3 o SECTIONAL ADDRESSES 



story of nineteenth-century physics, for in it Joule described the experi- 

 ments on which his famous C 2 R law is based, and enunciated the law. 1 

 Indeed the story of the identification of heat with energy, in its novelty 

 and the difficulty of its adoption, is as outstanding a feature of nineteenth- 

 century physics as is the story of the equivalence of mass and energy 

 in the physics of the twentieth century. 



No survey of the physical science of the last generation would be 

 complete did it contain no reference to radiation and to the nineteenth- 

 century concept of the mechanism by which radiation is conveyed. 

 Despite the difficulty of framing a theory of the ether which should 

 satisfy dynamical laws — ' Why should it ? ' we might remark incidentally 

 to-day — the concept of an ether of space was so brilliantly successful in 

 correlating and predicting so many and so diverse phenomena — we need 

 but instance that bending of light round corners which we call diffraction, 

 that alternate heaping up and destruction of light which we term inter- 

 ference, and that remarkable refraction of a ray of light by certain crystals 

 as a cone of rays — as to draw from Lord Kelvin the downright statement, 

 ' This thing we call the luminiferous ether ... is the only substance we 

 are confident of in dynamics. One thing we are sure of, and that is the 

 reality and substantiality of the luminiferous ether.' Strange reading, 

 to-day ; and reading which might well introduce a note of hesitation into 

 some of the confident declarations of present-day realities. 



Molar mechanics, the billiard-ball atom, the ether : the nineteenth 

 century had built on these apparently stable foundations an immense 

 structure of ordered knowledge. The closing years of the century were 

 fated to show cracks in the superstructure and weaknesses in the founda- 

 tions. The facts of radio-activity and the discovery of the electron showed 

 that the concept of the atom must increase in complexity were it to 

 remain competent to subsume the additional perceptual facts. And the 

 experimental study of the radiation from a hot body revealed a state of 

 affairs inexplicable on the lines of classical theory. A hot body radiates 

 energy, and if the radiations are passed through a prism they may be 

 drawn out into a spectrum. How is the energy of the radiation distri- 

 buted between the different wave-lengths of the spectrum ? Experiment 

 gives a clear answer to this question, and the undisputed fact is tftat, if 

 we plot a curve showing values of the energy associated with a certain 

 wave-length as ordinates against the corresponding wave-lengths as 

 abscissas, we obtain a curve of a cocked-hat shape with a definite maxi- 

 mum of energy associated with a certain wave-length. If we repeat the 

 experiment with the radiating body at a higher temperature, a similar 

 curve is obtained with the maximum shifted into the region of shorter 



1 In this paper, and in a paper published in the Philosophical Magazine in 1841, 

 Joule used the term resistance in its ordinary electrical sense (' the resistances of 

 the . . . wires were found to be in the ratio 6 to 5-51'). The term was used by 

 Cavendish (' therefore resistance is directly as velocity ') in his now famous 

 anticipation of Ohm's Law in January 1781 — though his words were not printed 

 until 1879. Wheatstone is sometimes quoted as an early user of the term in his 

 Bakerian Lecture for 1843. It is all the more curious, then, that the Shorter 

 Oxford English Dictionary should give i860 as the date at which the term was 

 first used in print. 



