Bertram Borden Boltwood | Biography

At the turn of the twentieth century, when the science of radioactivity was still young, Bertram Boltwood was considered to be the United States' foremost authority. This was a reputation he had earned, along with Great Britain's Ernest Rutherford, by advancing the experiments of Henri Becquerel, Marie Curie, and Pierre Curie. Among his accomplishments was the proof of a radioactive series (the transformation of one radioactive element into another) that led eventually to a reliable method for determining the age of the Earth.

Boltwood came from an academic family: his grandfather had helped found Amherst College in Massachusetts, and his cousin was the poet Ralph Waldo Emerson (1803-1882). Though his father died when Bertram was only two, his mother continued her husband's legacy of education. He entered the chemistry department of Yale University in 1889, graduating at the top of his class three years later. After spending two years in Germany at the Ludwig-Maximilian University in Munich, Boltwood enrolled in graduate classes at Yale, receiving his Ph.D. in 1897.

Boltwood spent the next few years as an academic consultant to Yale 's departments of analytical and physical chemistry. He also spent a good deal of time and effort designing new laboratory apparatus and teaching materials.

In 1900 Joseph Hyde Pratt, another Yale graduate, teamed up with Boltwood to form a partnership of "mining engineers and chemists," in which Pratt collected ore samples that Boltwood would later examine in the laboratory. While this enterprise earned the two young scientists a moderate living and, perhaps more importantly, gave Boltwood valuable experience in the handling of radioactive materials.

About this time Rutherford and his colleague Frederick Soddy proclaimed that radioactive elements decayed, and in that decaying process, transmuted into other elements, both active and inactive. The evidence they presented to support their claim was substantial; however, Boltwood felt that he could express the relationship between radioactive elements more clearly and irrefutably. Thus, in 1904, he turned his research efforts toward radioactivity, a direction from which he would never again stray.

Boltwood began by showing that, in a given amount of uranium, a constant amount of radium was present; if the amount of uranium were increased, the amount of radium would increase proportionally. It seemed clear, then, that radium must be a "daughter" element of uranium. Still, there seemed to be a missing link--an element that existed between uranium and radium in the radioactive chain. Boltwood never found this intermediate element, although it was later discovered to be a form of thorium.

Though other scientists had discovered the "parent of radium," Boltwood continued his experiments with radioactive isotopes. He began to notice that traces of lead could always be found in uranium-containing minerals. Using this as a springboard he theorized that lead was the ultimate final step in the transmutation of uranium; this was supported by the fact that the older a mineral sample was, the greater was the amount of lead.

The most important application of this research was the development of the uranium-lead method of geologic dating. Until this time, the best estimate for the age of the Earth was that by William Thomson (Lord Kelvin). Using the temperature of the Sun as a starting point, Thomson determined the amount of time necessary for our planet to cool to its present temperature. The age he arrived at was 40 million years--a number considered by most geologists to be far too low. Rutherford, too, attempted to date the Earth, this time by determining the amount of helium (also a radioactive decay product) in the crust; however, this method was found to be unreliable as well.

Boltwood first determined the amount of time it took for lead to be produced from uranium--about one billion years for one gram of uranium to produce 0.15 grams of lead. Using this formula he tested rock samples of many different ages, ranging from millions to billions of years old. However, none older than 3.7 billion years could be found; this number agreed with geologists' theories, and it became the accepted age of the Earth. Later experiments on meteorite and lunar rock samples showed them to be older than the Earth by 0.9 billion years. The explanation for this is that the solar system was formed nearly 5 billion years ago; the Moon and meteorites cooled 4.7 billion years ago, but the Earth remained a liquid ball for 0.9 billion years, with the first rocks forming about 3.7 billion years ago.

In 1910 Boltwood was given a full professorship at Yale; along with this position went the responsibility for the construction of a new physics laboratory. As his reputation grew he was also requested to speak to many different organizations, as well as act as a consultant to engineers and other laboratories. In 1918 he was made the director of the Yale chemical laboratory and was required to design its new Sterling Chemistry Laboratory. Though he completed this task, it exacted a heavy mental and physical toll on him. Boltwood suffered a nervous breakdown, and throughout his remaining years experienced periods of intense depression. It was during such a period in 1927 that he committed suicide.