I was approached by Arthur Compton, who was one of the two leading cosmic ray physicists, the other being Millikan, and the two had a constant feud. So Compton said 'You have proved that electrons should lose lots of energy in matter, in the atmosphere for instance. How could electrons get down to earth so easily? It must be something else. Cosmic radiation cannot be electrons and it cannot be gamma rays, because gamma rays would immediately make electrons. So,' suggested Compton, 'probably they are protons.' And I said 'Yes, probably they are protons.' And Compton went on to say 'Well, there is a phenomenon observed on cosmic radiation, namely that they are deflected in the earth's magnetic field,' I think to the east, I'm not sure, it could be to the west, 'and that indicates that they are positively charged and are... and therefore they ought to be protons.' So I think that was an important step by Compton, and we have a joint paper on... on that subject. Later on it turned out that many of the particles we see in the cloud chamber at sea level are neither protons nor electrons, but are mesons - but that's a different story, but I got interested in mesons very much.