12 THE ROYAL SOCIETY OF CANADA 



trating rays from radioactive substances, or as I suggest that we 

 call them, from radiants; the atomic heat of elements, so admirably 

 handled by Debye; the residual energy at low temperatures; and the 

 constitution of the atom. 



Space prevents us from considering more than the last of these. 



The first step towards the new method was taken by Planck 

 when he saw the necessity of explaining why the energy of short 

 wave radiation is some hundred millionth part of that demanded by 

 classical dynamics. He made the supposition that energy is not 

 indefinitely divisible; but he did not assume that it was atomic. 

 He actually imagined that energy was emitted from oscillators in 

 exact multiples of hn, where n is the frequency of the oscillation and 

 h is a universal constant (Planck's) with a value 6-5X10 -2 ' erg 

 second. The magnitude of the energy quantum is thus proportional 

 to the frequency. 



This quantum hypothesis has spread like fire during a drought. 

 It pervades the scientific journals. No physicist has pretended to 

 explain or understand it, for as Jean says, the lucky guess has not 

 yet been made. Nevertheless it appears that "&" has truth under- 

 lying it, and that it has come to stay, for the applications of the quan- 

 tum hypothesis have already achieved a great and unexpected measure 

 of success. In the meantime it is necessary to proceed with caution 

 checking every theory by experiment, for there is no other criterion 

 to guide the investigator, whether to hold to the old, or try the new. 



(7). The first steps towards the idea of the modern or Ruther- 

 fordian atom rest on an experimental basis, and are not therefore 

 open to suspicion. 



Rutherford and Geiger found that when the alpha particles from 

 a radiant, such as radium or polonium met a thin gold leaf, the bulk 

 of the alpha particles passed through with slight deflection, but about 

 one in eight thousand bounced back, or returned towards the side of 

 their source. Both large and small deviations of the alpha particles 

 in passing through matter were satisfactorily explained by ordinary 

 or Newtonian dynamics, with the law of repulsion inversely as the 

 square of the distance between similar electric charges. One charged 

 particle was the alpha particle with a positive charge twice as large, 

 numerically, as that of an electron. The other charged particle was 

 the nucleus of the atom of gold, and the magnitude of this charge was 

 about ^A where A is the atomic weight of gold. This view was sub- 

 jected to a searching series of experimental tests and emerged trium- 

 phant. 



(8). About this time C. T. R. Wilson skilfully obtained photo- 

 graphs of the mist-ladened, charged air molecules, marking the track 



