Contemporary Advances in Physics — XI 

 Ionization 



By KARL K. DARROW 



TONIZATION, in its most general sense, signifies a segregation of 

 -*■ positive from negative charge within the volume of a substance 

 which as a whole is (or initially was) electrically neutral. In practice 

 a gas (for instance) is said to be ionized if charges of either sign can 

 be extracted from it. Charged particles of both signs, electrons and 

 ions, can be drawn out from a gas in which an electrical discharge is 

 being maintained; in such a condition, therefore, a gas is ionized. 

 Millikan's droplets, floating around in a gas which had recently been 

 irradiated, absorbed charges of either sign out of the gas, which there- 

 fore was ionized by the radiation and remained ionized for some 

 time afterward. A negatively-charged electrode immersed in a 

 carefully-screened gas receives very little charge from it; this condi- 

 tion continues if the gas is bombarded with electrons having less than 

 a certain speed ; let the speed of the bombarding electrons be increased 

 past this limit, and the electrode begins to receive positive charge — 

 the gas is ionized by the electrons. Dilute electrolytic solutions are 

 evidently in a continual and spontaneous state of ionization. 



Observations on positive ions issuing from ionized gases have 

 been interpreted as meaning that all such ions are atoms or molecules 

 bearing charges of which the magnitude is e, or 2e, or some other 

 small-integer multiple of e; in other words, as meaning that positive 

 ions are atoms or molecules from which one or more electrons have 

 been detached. Generalizing from these to all cases, it is believed 

 that the first stage of ionization, in monatomic gases at least, is the 

 detachment of electrons from atoms. Whether the separated elec- 

 trons remain free, or attach themselves to other atoms, or become 

 the gathering-agents of clusters of atoms, is an interesting but at 

 present subsidiary question. Ionization in monatomic gases begins 

 by the detachment of electrons from atoms; and the word "ionization" 

 in fact is frequently used to mean this process alone. In diatomic 

 and compounded gases, the nature of the ions observed permits either 

 of two suppositions; the initial process of ionization may be the 

 detachment of electrons from molecules, or the splitting of molecules 

 into fragments each consisting of one or more atoms, some of these 

 fragments having an excess and the others a compensating deficit of 

 electrons. Special experiments must be performed to decide between 

 these suppositions. 



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