# Page:Popular Science Monthly Volume 78.djvu/582

A discovery of inestimable value is due to Kaufmann; he found that the mass of the ${\displaystyle \alpha }$ particle varied with the velocity. This discovery all but consecrated the old theory that mass, that gravitation, is a form of energy, a mode of motion. There can not be a manifestation of energy when the motion is uniform. No motion of energy can be evidenced without acceleration followed by arrest and a reversal of motion, or