92 THE EVOLUTION OF MATTER 



point to the boiling-point every three-quarters of an 

 hour. In the combustion of fuel, from which the 

 world draws by far the greater part of the energy it 

 needs, the heat evolved is sufficient to raise a weight 

 of water some 80 to 100 times the' weight of fuel 

 from the freezing-point to the boiling-point. Hence 

 radium, weight for weight, gives out as much heat as 

 the best fuel every three days, and in the fifteen years 

 that have elapsed since it was first isolated, a 

 quantity of energy nearly two thousand times as 

 much as is obtainable from fuel has been given out 

 by the radium, and the supply as yet shows no sign 

 of exhaustion. 



Before, however, these questions could be asked 

 in this definite quantitative form they had been 

 answered, from a detailed investigation of the 

 radioactivity of the element thorium. Professor, 

 now Sir Ernest, Rutherford, at M'Gill University, 

 Montreal, and now at Manchester University, was 

 one of the leading and most active physicists in the 

 investigation of the new property, and, when the 

 writer joined him in Montreal in 1901, had made 

 a large number of very startling and fundamental 

 discoveries, and had developed the refined methods 

 of investigation and measurement which, more than 

 anything else, contributed to the rapid solution of the 

 problem. The apparently steady and continuous 

 outpouring of the radiations from thorium was found 

 to be a most complex process, in which new substances 

 were being continually produced. These new sub- 

 stances are endowed with a temporary or transient 

 radioactivity, which in the course of time decays 

 away and disappears. Simple methods of chemical 

 analysis sufficed to remove from thorium altogether 

 infinitesimal quantities of substances, to which, 

 however, by far the greater part of the radioactivity 

 was due. After removal the activity of these sub- 



