ELECTRON, THE. One of the most important generaliza tions of all time is that of the electrical constitution of matter, for this conception underlies practically the whole of loth century physics. It is customary to consider that it was introduced during the last half of the last decade of the 19th century; yet, to choose any particular year, or even decade, for the introduction of this idea is to sacrifice accuracy, for, in general, advances in science do not take place at any given time. It is as difficult to determine when a particular conception arises as it is to fix the instant at which the babe acquires consciousness. The progress of science takes place, for the most part, by a process of infinitesimal accre tion. It is a process of growth, each experimenter adding a little to the structure reared by his predecessors. There is nothing in the history of science that illustrates this process more beauti