2 PHYSICAL SCIENCE 



workers trained by him and by his pupil and 

 colleague Sir Ernest Rutherford, in the Cavendish 

 Laboratory. Students from almost all civilised 

 countries have come to Cambridge as to the centre 

 of this branch of physical research, and many of 

 them are now carrying forward their investigations 

 elsewhere, by methods learnt in the University of 

 Newton, Clerk- Maxwell, and Stokes. 



When we again take up the story twenty 

 years later, we see a marvellous increase in 

 both these branches of knowledge. In thermo- 

 dynamics much progress has been made by 

 following principles already laid down, and apply- 

 ing them in new directions, especially in the 

 subject of physical chemistry. 



On the other hand, the study of the con- 

 duction of electricity through gases and the 

 allied domain of radio-activity has attracted most 

 of the work of experimental physicists. It has 

 led to an unprecedented output of new knowledge 

 and an amazing new insight into the secrets of 

 atomic structure. 



Moreover, two fresh subjects of inquiry 

 have appeared. One of these depends both on 

 thermodynamics and on atomic conceptions. 

 The examination of the known facts of optical 

 spectra had led Planck in 1901 to the view that 

 radiation was emitted only in definite units, 

 the constant unit being not energy but the 

 product of energy and time — a quantity now 

 called "action." 



This quantum theory was applied by Einstein 

 in 1907 to explain the specific heat of solids, and 

 by Bohr in 191 3 to give a new picture of the 



