CONTEMPORARY ADVANCES IN PHYSICS 211 



particle-momentum (equation 2). If by luck an experimenter should 

 happen upon two tracks having the same curvature but made by 

 particles having masses ^ standing to one another in the ratio (say) 

 100 : 1, the speeds would stand to one another in the ratio 1 : 100, 

 and this might well entail a perceptible difference in the primary 

 ionization. It would come to the same thing, if someone should take 

 the data for a large number of tracks, and plot primary ionization as 

 function of curvature: if there are really two kinds of particle differing 

 in mass, there should be two sets of points lying along two curves, 

 and from the ordinates of these curves at any abscissa the ratio of 

 the masses would be derivable. 



Perhaps the last sentence suggests that someone already has made 

 this correlation, and has found that the points for all of the single or 

 penetrating particles lie upon one curve, and all the points for shower- 

 particles and shower-producing particles lie on another. This has 

 not been done. The reason is, that many of the penetrating particles 

 exhibit no perceptible curvature of track at all, and most of the others 

 a very small curvature. The former are moving so fast that their 

 momentum cannot even be estimated, except as being beyond a certain 

 critical value. As for the latter, the speeds of even these are so great 

 as to approach the speed of light; for a given momentum-value the 

 speed varies only a little with the mass, and the primary ionization 

 varies too little to serve as an index of mass. To make a profitable 

 correlation, one must use only the particles of which the tracks are 

 notably curved. Nearly all of these are shower-particles, which 

 already are presumed to be electrons. To find a penetrating particle 

 with a highly-curved track, one must find it when it is near to the end 

 of its course and its energy wellnigh gone. Such is the principle 

 which directed some of the recent successful searches for particles 

 proclaiming themselves by their ionization to be more massive than 

 electrons. 



Before looking at the track of one of these particles, we ought to 

 notice a couple of questions concerning ionization. One of them is: 

 is the distinction between primary and entire ionization — or rather, 

 our lack of perfect ability to make it in practice — likely to lead to 

 trouble? Many observers are far from clear in reporting whether 

 what they observe is more like the one or more like the other; but it 

 seems probable that the second like the first is dependent only upon 

 the speed and the charge of the traversing particle, not on the mass 

 thereof; and this diminishes the dangers from confusing the two. 

 The question is implicated with the second: to what extent do experi- 



* Allowance being made for the relativistic dependence of mass on speed. 



