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 pair of pictures, alpha particles stream upwards through the chamber. One in each picture scatters a nucleus in the helium gas filling the chamber. The two particles move off at a 90-degree angle in each case, revealing that they have the same mass - alpha particles are helium nuclei. (The angle is in fact slightly less than 90 degrees because the tracks have been photographed at an angle).