A systematic presentation of experimental and theoretical work that establishes the discrete, atomic character of electric charge and explains techniques used to isolate and measure the elementary charge. It traces historical ideas about electricity, extends electrolytic laws to gas conduction, analyzes ionization by penetrating radiation, examines Brownian motion in gases, and considers whether the electron is divisible. Later chapters discuss atomic structure and the behavior of radiant energy. Mathematical proofs, experimental data, and technical derivations are collected in appendices to keep the main text accessible to non-specialist readers.