INTRODUCTION 9 



on the nature of electrolytic solutions, but on 

 many physiological questions of vital interest. 



A natural step leads from the conduction of 

 electricity in liquids to its conduction in gases, 

 and, on our stage, the ion is joined by the 

 corpuscle or electron. The dream of the old 

 philosophers of a common basis for matter is 

 realised by experimental investigation. 



Arising from these experiments and their 

 interpretation comes the theory of radio-activity, 

 the modern equivalent of the imagined trans- 

 mutation of the mediaeval alchemist. We see 

 and measure the gradual disintegration of the 

 chemical elements, and draw on the energy 

 stored within the atoms themselves. Rutherford 

 has even succeeded in inducing such changes 

 artificially in some few elements. 



The vibrations of electro-magnetic systems 

 produce the aethereal waves now used in wireless 

 telegraphy, and the vibrations of atomic systems 

 give rise to light. Thus atoms must be related 

 intimately to light, and light to electro-magnetic 

 phenomena. 



Our model of an atom must clearly explain 

 not only the facts of radio-activity, but those of 

 radiation also. If electrons radiate energy as 

 they revolve in planetary orbits, they should, on 

 ordinary dynamical principles, move faster and 

 faster and circle nearer and nearer to the nucleus. 

 Hence a number of atoms should emit waves of 

 all possible periods of vibration and wave lengths. 

 But this deduction is inconsistent with the well- 

 known bright line spectra of many elements, 

 spectra showing vibration in one or a few 

 definite periods only. This was the origin of 



