256 Mr. W. T. David on Thermal 



Summary, 



Ionization methods for measuring contact potential are- 

 described and found to be exceptionally efficient. 



A study of the rate of charging due to contact potential 

 in the case of an insulated plate in ionized gas leads to new 

 direct evidence of the presence of charged surface layers. 

 The time necessary for their formation and change is found 

 to be observable, and effects due to their capacity are shown 

 to be detectable. Their behaviour agrees with the conception' 

 that contact potential phenomena can be explained by the- 

 different amounts of work done by electrons in escaping from 

 different metals. 



The rate of charging or removal of the layers responsible 

 for contact potential is shown to be apparently independent 

 of the metal and identical with the behaviour of ordinary 

 charges imparted by external means. 



The influence of the dryness of the gaseous medium and 5 

 the nature of the gas is examined, and it would appear that 

 although chemical action may cause large contact effects, it 

 is not usually responsible for more than a small part of the 

 phenomenon. 



My best thanks are due to Professor Sir J. J. Thomson 

 for his suggestion of the research and kind interest in the- 

 experiments. 



The Cavendish Laboratory, 

 November 1, 1912. 



XXX. Thfrmal Radiation from Hot Gases. 

 By W. T. David, B.A., B.Sg,, Trinity College, Cambridge *. 



THE radiation emitted by a substance constitutes in* 

 general either a continuous spectrum or a line or band: 

 spectrum. 



Continuous spectra are generally emitted by solid bodies 

 when heated. It seems probable that these continuous 

 spectra are emitted, not by the intra-atomic corpuscles or 

 electrons, but by the free corpuscles which determine the 

 conductivity of metals. These corpuscles possess high velo- 

 cities, and during collision with an atom at the end of their 

 free flight, pulses are emitted along the Faraday tubes attached 

 to them. This is also the theory of the origin of Rontgen 

 rays. In this case, however, the velocity of the corpuscle- 

 (cathode rays) before striking the molecules is enormously 



* Comnmnicatod bv the Author. 



