In a series of classic cloud chamber experiments in the late 1920s, English physicist Patrick Blackett obtained clear evidence of how alpha particles scatter from atomic nuclei of different masses. In this picture, alpha particles stream upwards through the chamber. One of them scatters a proton in the hydrogen gas filling the chamber. The lighter proton shoots up the picture, leaving a fainter track, while the heavier alpha is deflected only slightly, so that the angle between the tracks is noticeably less than 90 degrees.