PRESIDENTIAL ADDRESS. 11 



for the use of mankind atomic energy and other energies 

 not at present immediately available. 



It may be noticed that unless the energy within the atom 

 were liberated quickly the energy would not be of much 

 use to us. Radium on decomposing to Niton and Helium 

 liberates energy of an order of about one million times 

 greater than could be obtained by burning radium to radium 

 oxide, but this large intra-atomic energy is only made 

 available over a period extending to many thousands of 

 years. In this particular case, however, the Niton is 

 unstable and has a short life, so that its energy is given up 

 rapidly, and this adds considerably to the energy more 

 immediately obtainable from the radium. 



Botli these cases of transmutation are accompanied by 

 the ejection of an a-particle (Helium). The energy liber- 

 ated by the ejection of # particles is not quite so great as 

 that involved in the production of a particles, but would be 

 well worth obtaining provided the energy were liberated 

 quickly enough. 



The sudden character of the change when an atom of one 

 element changes into another is remarkable for its abrupt- 

 ness; a moment comes in the life of the atom when the 

 situation is ripe for the explosion, but this moment is not 

 the same for every atom of the same element, and the 

 period of life for the atom does not depend on the time it 

 has already been in existence. So far as is known, there 

 is no variation in the properties of the atom up to the 

 moment of explosion. When the explosion takes place 

 there is the greatest precision (5) in this act, for when an 

 a-particle is ejected, the a-particle is always detectable for 

 the same distance (30 milimetres) and no further. 



The Periodic Law. 

 It will repay us to study here the Periodic Classification 

 of the elements, and the arrangement, as presently known, 



