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LXX. Dissociation of Hydrogen and Nitrogen by Electron 

 Impacts. By A. Ll. Hughes, JD.Sc, .Research Professor 

 of Physics, Queens University, Kingston, Canada*. 



Hydrogen. 



LANGrMUIR t found thai hydrogen could be dissociated 

 by contact with a tungsten (or other metallic) wire 

 when its temperature was raised above 1300° K. He cal- 

 culated from his experiments that the work necessary to 

 dissociate a gram molecule of hydrogen was 84,000 calories. 

 This may be expressed in terms of the energy necessary to 

 dissociate a single molecule. It is the energy which an 

 electron would acquire in falling through a potential dif- 

 ference of 3' 6 volts. 



It occurred to the writer that possibly the molecules 

 could be dissociated by direct impact of electrons possessing 

 energy in excess of that corresponding to 3*6 volts. The 

 experiment failed to show any appreciable dissociation by 

 electrons whose energy corresponded to 3*6 volts. It, how- 

 ever, led to an investigation of the disappearance of hydrogen 

 when subjected to bombardment by electrons possessing 

 higher energies. A number of investigations have from 

 time to time been carried out on the reduction in pressure 

 when an electric discharge is passed through a gas. In some 

 respects, the central idea of the present investigation is new. 

 The experimental conditions are simplified so as to give as 

 direct information as possible as to the ratio of the number 

 of molecules disappearing from the gas in terms of the 

 number of collisions between electrons and molecules, for 

 different values of the electron velocities. A stream of 

 electrons is passed through hydrogen and the progressive 

 decrease in pressure noted. It is assumed, for reasons 

 discussed later, that the hydrogen which disappears, does so, 

 because it is dissociated into atoms which condense on the 

 walls when they strike them. 



Apparatus. — The final form of apparatus used is shown in 

 fig. 1. The earlier experiments showed the necessity for 

 trapping the atomic hydrogen as completely as possible. 

 This was secured (as in some of Langmuir's experiments) 

 by keeping the experimental tube in liquid air throughout a 

 set of observations. This called for a source of electrons 



* Communicated by the Author. 



f Langmuir, Journ. Amer. Chem, Soc. xxxvii. p. 451 (1915), 



