352 SECTIONAL TRANSACTIONS.— A. 



rotation per molecule has different values in the solid, liquid and gaseous states, yet 

 the ratios of these rotations for three widely separated homogeneous radiations remain 

 the same. Such a result could arise from the condensation, which reduces the effective 

 electric force P of the formula in the ratio 3/ ([j.'^+2) where [i is the refractive index, 

 and the molecular rotation in the same ratio. If this does not wholly account for the 

 difference we seem to be driven to the conclusion that for these substances, the formula, 

 whatever it be, which represents the rotatory dispersion, contains only one term, as 

 Lowry found in other cases ; and that, moreover, in these latter cases that term is 

 preponderantly of molecular type, and not due to any ionic crystalline structure. 



3. Prof. A. W. Porter, F.E.S., and Mr. J. J. Hedges.— 77ie Law 



of Distribution of Particles in a Colloid Suspension. 



4. Prof. E. Whiddington. — Tlie Ultramicrometer in Minute 



Physical Measurements. 



5. Joint Discussion (Cosmical Physics Sub-section) with Sec- 



tions F and M on Weather Cycles in Relation to Agricidturc and 

 to Industrial Fluctuations. Opener: Sir ^Y. Beveridge, K.C.B. 



6. Prof. Sir William Bragg, F.R.S. — Lecture on The Significance 



of Crystal Analysis. 



Friday, September 8. 



7. Presidential Address by Prof. G. H. Hardy, F.E.S., on 



llie Theory of Nmnbers. (See p. 16.) 



8. Prof. J. C. McLennan, F.R.S. — X-Rays from Light Atoms. 



9. M. le Due de Broglie. — X-Rays and Beta Rays. 



There is a close connection between electrons and light, and this relation 

 reveals itself still more plainly in the consideration of corpuscles of high velocity 

 and vibrations of great frequency. 



The equation which appears to govern the interactions between the two 

 phenomena is the quantum equation in the form due to Planck and Einstein, 

 that is to say, we have as between the energy of the corpuscles and the frequency 

 of the vibrations a relation of the form : 



W = h:-. 



I shall only say a word concerning the excitation of X-rays by the impact 

 of cathode rays on the anticathode of a tube ; to-day it is well known that the 

 continuous background of the spectrum emitted by an anticathode bombarded 

 by electrons of definite energy W begins on the short wave-length side at a 

 maximum frequency v, which is precisely equal to 



W 

 h' 



This is a result which Webster has made prominent, and it is completed 

 by the following proposition when the point in question is the excitation of 

 characteristic rays; all the fluorescent rays of a certain series appear simul- 

 taneously in a tube when the energy of the cathode rays reaches a value which 

 exceeds the quantum hi' of the discontinuity of absorption relative to this 

 series. The beautiful experiments of Whiddington formerly gave a first 

 approximation to this law by showing that the emission of Barkla's secondary 

 rays was closely connected with critical values of the velocity of the cathode 

 rays. 



The converse phenomenon is now beginning to be well known ; here it is a 

 question of the rapidly moving electrons liberated by matter when illuminated 

 by a beam of X-rays. This phenomenon obeys the general law of the photo- 

 electric effects, namely, that the individual energy of the corpuscles expelled 

 only depends upon the frequency and not upon the intensity of the exciting 

 radiation ; it is even more remarkably bound up with the levels of energy which 

 the theory of Bohr has pictured in the structure of atoms. 



