The American radiochemist Bertram Borden Boltwood (1870-1927) discovered the parent of radium and developed a method of geological dating.
Bertram Boltwood was born on July 27, 1870, in Amherst, Massachusetts. His paternal ancesters had come from Great Britain two centuries earlier and were a prominent family in New England. His father, a lawyer, died when "Bolty" was two years old, and the boy, an only child, was raised by his mother in her home town of Castleton-on-Hudson, New York. His mother's family had come to America from Holland, also in the 17th century.
Boltwood's mother was not wealthy, but she was affluent enough to send him to private school and, befitting her social position, destined him to attend Yale, his father's college. He entered Yale's Sheffield Scientific School in 1889 and upon completion of the three-year program took highest honors in chemistry. Next came two years of postgraduate work in Munich, where he specialized in analytical techniques and in the chemistry of the rare earth elements. In 1894 Boltwood returned to Yale, working as a laboratory assistant while studying for his doctoral degree. This program was enriched by a semester of physical chemistry in Ostwald's laboratory in Leipzig, and Boltwood received the Ph.D. from Yale in 1897.
Even before graduation Boltwood had served as an instructor in analytical chemistry in the Sheffield Scientific School. He continued in this post, and later in physical chemistry with the same rank, until 1900, when he established a mining engineering and chemistry partnership with a schoolmate. While at Yale his mastery of chemical techniques made him a great resource for his colleagues. Those years were also devoted to translating German chemical texts and to improving laboratory apparatus: his automatic Sprengel pump, new design for a water blast, and lead fume pipe for the Kjeldahl nitrogen determination apparatus date from this period, while his invention of Boltwax, a product useful for vacuum seals, came later.
As a businessman with a private laboratory in New Haven in 1900, Boltwood analyzed ore samples sent by his partner working in the field in the Carolinas. Many of these samples contained rare earth elements and also uranium and thorium. The radioactivity of uranium had been discovered by Henri Becquerel in Paris in 1896, and that of thorium independently by Gerhard C. Schmidt and Marie Curie in 1898. Boltwood's interest in the rare earths, his expertise in analytical and physical chemistry, and his familiarity with such ores as monazite and uraninite made it almost inevitable that he would become fascinated with the popular science of radioactivity.
In a series of papers in 1902 and 1903 Ernest Rutherford, a physicist, and Frederick Soddy, a chemist, explained the phenomenon of radioactivity as the spontaneous disintegration of an atom and its transmutation into another element. The evidence for their theory was compelling, but mostly of a physical character. When Boltwood first began his radioactivity research in 1904, he felt that chemical evidence would provide essential confirmation. If the ratio of the amounts of radium and uranium in old, unaltered minerals was constant, this would imply a genetic relationship wherein uranium decayed in a number of steps to form radium, and this element in turn itself decayed to form other products.
Separating and measuring the minute traces of radium was all but impossible. But radium's first daughter product is an inert gas called emanation, which could easily be collected and which would give an accurate indication of the quantity of its parent that was present. Boltwood used a gastight electroscope to measure the radioactivity of the radium emanation and showed it to be directly proportional to the amount of uranium in each of his many samples.
He next decided to go beyond this circumstantial evidence of a connection between these two elements: he would seek direct proof by trying to "grow" radium. Uranium X was the only radioelement known to be between them, and its short half life meant that radium should be formed rather quickly. Yet, after a year, Boltwood remained unable to detect any radium emanation in his uranium solution. Since his faith in the disintegration theory of radioactivity did not waver, he could only conclude that an unknown, long-lived product lay between uranium X and radium, preventing rapid accumulation of the latter.
Boltwood's search for the parent of radium was interrupted by his appointment as assistant professor of physics at Yale College in 1906 and by the responsibility that fell upon him to supervise extensive renovations in the old laboratory. When he resumed his investigations he was inclined to accept actinium, one of the several new radio-elements discovered in that period, as the parent. The problem he faced, however, was that his own work, and that of such other radiochemists as Soddy and Herbert N. McCoy at the University of Chicago had determined the activities of members of the decay series relative to the first element in each series; if Boltwood placed actinium in the uranium series the sum of the constituents' activities would be greater than that of the mineral that contained them. A closer look at actinium showed it to have mixed with it a tiny amount of another radioelement bearing the chemical properties of thorium. Named ionium by Boltwood in 1907, it was indeed the immediate parent of radium.
This intensive study of the radioelements highlighted several examples of products that differed in origin, in half life, and perhaps in emissions, yet seemed to have identical chemical properties. Radiothorium, ionium, and uranium X, for example, were to the chemist thorium. Boltwood did not contribute to the conception in 1913 by Kasimir Fajans, and subsequently by Soddy, of isotopes and of the group displacement laws that regulated radioactive decay in the periodic table of the elements, but these theories were rooted in his experimental discoveries.
Other results of his familiarity with the radioelements were his belief that inactive lead was the end product of at least the uranium decay series and his observation that the geologically older rocks contained greater amounts of lead. From this he could estimate the rate at which lead accumulated. With encouragement and ideas from Rutherford, who by now was a good friend and a collaborator-by-post, Boltwood "inverted" the method and used the measured quantity of lead to calculate the age of rocks. This was a striking application of science, all the more so because his billion-year span for the age of the earth contradicted the conventional wisdom of only some tens of millions of years. By the 1930s, more data and a better understanding of isotopes brought widespread acceptance of this technique.
Except for a year (1909-1910) in Rutherford's Manchester laboratory, Boltwood remained at Yale for the rest of his life. He was made a full professor of radiochemistry in 1910, and in 1918 he was appointed director of the Yale College chemical laboratory. In that capacity he not only presided over the merging of the college and Sheffield chemistry departments, but over the design of a new university laboratory. His fame as America's leading investigator of radioactivity brought him election to the National Academy of Sciences and other organizations. Overwork seems to have caused a breakdown in his health, and his normally exuberant personality was clouded increasingly by periods of depression, culminating in his suicide in the summer of 1927.
The most extensive treatment of Boltwood's work and where it fits into contemporary science may be found in Lawrence Badash, Radioactivity in America: Growth and Decay of a Science (1979). The story of the lead-uranium method of determining geological age is told in Badash, "Rutherford, Boltwood, and the age of the earth: The origin of radioactive dating techniques," Proceedings of the American Philosophical Society, 112 (1968). Biographical and obituary notices by colleagues who knew him well include Rutherford, in Nature, 121 (1928); Alois Kovarik, in American Journal of Science, 15 (1928); and Kovarik, in Biographical Memoirs of the National Academy of Sciences, 14 (1930). A historian of science's perspective may be found in Badash, Dictionary of Scientific Biography, 2 (1970).