In 1917 (in experiments reported in 1919 and 1925), Rutherford proved that the hydrogen nucleus is present in other nuclei, a result usually described as the discovery of protons. These experiments began after Rutherford had noticed that, when alpha particles were shot into air (mostly nitrogen), his scintillation detectors showed the signatures of typical hydrogen nuclei as a product. After experimentation Rutherford traced the reaction to the nitrogen in air and found that when alpha particles were introduced into pure nitrogen gas, the effect was larger. In 1919 Rutherford assumed that the alpha particle merely knocked a proton out of nitrogen, turning it into carbon. After observing Blackett's cloud chamber images in 1925, Rutherford realized that the alpha particle was absorbed. After capture of the alpha particle, a hydrogen nucleus is ejected, so that heavy oxygen, not carbon, is the end result i.e. Z is not decremented but incremented (see initial proposed reaction below). This was the first reported nuclear reaction, 14N + α → 17O + p. Rutherford at first thought of our modern "p" in this equation as a hydrogen ion, H+.