Radioactivity was first observed in 1896 by Henri Becquerel who demonstrated that uranium would fog photographic film. Shortly thereafter, it was learned that thorium and actinium were also radioactive and that there were long series of radioactive substances associated with each of these three elements. It was found that the parent elements changed successively from element to element by the emission of alpha, beta, and gamma radiations until some stable end-element was produced; in the case of these three series, the stable end-element was an isotope of lead. These naturally occurring series elements, through their radioactive daughter gases radon and thoron, have an appreciable effect upon background or air activity. These activities are washed down with rain and are carried into water supplies.