From Today in Science History:
Joly was an Irish geologist, physicist and inventor whose interests spanned several fields. Using Edmond Halley's method of measuring the degree of salinity of the oceans, and then by examining radioactive decay in rocks, he estimated Earth's age at 80-90 million years (1898). Later, he revised this figure to 100 million years. He published Radioactivity and Geology (1909) in which he demonstrated that the rate of radioactive decay has been more or less constant through time. He also developed a method for extracting radium (1914) and pioneered its use for cancer treatment.
