indefinitely. This 

 constitutes the first difference in properties between the 

 individual atoms of the same element that has ever been 

 discovered. It may be likened to the individual differ- 

 ences of velocity that exist between the molecules of a 

 gas at constant temperature according to the kinetic 

 theory. It is open to question whether all atomic proper- 

 ties are not really average properties, the individual atoms 

 continually passing with great rapidity through phases 

 varying widely among themselves in chemical and phys- 

 ical nature. 



In the following table the average lives of the best 

 known metabolons are arranged in order, together with the 

 average lives of the atoms of the so-called permanent 

 radio-elements. The latter will be asfain referred to. 



