Mafichester Memoirs , Vol. xlviii. (1904), No. 8. 29 



part of the radium disintegrates in 5"3 days. The value 

 of X is therefore about 2 x io~^\ and the average life of the 

 atom is 1,500 years. Although this value is not likely to 

 be very exact, and must be regarded as a preliminary 

 estimate, it is probable that at least it approximates to the 

 truth. If it is accepted a further important step may be 

 made. 



It is possible to deduce the average life of the radium 

 atom from the disintegration theory alone without the 

 datum just considered, at least in so far as to indicate a 

 maximum and minimum value between which it must lie. 

 The uncertainty arises from the fact that we do not as yet 

 know the number of a particles expelled from each atom 

 at each disintegration it undergoes. Obviously it cannot 

 be less than one, and this fixes the minimum estimate of 

 the life. On the other hand, if the radium atom is com- 

 pletely resolved into a particles not more than about 150 

 in all could be produced, which gives the maximum 

 estimate. The method employed is as follows (compare 

 for example Rutherford, Nature, Aug. 20th, 1903). The 

 energy of the individual particle is about 5 x io~^ erg- 

 It can be argued that the radiant particle will possess by 

 far the greater part of the energy of atomic disintegration,, 

 in the same way as when a gun is fired the major part of 

 the energy is communicated to the bullet, and but a small 

 part is used in the recoil. This holds good whatever the 

 number of particles projected at each disintegration. If 

 the energy emitted by a gram of radium per second 

 (Curie and Laborde) is divided by the energy of the 

 individual particle, the result, 2*4 x 10", is the number of 

 a particles expelled by a gram of radium per second. If 

 but one particle is projected at each stage, then since 

 there are four stages in the disintegration of radium, at 

 each of which a particles are expelled, one-fourth of the 



