CONTEMPORARY ADVANCES IN PHYSICS 



193 



At the beginning I place, as Fig. 1, a picture of the track of a cosmic- 

 ray particle believed to be an electron. Anyone who has ever studied 

 the pictures of cloud-chamber tracks will at once be impressed by 

 seeing how distinctly the droplets stand apart. This separation was 

 achieved by letting half a second elapse from the instant when the 

 electron shot through, to the instant when by expansion the gas of 

 the chamber grew suddenly cool and the water-vapor suspended in the 

 gas condensed itself as dewdrops on the ions. These ions, formed by 

 the passage of the electron, had been diffusing through the gas during 

 the half-second intervening, and the diffusion-process had served in 

 the main to carry them apart (though there must also have been cases 

 of ions approaching and possibly even combining with each other). 

 The counting of these droplets is germane to the question as to whether 

 the traversing particle was or was not an electron. This question, 

 however, we leave till later, and turn to photographs in which the 

 droplets of the tracks lie close together and are uncountable, because 

 the expansion took place before there had been time for much diffusion. 

 Tracks so formed have the advantage of sharpness over what they 

 lose in detail. 



Fig. 2 — ^Track of a particle, presumably a mesotron, traversing a metal plate without 

 sensible deflection. (Auger; Universite de Paris) 



Figure 2 presents the track of a particle which traversed a plate of 

 lead as it shot across the chamber. In passing through the lead, it 

 underwent no sensible deflection; no other particle sprang from the 

 lead; and there is nothing in the aspect of the track which differs on 

 the two sides of the metal. It would be more impressive yet to present 

 a similar picture for a particle traversing ten or fifty centimetres of 

 lead, but here the practical limitations on the size of a Wilson chamber 

 defeat the physicist, or at any rate no one has overcome them yet. 

 Ehrenfest has lately circumvented them by the laborious scheme of 

 setting up two Wilson chambers, one above the other, with as much as 

 9 cm. of lead or gold between them. However, the passage of single 



