JoLY — Radio- Therapy : its Scientific Basis ami its Teachiiifj. 493 



the greater the frequency the greater the energy of the photo-i'lectrons. 

 Or, we might saj^ the sljorter tlie wave-length the greater tlie velocity of the 

 electron. 



(5) The effects are independent of the temperature, taking place freely 

 at the temperature of liquid air.' The effect is, therefore, an inter-atomic 

 and hot a chemical one. Chemical actions cease at such temperatures. 



These are the fundamental laws of photo-electricity. To-daj' we are in 

 a position to extend these laws to the much shorter and more rapid 

 oscillations of X and y rays. 



Tt much conduces to a clear understanding of tlie relations of the 

 several parts of the subject before us if we take into account a comparatively 

 recent generalization which ajDplies to all aethereal radiation — heat, light, 

 X ray, or y vny. The generalization is due to Planck and Einstein. 



According to Planck, the euergy of radiation is not sent into the aether 

 as a continuous stream, but in " fits " or discreet rays. The total energy, for 

 instance, which reaches the leaf of a plant from the sun in a second of time 

 is composed of an integral number of definite elementary units of radiation 

 or vibration. The energy associated with each unit of radiation he calls a 

 quantum of energy ; and he shows that this is always equal to a certain 

 universal constant multiplied by the frequency. This constant (Planck's 

 constant) has the value 6'55 x 10"-' erg seconds. If u is the frequency of 

 any aethereal radiation — light, X rays, or 7 rays — we can say that the energy 

 of the quantum is n x 6-55 x 10"" light unit. Such units of energy move 

 in space without spreading or dividing. They keep their discrete and 

 separate character wherever they travel. 



The important matter for us to notice here is that the energy increases 

 directly with the frequency. Now, the frequency of an X ray may be 10,000 

 times that of a light-wave ; hence the quantum of energy associated with it 

 may be 10,000 times greater. And as a y ray of radium G may be of 100,000 

 times the frequency of a light-ray, the quantum of energy associated with it 

 may be 100,000 tiuies greater, and 10 times as much as accompanies the 

 X ray. 



These facts enable us to assign an explanation to some of the laws of 

 photo electricity which we have recorded above. 



We have stated, as matter of observation, that light gives rise to a 

 discharge of electrons, or /S rays, from substances in which it is absorbed, and 

 that the velocity and energy of the discharged electrons are proportional to 

 the frequency of the light employed, and are not in any way dependent on 



' Joly, "Nature," vol. Ixxii, p. 308, 1905. 



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