July 27th

Bertram Borden Boltwood was born on this day in 1870. He was  an American chemist and physicist whose studied the radioactivity of uranium and thorium, and their resulting products, which laid a foundation for the concept of isotopes. Boltwood traced each “radioactive series” of radioactive elements in rocks as they sequentially disintegrated into other isotopes or elements. He noted (1905) that the lead, a stable element, always found in such ores, was apparently the ending product of radioactive decay. Since each step in the decay occurs with a known half-life rate, he proposed (1907) that the ratios of the original radioactive elements to their various decay products could be used for dating rocks. Thus, with these measurements of the composition of such ores in the earth’s crust, the age of the planet could determined. He estimated 2.2 billion years. Overwork, stress and depression led to suicide.