SOME co.v//-:.u/'()/c.;/v'j' .inr.ixcrs ix riiysics ii \n 



anil tii.iiu- olliors lia\i- measured the charm's " of iniii/.ed atcinis in 

 (liM-harKe-lul)es, and found them sontelimes single and sometimes 

 multiple elect ron-eliar^rs, hut have not measured the minimimi energy 

 re<|uiriHl to produce a (larticidar kimi of ionization. H. i). Smyth. 

 .It I'rinceton and Ca\endi>h. was the first to cond)ine hoth methixis; 

 he ionized atoms by electron-impacts in a rube designe<l for determin- 

 ing ionization-poteiitials in the accepted manner, and after further 

 accelerating the ions drew them through a channel into a second 

 tulK' where they were deflected in a magnetic held so that their charges 

 could Ik- measured. The difficulty to be surmounted is that in the 

 first tul>e the pressure of the gas must be high enough to yield a satis- 

 facti>r>- number of ions, and in the second tube it must be low enough 

 not to interfere with the arcs described by the ions in the inagnetic 

 field. At first he sent a beam of mercury vajior rushing transverseh' 

 across his first tube from a boiler into a liquid-air trap: b>' first sending 

 the atoms down a long tube with a system of dia[)hragms and so 

 stopping the obliquely-mo\-ing ones, he was able to prevent atoms 

 from straying out of the beam in the critical zone. Later he attacked 

 a more tlifficult case, that of nitrogen; the gas was continuously fed 

 into the first tube and a powerful pump drew it out befori' it could 

 difTuse seriously^ into the second tube. 



While it is interesting to have direct cont'irm.iiidii iliat tlu- first 

 and easiest ionization is the extraction of a single electron, Smyth's 

 most important results refer to the later ionizations. Mercury 

 atoms that had lost two electrons appeared in the second tube when 

 the bombarding potential attained 10 \T)lts, nine volts more than 

 the fir>t ionizing-potential; at a much higher voltage, triply-charged 

 atoms were detected, or at least suspected. In nitrogen, the earliest 

 ionization, at about It) volts, docs not involve dissociation, but at a 

 potential 8 volts higher, a doubly-ionized single nitrogen, atom makes 

 its appearance, and a little further along, Smyth delects an ion which 

 may be a singly-ionized nitrogen atom or a doubly ionized molecule 

 (the two possibilities cannot be discriminated by this method, but 

 the second seems improbable). Valuable knowledge about the rela- 

 tions between ionization and dissociation — between, that is, the 

 removal of an electron from a molecule, and the breaking of the bonds 

 that hold the atoms of the molecule together — may be expected from 

 experiments of this type. 



Something more is to be said on two of the topics of the last article 

 in this series. A. H. Compton's discovery that scattered X-rays 

 consist of two distinct radiations, one with the fre(|uency of the 



" Actually, the charge-mass ratios. 



