<P> Dalton also believed atomic theory could explain why water absorbs different gases in different proportions . For example, he found that water absorbs carbon dioxide far better than it absorbs nitrogen . Dalton hypothesized this was due to the differences between the masses and configurations of the gases' respective particles, and carbon dioxide molecules (CO) are heavier and larger than nitrogen molecules (N). </P> <P> In 1827, botanist Robert Brown used a microscope to look at dust grains floating in water and discovered that they moved about erratically, a phenomenon that became known as "Brownian motion". This was thought to be caused by water molecules knocking the grains about . In 1905, Albert Einstein proved the reality of these molecules and their motions by producing the first Statistical physics analysis of Brownian motion . French physicist Jean Perrin used Einstein's work to experimentally determine the mass and dimensions of atoms, thereby conclusively verifying Dalton's atomic theory . </P> <P> The physicist J.J. Thomson measured the mass of cathode rays, showing they were made of particles, but were around 1800 times lighter than the lightest atom, hydrogen . Therefore, they were not atoms, but a new particle, the first subatomic particle to be discovered, which he originally called "corpuscle" but was later named electron, after particles postulated by George Johnstone Stoney in 1874 . He also showed they were identical to particles given off by photoelectric and radioactive materials . It was quickly recognized that they are the particles that carry electric currents in metal wires, and carry the negative electric charge within atoms . Thomson was given the 1906 Nobel Prize in Physics for this work . Thus he overturned the belief that atoms are the indivisible, ultimate particles of matter . Thomson also incorrectly postulated that the low mass, negatively charged electrons were distributed throughout the atom in a uniform sea of positive charge . This became known as the plum pudding model . </P> <P> In 1909, Hans Geiger and Ernest Marsden, under the direction of Ernest Rutherford, bombarded a metal foil with alpha particles to observe how they scattered . They expected all the alpha particles to pass straight through with little deflection, because Thomson's model said that the charges in the atom are so diffuse that their electric fields could not affect the alpha particles much . However, Geiger and Marsden spotted alpha particles being deflected by angles greater than 90 °, which was supposed to be impossible according to Thomson's model . To explain this, Rutherford proposed that the positive charge of the atom is concentrated in a tiny nucleus at the center of the atom . Rutherford compared his findings to one firing a 15 - inch shell at a sheet of tissue paper and it coming back to hit the person who fired it . </P>

Who determined the mass of the electron (points 3)