122 CONCEPTION OF THE CHEMICAL ELEMENT 



in /3-ray changes. The period of both substances 

 is long, and it is probable that the /3-particle is 

 expelled, but is undetectable by ionisation methods. 

 For the slowest /3-ray change, that of radium-Z?, 

 with a period of twenty-four years, the /3-radiation 

 is of such low velocity as to be only capable of 

 detection by special care, and is far less penetrating 

 than average a-rays. These facts serve to show 

 that changes may be going on in the non-radio- 

 active elements which at present are beyond experi- 

 mental means of detection. 



PERIOD OF AVERAGE LIFE. 



The law of radioactive change, which is the 

 same for all cases, is that of unimolecular reaction, 

 the rate of change, or quantity changing in unit of 

 time, being a fraction, designated by X and known 

 as the radioactive constant, of the amount present. 

 The value of X, although vastly different for different 

 radio-elements, is an absolute constant, so far as is 

 known, for any one element, independent of every 

 consideration whatever. The period of average life 

 is the reciprocal of this constant, but the actual life 

 of any one atom may assume any value. This is an 

 experimental fact very difficult to account for. For 

 example, it is quite easy to compare the value of X 

 for a collection of atoms ( i ) only just produced and 

 not in existence a short interval before, and (2) that 

 have remained undistinguished from an originally 

 very much greater number, and each of which has 

 been in existence many times the period of average 

 life. In both cases the value of X is the same. This 

 fact excludes from consideration as a conceivable 

 cause of disintegration any gradual progressive altera- 

 tion in the atom during its period of existence, as, 

 for example, was at one time suggested, a gradual 



