August 26, lyogj 



NA TURE 



255 



Take, for example, the pressure exerted by light. This 

 would follow as a matter of course if we supposed light 

 to be small particles moving with great velocities, tor 

 these, if they struck against a body, would manifestly 

 tend to push it forward, while on the undulatory theory 

 there seemed no reason why any effect of this kind should 

 lake place. 



Indeed, in 17921 this very point was regarded as a test 

 between the theories, and Bennet made experiments to see 

 whether or not he could find any traces of this pressure. 

 We now know that the pressure is there, and if Bennet's 

 instrument had been more sensitive he must have observed 

 it. It is perhaps fortunate that Bennet had not at his 

 command more delicate apparatus. Had he discovered the 

 pressure of light, it would have shaken confidence in the 

 undulatory theory and checked that magnificent work at 

 tile beginning of the last century which so greatly in- 

 creased our knowledge of optics. 



.'^s another example, take the question of the distribu- 

 tion of energy in a wave of light. On the emission theory 

 the energy in the light is the kinetic energy of the light 

 particles. Thus the energv of light is made up of distinct 

 units, the unit being the energy of one of the particles. 



The idea that the energy has a structure of this kind 

 has lately received a good deal of support. Planck, in a 

 very remarkable series of investigations on the Thermo- 

 dynamics of Radiation, pointed out that the expressions 

 for the energy and entropy of radiant energy were of 

 such a form as to suggest that the energy of radiation, 

 like that of a gas on the molecular theory, was made up 

 of distinct units, the magnitude of the unit depending on 

 the colour of the light ; and on this assumption he was 

 able to calculate the value of the unit, and from this 

 deduce incidentally the value of Avogadro's constant — 

 the number of molecules in a cubic centimetre of gas at 

 standard temperature and pressure. 



This result is most interesting and important, because if 

 it were a legitimate deduction from the Second Law of 

 Thermodynamics, it would appear that only a particular 

 type of mechanism for the vibrators which give out light 

 and the absorbers which absorb it could be in accordance 

 with that law. 



If this were so, then, regarding the universe as a 

 collection of machines all obeving the laws of dvnamics, 

 the Second Law of Thermodynamics viiould only be true 

 for a particular kind of machine. 



There seems, however, grave objection to this view, 

 which I may illustrate by the case of the First Law of 

 Thermodynamics, the principle of the Conservation of 

 Energy. This must be true whatever be the nature of 

 the machines which make up the universe, provided they 

 obey the laws of dynamics, any application of the prin- 

 ciple of the Conservation of Energy could not discriminate 

 between one type of machine and another. 



Now, the Second Law of Thermodynamics, though not 

 a dynamical principle in as 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 directlv affect the 

 individual machines, but could only observe the average 

 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 

 p.Trticular type : so that investigations founded on thermo- 

 dynamics, though the expressions they lead to mav suggest 

 — cannot, I think, be regarded as proving — the unit struc- 

 ture of light energv. 



It would seem as if in the application of thermodynamics 

 to radiation some additional assumption has been implicitly 

 ir.lroduced. for these applications lead to definite relations 

 between the energv of the light of any particular wave- 

 leiTth and the leninerature of the luminous bodv. 



Now a possible way of accounting for the light emitted 

 hv hof bodies is 10 suppose that it arises from the collisions 

 of corpuscles with the molecules of the hot bodv. but it 

 is only for one particular law of force between the cor- 

 puscles and' the molecules that the distribution of energv 

 would be the same as that deduced by the Second Law 

 of Thermodvnamics. so that in this case, as in the other, 

 the results obtained bv the application of thermodynamics 

 to radiation ivoulH rr.oiprr. ^^- to suppose that the Second 



NO. 2078, VOL. 81] 



Law of Theiniodynamics is only true for radiation when 

 the radiation is produced by mechanism of a special type. 



Quite apart, however, from considerations of thermo- 

 dynamics, we should e.xpect that the light from a luminous 

 source should in many cases consist of parcels, possess- 

 ing, at any rate to begin with, a definite amount of 

 energy. Consider, for example, the case of a gas like 

 sodium vapour, 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 energy originally possessed by such a syste:^ will be 

 the electrostatic energy of the charged jar. Whe# the 

 vibrations are started, this energy will be radiated away 

 into space, the radiation forming a comple.x system, con- 

 taining, 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 geometrically similar 

 Leyden jars the energy for a given charge will be pro- 

 portional to the frequency of the vibration ; thus, the 

 energy in the bundle of radiation will be proportional to 

 the frequency of the vibration. 



We may picture to ourselves the radiation as consisting 

 of the lines of electric 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 undukitions, 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 fractions of this charge 

 do not exist, and each unit charge will anchor a unit 

 tube of force, which must come from the parcel of radia- 

 tion falling upon it. Thus a tube in the incident light 

 will be anchored bv the condenser, and the parcel formed 

 bv this tube will be anchored and withdrawn as a whole 

 f.'om the pencil of light incident on the condenser. If the 

 energy required to charge up the condenser with a unit 

 of electricity is greater than the energy in the incident 

 parcel, the tube will not be anchored and the light wilt 

 pass over the condenser and escape from it. These prin- 

 ciples that radiation is made up of units, and that it re- 

 quires a unit possessing a definite amount of energy to 

 excite radiation in a body on which it falls, perhaps receive 

 their best illustration in the remarkable laws governing 

 Secondary Rontgen radiation, recently discovered by Prof. 

 Barkla. Prof. Barkia has found that each of the different 

 chemical elements, when exposed to Rontgen rays, emits 

 a definite tvpe of secondarv radiation whatever may have 

 been the type of primary ; thus lead emits one type, copper 

 another, and so on ; but these radiations are not excited 

 at all if the primary radiation is of a softer type than 

 the specific radiation emitted by the substance; thus the 

 secondary radiation from lead being harder than that 

 from copper, if copper is exposed to the secondary radia- 

 tion from lead the copper will radiate, but lead will not 

 radiate when exposed to copper. Thus, if we suppose 

 that the energy in a unit of hard Rontgen rays is greater 

 than that in one of soft, Barkla's results are strikingly 

 analogous to those which would follow on the unit theory 

 of light. 



Though we have, I think, strong reasons for thinking 

 that the energv in the light waves of definite wave-length 

 is done up into bundles, and that these bundles, when 

 emitted, all possess the same amount of energy, I do not 

 think there is any reason for supposing that in any casual 

 specimen of light of this wave-length, which mav sub- 

 sequent to its emission have been many times refracted 

 or reflected, the bundles possess any definite amount of 

 energy. For consider what must happen when a bundle 

 is incident on a surface such as glass, when part of it is 

 reflected and part transmitted. The bundle is divided into 

 two portions, in each of which the energy is less than 

 the incident bundle, and since these portions diverge and 

 may ultimately be many thousands of miles apart, it 



