CONTEMPORARY ADVANCES IN PHYSICS 155 



carry the screen of lead over the top of the air-chamber — or, one may 

 float the apparatus on or just below the surface of the lake — or, one 

 may rely exclusively on observations made in the upper air. But it is 

 likely that part of the unknown radiation comes very obliquely down- 

 ward through the atmosphere, and therefore is liable to be obstructed 

 by lateral walls of lead which cannot be removed without admitting to 

 the apparatus other rays which come very obliquely upward from the 

 earth. So, every sort of screening by water or by metal probably keeps 

 out some of the rays which are wanted, along with those which are 

 unwanted. Only the observations in the uppermost air may give the 

 full effect of the mysterious radiation, and perhaps not even they, for 

 the walls of the chamber cannot be reduced to infinite thinness. 



How great is the ionization left over to be ascribed to cosmic rays, 

 after Restgang and radioactivity are allowed for? The absolute value 

 is still a subject of controversy; fortunately it is a minor matter, 

 knowledge of which is not required for solving the major problems; 

 thus, to determine the penetrating power of the rays one needs only 

 relative values at various depths of water. Concerning it I will merely 

 state, that in the atmosphere near sea-level, the nntnber of ions of either 

 sign appearing per cubic centimetre per secoyid, and attributed to cosmic 

 rays, averages about one and o?ie-half. Say we have an ionization- 

 chamber of volume U (in cubic centimetres), at sea-level, filled with air 

 at atmospheric pressure. Its charged electrodes will be discharged 

 at a certain rate; part of this rate of discharge can be explained in the 

 ways aforesaid; the rest, the inexplicable residue, will amount to 

 about l.5Ue. This residue is the evidence for the cosmic rays. The 

 electron-charge e is so small, that even when the volume of the chamber 

 is thousands of cubic centimetres the product (l.SUe) is no great 

 quantity of electricity. High up, the effect is much greater; according 

 to Piccard it is so great at sixteen kilometres over sea-level, that if the 

 air up there were as dense as it is by the sea, there would be 200 ions of 

 either sign appearing per cc. per second. 



Stating the effect in ions per cc. per second has one disadvantage: it 

 suggests that in each cubic centimetre of an ionization-chamber the 

 ions are created at random, — one in the middle, perhaps, at a certain 

 instant, the next somewhere else two-thirds of a second later, the next 

 yet somewhere else two-thirds of a second later yet, and so on. What 

 happens, however, is this: at practically a single instant hundreds, or 

 even thousands, of ion-pairs are created along a single straight line 

 traversing the chamber; this is followed by many seconds, or even 

 minutes, during which nothing happens; then there is another such 

 event, a train of ions suddenly appearing along a straight line, not 



