11-1 Prof. W. C. McC. Lewis on the Relation of the 



not revolving with a speed comparable with that of light, its 

 mass may be regarded as constant, and the constancy of 

 angular momentum leads to the relation that the frequency 

 of revolution x (orbit) 2 is a constant independent of the 

 nature of the substance as well as of T and V. Now for 

 a given substance Pascal's relation leads to the conclusion 

 that the orbit r is a constant independent of T and V. 

 Hence it would follow that for any given substance the 

 frequency of revolution of the electron is constant inde- 

 pendent of T and V. This refers in the first instance to 

 the steady revolution of an electron round the some orbit, 

 no radiant energy being emitted or absorbed during the time 

 considered. According to Bohr, however, there is a very 

 close connexion between this frequency of revolution and 

 the vibration frequency v of the light which the given sub- 

 stance absorbs or emits. Bohr in fact assumes that the 

 frequency of emission is just half the rate of revolution. 

 Whether this simple relation exists or not, it is fairly certain 

 that constancy in the rate of revolution in any of the available 

 orbits will also lead to constancy in the frequency of the light 

 absorbed or emitted by the substance due to its revolving 

 electrons. That is, if Pascal's and Bohr's relations be simul- 

 taneously true, one would expect the vibration frequency — 

 or frequencies — in the absorption or emission spectrum of a 

 given substance to be independent of temperature and pres- 

 sure. Experiment shows that this is approximately but by 

 no means strictly true. The absorption band of a given 

 substance in the liquid state is nearly but not quite identical 

 in position with the band exhibited by the vaporized sub- 

 stance. Thus the head of the absorption band for toluene 

 vapouris\ = 2610 (A.U.), for liquid toluene, \ = 2690 (A.U.). 

 For paraxylene vapour, A, = 272S (A.U.) for liquid, 

 X = 2750 (A.U.); the values are accurate to 5-7 A. units*. 

 If we take Pascal's relation as true, and assume at the same 

 time that nY is independent of T and P, then the variations 

 in X from liquid to vapour indicate that Bohr's relation 

 regarding constancy of angular momentum cannot be quite 

 true. Conversely, taking the X values to be approximately 

 independent of the physical state and assuming the con- 

 stancy of angular momentum, we are led to a theoretical 

 basis for Pascal's relation. Actually, however, it would 

 seem that neither the constancy of A nor the constancy of 

 the angular momentum is to be regarded as strictly true. 



* It has been shown also that emission spectra are only slightly 

 affected by pressure. 1 am indebted to Prof. K 0. C. Baly, J\B,S., for 

 the above information respecting the numerical values. 



