Boltwood, Bertram Borden

(b. Amherst, Massachusetts, 27 July 1870; d. Hancock Point, Maine, 14/15 August 1927)

radiochemistry.

Scientists spent the first several years following Henri Becquerel’s discovery of radioactivity in 1896 largely in studying the physical properties of the radiations. By 1904, however, enough radioelements had been found to shift their interest to the bodies emitting these radiations. In the chemical identification of the radioelements and in positioning them in proper sequence in the decay series, Boltwood was an equal among such first-generation radiochemists as Otto Hahn, frederick Soddy, Friedrich Giesel, and Herbert N. McCoy.

Boltwood’s paternal ancestors came to America from Great Britain in the mid-seventeenth century, They settled in New England, where for several generations they were farmers, millers, and blacksmiths. One was able to work his way through Williams College, graduating in 1814 and later becoming a lawyer in Amherst, Massachusetts. This was Lucius Boltwood, Bertram’s grandfather, who was active in the founding of Amherst College and served as its secretary from 1828 to 1864. He also was a candidate for the governorship of Massachusetts in 1841. Bertram’s father, Thomas Kast Boltwood, graduated from Yale College in 1864, received a degree from the Albany Law School in 1866, and practiced his profession until his untimely death in 1872. Bertram, an only child, thereafter was raised entirely by his mother, Margaret Van Hoesen Boltwood, in her native village of Castleton-on-Hudson, New York. The Van Hoesens, of Dutch stock, were among the early settlers of Rensselaer County during the seventeenth century.

Bertram Boltwood grew up in comfortable surroundings, attended a private school, and from 1879 to 1889 prepared at the Albany Academy for Yale. The intellectual stature of his family, represented by cousin Ralph Waldo Emerson and uncle Charles U. Shepard, a professor of mineralogy at Amherst, presumably had great influence on him, although his childhood was characterized more by fun and practical jokes than by scholarship. Nevertheless, he entered Yale’s Sheffield Scientific School in 1889, majoring in chemistry. Upon completion of the three-year course, Boltwood took highest honors in his subject and then departed for two years of advanced study at the Ludwig-Maximilian University in Munich. The training he received there under Alexander Krüss in special analytical methods and in the rare earths was to prove valuable in later years.

Boltwood returned to Yale in 1894 as a laboratory assistant in analytical chemistry and also to pursue graduate research. His work on double salts was accomplished under the direction of Horace L. Wells, who became his thesis adviser. In 1896 Boltwood spent a semester at the University of Leipzig, where he studied physical chemistry in Ostwald’s laboratory, and then returned to Yale, where he received the Ph.D. in June 1897. A strong attachment brought Boltwood back to Europe several times in later years; his exuberant personality, his lifelong bachelorhood, and his height—well over six feet—made him both distinctive and welcome there.

Following graduation, Boltwood remained at the Sheffield Scientific School as an instructor in analytical chemistry, a position he had assumed a year earlier; later he was an instructor in physical chemistry. Until 1900, when he established a private laboratory as a consulting chemist. Boltwood devoted himself to perfecting laboratory apparatus and techniques and supplying teaching materials for students. He devised a simple automatic Sprengel pump, a new form of water blast, a lead fume pipe for the Kjeldahl nitrogen determination apparatus, and, somewhat later. Boltwax, a wax with low melting point, useful for vacuum seals. He also translated German texts on physical chemistry and quantitative analysis by electrolysis. Boltwood’s eager acquisition of new techniques made him a storehouse of information upon which his colleagues often drew. In later years he conducted demonstration classes in laboratory arts for research students.

In 1900 Boltwood left Yale and, with Joseph Hyde Pratt, also a Sheffield graduate established a partnership: Pratt and Boltwood. Consulting Mining Engineers and Chemists. Pratt worked in the field, mostly in the Carolinas, and sent ore samples for analysis to Boltwood’s private laboratory in downtown New Haven. Many of these samples contained rare earth elements and uranium and thorium, with which they commonly are associated. In 1896 Becquerel had discovered the radioactivity of uranium, and in 1898 Gerhard C. Schmidt and Marie Curie independently found thorium to be radioactive. It was perhaps inevitable that Boltwood’s interest would turn in this direction, considering his early training in the analysis of rare earths, his inclination toward analytical and physical chemistry, his current familiarity with such ores as monazite and uraninite, and the challenge offered to his laboratory skill by work in radioactivity. He was not a total stranger to radioactivity, moreover, for in a senior thesis written under his direction in 1899, a student had repeated the Curies’ separation process for radium and had narrowly missed the discovery of actinium in the pitchblende residues. Upon André Debierne’s announcement of this new radioelement, Boltwood tested his student’s substance and confirmed actinium’s presence.

In April 1904, Boltwood began research on radioactivity. Not long before. Ernest Rutherford and Frederick Soddy had advanced a revolutionary new interpretation of this phenomenon: that radioactive atoms decay and transmute into other elements. While the evidence supporting this theory already was impressive, Boltwood reasoned that he could more strongly confirm it by showing a constant ratio between the amounts of radium and uranium in unaltered minerals. Such uniformity in composition would have to be accepted as proof of a genetic relationship, wherein the uranium decayed in several steps to form radium, which in turn decayed to from a series of several daughter products.

