FOOTE AND MOHIvER: IONIZATION 439 



(i) A positively charged molecule (Hg Cl2) + 



(2) A positively charged molecule of mercurous chloride and 

 a negatively charged chlorine atom (Hg CI) + + (CI) ~ 



(3) A doubly positively charged mercury atom and two nega- 

 tively charged chlorine atoms (Hg)++ + (Cl)~ + (Cl)~ 



Since the chlorine is undoubtedly bound to the mercury as 

 atoms rather than as a molecule there would not be an imme- 

 diate production of molecular chlorine. If molecular chlorine 

 were produced it would require a secondary reaction of two 

 chlorine atoms, quite apart from the phenomenon of ionization. 

 The ionization by method (3) would appear improbable in low 

 voltage arcs. The two chlorine atoms are probably joined to 

 opposite sides of the mercury atom. Hence in order that the 

 impacting electron may eject both chlorine ions, it must first 

 collide with the molecule and eject one chlorine ion, then pass 

 through the mercury atom before its electric field may exert an 

 appreciable influence on the second chlorine ion. But to pass 

 through the mercury atom would require a velocity approaching 

 that of a beta particle. Hence while this type of ionization might 

 exist in cathode ray phenomena it would not appear possible 

 at one collision in ordinary arcs. 



The production of a doubly charged mercury atom, however, 

 might be developed by successive collision or by absorption of 

 radiation followed immediately by collision in an arc of high 

 current density. The process occurs in two steps. A colli- 

 sion with an impacting electron of suitable velocity or the ab- 

 sorption of radiation of the proper frequency may cause the 

 ejection of one chlorine ion resulting in ionization by method 

 (2). Before the positively charged mercurous chloride thereby 

 produced, recombines, it may collide with a second electron 

 and lose the second chlorine ion. Hence even if ionization 

 finally resulted in a doubly charged mercury ion and two chlorine 

 ions, measurements of ionization potential of mercuric chloride 

 would show two inelastic collisions, the first giving the energy 

 required for the step (HgClo) — ^ (HgCl)+ -f (CI)- and the 

 second for the step (HgCl)+ — ^ (Hg)++ + (CI)-. 



In general, since this latter step is a secondary process which 



