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SCIENCE 



[N. S. Vol. XXXVII. No. 943 



those to be emitted, or by the direct im- 

 pact of j8-rays of a corresponding energy; 

 and again, there is now good evidence to 

 show that whenever an electron ionizes a 

 gas its energy of impact must exceed hv 

 where v is now the natural period of 

 vibration of the resonator within the 

 atom which is responsible for the selective 

 photo-electric effect.*^ All of these results 

 are certainly successes of Planck's "Wirk- 

 ungs quantum" h, though in directions 

 scarcely contemplated originally by the 

 theory; for in Planck's theory it is the 

 natural period of the oscillator which deter- 

 mines the emission of energy in units of 

 size hv, but in the normal photo-electric 

 effect the emitted electron has an energy 

 which has nothing to do with its natural 

 period, if it has one. It is rather the period 

 of the incident waves which determine the 

 energy with which the electron is ejected. 



I think I have now stated inost of the 

 important experimental facts which we 

 proposed at the outset to review in the 

 light of atomistic theories of radiation. 

 When we look back over these experimental 

 data there are two main results which 

 stand out conspicuously through it all. 

 TJie first is that neither atoms nor electrons 

 appear to he able to absorh any energy 

 until it conies to them in a certain degree 

 of intensity, and this degree varies with 

 different substances. "We see this in the 

 realm of low intensity heat waves where, in 

 the measurement of atomic heats, different 

 kinds of atoms seem to take on their nor- 

 mal energy load at different stages, as tem- 

 perature rises, the lighter atoms taking it 

 on in this case last; we see it in the realm 

 of high intensity heat waves, such as are 

 dealt with in finding black-body radiation 

 curves; we see it in the realm of photo- 

 chemical or photo-electric radiations, where 



"Franck u. Hertz, Ber. d. D. Flujs. Ges., :3, 

 p. 967, 191], and 14, p. 167, 1912. 



different substances begin to emit electrons 

 at different frequencies of the incident 

 light; and finally we see it in the realm of 

 X-rays, where different substances are ex- 

 cited to emit characteristic X-radiations at 

 diffei'ent hardnesses, the heavy atoms in 

 this case responding last, instead of first. 

 We see further that one intensity factor 

 h proves itself, to say the least, exceedingly 

 useful in every one of these domains. 



The second important fact that stands 

 out is this, that in all types of experi- 

 ments in which the absorption of energy 

 results in the emission of electrons there is 

 apparently a complete, or nearly complete, 

 inter-convertibility of energy between an 

 electron and a so-called ether ray, whether 

 it be an X-ray or a light ray. Now the first 

 of these two facts is the one upon which one 

 group of quantum theorists is focusing its 

 attention and demanding a unitary tlieoiy 

 which emphasizes primarily an emission of 

 energy which is discontinuous in time. The 

 second fact, and it is the one which is the 

 more striking and the better established, is 

 that upon which the other group of theo- 

 rists is focusing its attention and demand- 

 ing an atomistic theory of radiation as re- 

 gards space relations. Now the fifth and 

 last of the quantum theories which I have 

 presented is that which, in view of both of 

 these groups of facts, demands a quantum 

 theory which combines both of these char- 

 acteristics. The facts which have been here 

 presented are obviously most completely 

 interpreted in terms of such a theory, how- 

 ever radical it may be. Why not adopt it 1 

 Simply because no one has thus far seen 

 any way of reconciling such a theory with 

 the facts of diffraction and interference so 

 completely in harmony in every particular 

 with the old theory of ether waves. Lorenz 

 will have nothing to do with any ether- 

 string theory, or spotted wave-front theory, 

 or electro-magnetic corpuscle theory. 



