166 



One of the most useful theorems in thermodynamics is a sort of supple- 

 ment to the Second Law. and is due to Nernst. It states that the entropy 

 of a substance at the absolute zero of temperature is zero. This theorem is amply 

 justified by experiments on specific heats, thermo-neutral chemical reactions, 

 and so forth. Boltzmann has shown that if the energy be subdivided into a 

 large number of ecjual parts a quantity can be calculated, by means of the 

 theory of probability, which is projiortional to the entropy as deduced with 

 the aid of Nernst's theorem, the proportionality factor depending upon the 

 magnitude of the elementary amounts of energy. Some value of this amount 

 should give the value of the entropy exactly. This value, according to Planck, 

 would be the "elementar-quantum." Nernst's theorem may then be con- 

 sidered as another ground for b(>lief m the basic truth of Planck's tlieory.'" 

 Additional evidence in favor of the theory is to be found in the jihenomena 

 of fiuorescence, the photo-electric effect, and others. 



We shall now exaniinc Planck's assumptions and attempt to interpret 

 them physically. We know that Hertzian waves onlj' differ in wave-length 

 from the luminous waves emitted by an incandescent body. What is then 

 more natural than to assume that the atoms of Ixxlics contain tiny Hertzian 

 resonators, or oscillators? We say "atoms" because the spectral lines of an 

 element ap[)ear in the spectra of its compounds. The "))erfectly reflecting 

 walls" may be notiiiiig more than a useful mat lu-niat ic;il fiction, or may rep- 

 resent true physical boundaries corresponding to the cell walls of barmor's 

 elementary receptacles of energy. The assumption of a nieclianicai model is 

 of vastly less interest than those implying at least an atomic structure of 

 energy. If there are atoms of energy do tlu\y i)reserve their identity'? Are 

 they invariable? Planck assumes a discontinuous emission but a more or 

 less continuous absorpt ii>n of energy. May we not ask the (luestion: "Are 

 these discontinuities due to the oscillator or to the energy itself"? If the 

 energy exists in discrete (juantities, why is it not absorbed as well as emitted 

 in multiples of the "elemental' (luantum'"? If absoi'lx'd in discrete ainoimts 

 are these identical with those emitted? .Vjjparently not, for in the i)henomena 

 of fluorescence the fluorescent light is nearly always of lower frecpiency than 

 the light which causes the fluorescence, or, speaking in terms of Planck's 

 hypothesis, the emergent cjuanta are smaller than the incident (juanta, for 

 the so-called atoms of encrf/i/ are larger in ])roportioii to the frcHjueiicy of the 

 light, so that "atoms" corresponding to l)lue light are larger than those cor- 

 responding to red light. If, then, w(> are to account for (luorescence by means 

 '"Prejss. Akail. Wi.-is. Ro.lin, .Siz. Her., 4. pp. O.i-!!), I'.Ul. 



