oj- Physical Knowledge. 67 



deep into the roots of the Theory to leave them unhurt. So 

 again nothing remains but to re-examine the foundations of 

 the Theory. And again we must admit that the principles 

 of Thermodynamics have shown themselves to be unshakable. 

 For the only method so far found to promise a complete 

 solution of the riddle depends directly upon the two laws 

 of Thermodynamics; though it combines with them a new 

 and peculiar hypothesis, which, if we utilize the two illus- 

 trations above mentioned, can be expressed somewhat as 

 follows : — 



In the case of the Water waves, the disintegration of the 

 energy of motion is limited by the fact that the atoms hold 

 the energy together, in a way, each atom representing a 

 certain finite material Quantum which can only move as a 

 whole. In the same sort of way certain processes must be 

 at work in the case of light and heat rays, although they are 

 quite of an immaterial nature, which shall hold together the 

 energy of radiation in definite finite Quanta, and shall unite 

 it the more strongly the shorter the waves and the quicker 

 therefore the frequency of the oscillations. 



In what way we are to conceive the nature of quanta of a 

 purely dynamical nature, we cannot yet say for certain. 

 Possibly such quanta might be accounted for if each source 

 of radiation can only emit energy when that energy attains 

 at least a certain minimum value ; just as a rubber pipe, into 

 which air is gradually compressed, bursts and scatters its 

 contents only when the elastic energy in it attains a certain 

 quantity. 



In any case, the hypothesis of Quanta has led to the idea 

 that there are changes in Nature which do not occur con- 

 tinuously but in an explosive manner. I need hardly remind 

 you that this view has become much more conceivable since 

 the discovery and investigation of Radio- Active Phenomena. 

 Besides, all difficulties connected with detailed explanation 

 are at present overshadowed by the circumstance that the 

 Quantum Hypothesis has yielded results which are in closer 

 agreement with radiation-measurements than are all previous 

 theories. 



Moreover, if it is a good sign for a new hypothesis that it 

 is found applicable to regions for which it was not originally 

 devised, then the Quantum Hypothesis can surely claim a 

 favourable testimony. I shall only refer to a quite particu- 

 larly striking point. Since we have succeeded in liquefying 

 Air, Hydrogen, and Helium, a new field of activity has been 

 opened for experimental investigation in the region of low 

 temperatures, and in this region a number of new and in some 



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