One of the cloud chamber photographs which in 1925 demonstrated the artificial transmutation of elem- ents. Alpha particles from a radioactive source travel up the picture. The alpha at left interacts with an atom of the nitrogen gas filling the cloud chamber. The alpha is captured by the nitrogen nucleus & transmutes it into a nucleus of a heavy isotope of oxygen (oxygen-17), together with a proton. The proton shoots off, leaving a faint track. The recoiling oxygen nucleus produces the thick track. The picture was one of 8 showing transmutation among the 23,000 cloud chamber photos taken by Patrick Blackett at the Cavendish Laboratory, Cambridge, in 1921-24.