Discontinuity in the Phenomena of Radiation 45 



It is to this particular difficulty that I wish to direct your 

 uttention this evening, and to make the matter as intelligible as 

 possible to the non-physicists among you, I must recapitulate 

 certain facts and definitions ; and in this recapitulation the 

 exigencies of time prevent me paying too much attention to the 

 historical and logical order of the development of the ideas 



The nineteenth century witnessed the foundation of the 

 energy principle. In a limited form conservation of energy may 

 be said to date back to Galileo, and Newton made a clear state- 

 ment of it for a restricted range of phenomena ; but it is on the 

 work of Carnot, Joule, Clausins, Rankine, Thomson and Helm- 

 holtz that its acceptance in the widest sense reposes to-day. 

 That form of energy which we most readily apprehend is the 

 energy of moving matter — so-called kinetic energy — and the 

 mathematical physicist demonstrates for us how it is to be 

 measured, viz., by multiplying half the mass of the body by the 

 square of the velocity, and, if need be, by certain numerical 

 factors also, according to the particular units in which we desire 

 to express the result — ergs, joules, foot-pounds, etc. Another 

 form 'of energy possessed by matter is that known as energy of 

 position or energy of configuration. A body possesses this by 

 reason of its occupying a position of advantage with respect to 

 other matter in the universe which is exerting force on it, or 

 by reason of the fact that it is strained from what may be called 

 its natural shape or configuration. Such energy is generally re- 

 ferred to as " potential " ; for if the body leaves its position of 

 advantage or returns towards its natural shape it does so with 

 increasing movement, i.e., with gradual increase of kinetic energy, 

 so that this acquired energy of motion may be said to be latent 

 or potential in the matter as it was originally situated or strained. 

 The important feature about the possession of this type of energy 

 is that it depends upon the action on the body, by other matter, 

 of forces which arise solely from the position of the body or from 

 elastic stresses set up in the body by its deformation — forces and 



