536 SECTIONAL TRANSACTIONS.— A. 



The curves obtained depend upon both absorption and emission variations, and 

 an attempt is made to estimate the relative importance of the two efEects, and so to 

 separate them. The evidence so far collected appears to indicate that the radiations 

 are due to the impact of ions on neutral molecules. 



Keport of Committee on Seismological Investigations. (See p. 237.) 



Monday, September 10. 



Presidential Address by Prof. A. W. Porter, F.R.S., on The Volta 

 Effect : Old and New Evidence. (See p. 21.) 



Prof. P. Zeeman. — The Spectrum of Ionised Argon and its Resolution in 

 the Magnetic Field. 



Mr. R. A. Watson Watt. — The^Present State of our Knowledge of Atmo- 

 spherics. 



This communication reviews, in a series of illustrative diagrams, the available 

 evidence as to the origin and properties of naturally occurring electromagnetic waves 

 of radiotelegraphic frequency. 



The relative frequency of occurrence of atmospherics of difierent wave-forms, 

 the mean peak field strengths and durations and the high frequency structure are 

 examined. The number per unit time passing a selected threshold value is measured 

 in temperate and tropical latitudes. 



The mean directions of arrival of the predominant streams of atmospherics at 

 various stations are studied with respect to their temporal variations, and the general 

 distribution of sources of atmospherics is indicated by the intersections of these 

 directions. 



Specimens of the location of thunderstorms and frontal phenomena by radio- 

 telegraphic direction finding on atmospherics are shown, together with distribution 

 charts showing the places of origin of individual atmospherics on selected days. 



Experiments for the determination of the effective disturbing range of atmospherics, 

 in which aural reception of the atmospherics accompanymg broadcast matter is 

 combined with the direction-finding methods outlined, are discussed. 



Finally some special features of the diurnal and seasonal variations in intensity 

 of atmospheric disturbance in relation to solar and to terrestrial-magnetic influences 

 are noted. 



Mr. R. W. James. — The Study of the Heat Vibrations of a Crystal Lattice 

 by means of X-rays, and its Bearing on the Question of Zero Point 

 Energy. 



From measurements of the intensity of reflexion of X-rays from a crystal face at 

 a series of angles, it is possible, in some simple cases, to determine the average scattering 

 power for X-rays of the atoms of which the crystal is composed, as a function of the 

 angle of scattering, and hence to deduce the distribution of the electrons within the 

 average atom. This distribution is affected by the heat vibrations of the crystal 

 lattice, which must be taken into account in any comparison of the electron distribu- 

 tion, deduced in this way, with those given by different atomic models. Measure- 

 ments over a wide range of temperatures have shown that, in the case of rock salt, 

 the formula due originally to Debye and modified later by Waller, does represent the 

 effect of the heat-motions on the intensity of reflexion. We are therefore justified 

 in using a formula of this type to calculate from the observed scattering powers those 

 corresponding to the atoms in a state of rest. If the atoms have energy of vibration 

 at the absolute zero of temperature, a larger correction will be necessary than if they 

 are at rest. The correction can be made for the two cases if we assume the zero point 

 energy to be half a quantum per degree of freedom. Waller has shown how to calculate 

 the scattering power of an atom, given the Schrodinger charge distribution for it, and 

 Hartree has determined the approximate Schrodinger distributions for the ions 

 Na* and Cl~. The calculated scattering curves deduced from these results agree very 



