CONTEMPORARY ADVANCES IN PHYSICS 209 



with which to go wandering around in the gas. They had not speed 

 enough to wander far, even in the half-a-second afiForded them before 

 the condensation. Probably they had already adhered to molecules 

 before the condensing water immobilized them. One speaks of the 

 droplets as being condensed partly on negative, partly on positive 

 ions; the last-named are the molecules from which the electrons were 

 reft. (If, during the half-a-second, an electric field of suitable strength 

 is applied, the ions of the two signs drift in opposite ways, and when 

 the water-vapor comes down there are seen two parallel trails of 

 droplets with an empty space between.) 



The simplest idea is that the traversing particle tears off one electron 

 from each of many molecules through or near which it passes, and 

 that half of the droplets are formed on these electrons and the other 

 half upon the molecules bereft. This is too simple to be true. It is 

 likely that sometimes the particle removes two electrons or more from 

 a single molecule, so that there well may be more negative ions than 

 positive. Much more serious is the certain fact that often when an 

 electron is thus released by the direct action of the traversing particle, 

 it shoots away with speed and energy enough to enable it to release 

 one or several more from neighboring molecules. Now and then one 

 comes on a cloud-chamber photograph in which there appears a track 

 with branches (Fig. 11) ; each of these is the trail of an electron which 



Fig. 11 — Tracks of a charged particle bristling with short branching tracks, made by 

 electrons ejected from atoms with energy sufficient to ionize. (Auger) 



has received a truly abnormal and extraordinary amount of energy. 

 Much commoner, in fact universal, is the "beaded" appearance of 

 such trails as appear in most of the pictures of this article: it is pre- 

 sumed that each of the beads is an unresolved cluster of droplets 

 formed on a cluster of ions, all but one pair of them made in the 

 indicated way. Occasionally one sees a picture in which the interval 

 allowed for diffusion has been so happily chosen that the droplets in 

 the clusters are far enough apart for counting, and yet consecutive 

 clusters do not overlap. In making Fig. 1 the interval allowed was a 

 little too long, and yet perhaps it is possible to think that the ions 

 are denser in some parts of the trail than in others, as though they had 

 been formed in clusters which have broadened almost but not quite 

 to the point of losing their identity. 



