August 27, 1909] 



SCIENCE 



273: 



strict a sense as the law of the conservation 

 of energy, is one that we should expect to 

 hold for a collection of a large number of 

 machines of any type, provided that we 

 could not directly affect the individual 

 machines, but could only observe the av- 

 erage effects produced by an enormous 

 number of them. On this view, the second 

 law, as well as the first, should be incapable 

 of saying that the machines were of any 

 particular type: so that investigations 

 founded on thermodjmamics, though the 

 expressions they lead to may suggest— can 

 not, I think, be regarded as proving — the 

 unit structure of light energy. 



It would seem as if in the application of 

 thermodynamics to radiation some addi- 

 tional assumption has been implicitly in- 

 troduced, for these applications lead to 

 definite relations between the energy of the 

 light of any particular wave-length and the 

 temperature of the luminous body. 



Now a possible way of accounting for the 

 light emitted by hot bodies is to suppose 

 that it arises from the collisions of cor- 

 puscles with the molecules of the hot body, 

 but it is only for one particular law of 

 force between the corpuscles and the mole- 

 cules that the distribution of energy would 

 be the same as that deduced by the second 

 law of thermodynamics, so that in this case, 

 as in the other, the results obtained by the 

 application of thermodynamics to radia- 

 tion would require us to suppose that the 

 second law of thermodynamics is only true 

 for radiation when the radiation is pro- 

 duced by mechanism of a special type. 



Quite apart, however, from considera- 

 tions of thermodynamics, we should expect 

 that the light from a luminous source 

 should in many cases consist of parcels, 

 possessing, at any rate to begin with, a 

 definite amount of energy. Consider, for 

 example, the case of a gas like sodium 

 vapor, emitting light of a definite wave- 



length; we may imagine that this light, 

 consisting of electrical waves, is emitted by 

 systems resembling Leyden jars. The en- 

 ergy originally possessed by such a system 

 will be the electrostatic energy of the- 

 charged jar. "When the vibrations are- 

 started, this energy will be radiated away 

 into space, the radiation forming a com- 

 plex system, containing, if the jar has no 

 electrical resistance, the energy stored up- 

 in the jar. 



The amount of this energy will depend 

 on the size of the jar and the quantity of 

 electricity with which it is charged. With 

 regard to the charge, we must remember- 

 that we are dealing with systems formed 

 out of single molecules, so that the charge - 

 will only consist of one or two natural 

 units of electricity, or, at all events, some- 

 small multiple of that unit, while for geo- 

 metrically similar Leyden jars the energy 

 for a given charge will be proportional to 

 the frequency of the vibration; thus, the 

 energy in the bundle of radiation will be 

 proportional to the frequency of the vibra- 

 tion. 



We may picture to ourselves the radia- 

 tion as consisting of the lines of electrie 

 force which, before the vibrations were-^ 

 started, were held bound by the charges on 

 the jar, and which, ■when the vibrations 

 begin, are thrown into rhythmic undula- 

 tions, liberated from the jar and travel' 

 through space with the velocity of light. 



Now let us suppose that this system 

 strikes against an uncharged condenser and 

 gives it a charge of electricity, the charge 

 on the plates of the condenser must be at 

 least one unit of electricity, because frac- 

 tions of this charge do not exist, and each 

 unit charge will anchor a unit tube of" 

 force, which must come from the parcel of" 

 radiation falling upon it. Thus a tube in 

 the incident light will be anchored by 

 the condenser, and the parcel formed by- 



