The author presents a systematic account of experimental investigations into radio-active substances, tracing early observations of penetrating emissions from uranium compounds and the methodological shift that treats radioactivity as an atomic property. The work describes the isolation and chemical preparation of a highly radio-active element and its salts, procedures for determining its atomic weight, and comparative measurements across related substances. It also reviews contemporaneous physical studies on emitted radiations, experimental techniques, and unresolved questions, offering both detailed laboratory results and a concise survey of the field's developing problems.