Boltwood quickly saw that the minute traces of radium, with chemical properties of its own, would be difficult to separate and test quantitatively. He therefore chose to measure radium’s first daughter product, emanation, as an indication of the amount of radium present. Emanation, being chemically inert and a gas, required only mechanical separation; its activity thus was easier to measure. Within a few months, Boltwood’s gas-light gold-leaf electroscopeyielded data showing that the activity of radium emanation was directly proportional to the amount of uranium in each of his samples. Rutherford, delighted with this news, encouraged Bolt wood to perform the same tests on minerals with much smaller percentages of uranium. Yet even with this further confirmation in hand, Boltwood decided that direct proof that uranium decays into radium was desirable-he would try to “grow” radium.

One of the steps in this effort was to determine the equilibrium amount of radium. Boltwood’s voluminous correspondence with Rutherford had ripened into a warm friendship: and the two collaborated, by mail, in the 1906 announcement that “the quantity of radium associated with one gram of uranium in a radio-active mineral is equal to approximately 3.8 × 10^-7) gram.” (The figure accepted today is 3.42 × 10^-7 gram.) But Boltwood’s attempts to grow radium were unsuccessful. Only one product between uranium and radium was then known; this was uranium X, whose short half-life should allow detectable quantities of radium to form within reasonable time limits. Yet even after more than a year, he was unable to observe any radium emanation in his uranium solution. Since Boltwood’s faith in the disintegration theory did not waver, he concluded that there must be a long-lived decay product between uranium X and radium that was preventing the rapid accumulation of the daughter product.

Boltwood’s search for this “parent of radium” was interrupted by his appointment as assistant professor of physics at Yale College. During the summer of 1906 he moved his apparatus into the Sloane Physics Laboratory and prepared to undertake his new academic duties. These responsibilities proved more extensive than anticipated, since, owing to the illness of the laboratory’s director, Boltwood’s close friend Henry A. Bumstead, he was left in charge of extensive renovations in the old building. Resumption of the search led him to Debierne’s actinium, and Boltwood indeed believed for a while that he had properly placed actinium in the decay series. Among others, Soddy in Glasgow was working on the same problem, and the two carried on a heated controversy in the pages of Nature. Rutherford also was disinclined to accept actinium as the parent of radium and based his objection on the relative activities of actinium’s products, a field in which Boltwood and McCoy had done basic work. The activities of many radioelements had been determined, relative to that of uranium, and if those of actinium and its products were added to those of uranium, uranium X, radium, radium emanation, radium A, B, and so on, the total would be far greater than that of the mineral which supposedly contained them all in secular equilibrium.

Further investigation showed Boltwood that his difficulty lay in accepting Debierne’s work on actinium as correct. In fact, there were other constituents in the Frenchman’s radioelement, one of them having chemical properties similar to those of thorium. It was this substance, named “ionium” by Boltwood in 1907, that was the immediate parent of radium. He had now proved that ionium grows radium; that uranium grows ionium had still to be shown to complete the direct proof of this relationship. Tests a few years later were unsuccessful due to the small quantity of ionium accumulated. Finally, in 1919, Soddy conclusively proved this relationship, using uranium purified many years earlier.

An outgrowth of this work was a superior method for the determination of the half-life of radium. Under Boltwood’s direction during the 1913–1914 academic year, a Norwegian chemist, Ellen Gleditsch, who had previously worked in Madame Curie’s laboratory, obtained a value of slightly under 1,700 years. Another result of this intensive study of the chemistry of the radioelements was the realization that many of these substances, which differed in type and intensity of radiation, nevertheless could not be separated chemically. Thus, beginning about 1907, Boltwood, as well as Hahn, McCoy, and most other radiochemists, recognized the inseparability of, for example, thorium, radiothorium, ionium, and uranium X. But it was not until 1913 that Kasimir Fajans and Soddy declared them to be chemically identical isotopes, and explained the decay sequence by the group displacement laws.

Just as the radioelements were related, Boltwood’s research activities bore logical connections. His first foray into the intricacies of the radioactive decay series in 1904 soon led him to examine the question of the inactive end products. Earlier analyses of uranium minerals showed that lead invariably appeared with the uranium. Between 1905 and 1907 Boltwood extended these observations and noted further that the geologically older minerals contained higher proportions of lead, as would be expected if this end product were accumulating over the ages. The thorium series was less well understood, and Boltwood at first doubted that it ended in lead, while actinium was not then recognized as forming part of a distinct series.

A direct result of this work was a striking application of science, the method of radioactive dating of rocks. If the rate of formation of an inactive decay product could be determined, the total amount found in a mineral would immediately yield its age. Both lead and helium (believed by most to be the alpha particle) were seen as suitable elements and, indeed, served in radioactive dating techniques. The helium method, pioneered in England by R. J. Strutt (later the fourth Baron Rayleigh), could not however, give more than a minimum age because a variable portion of the gas would have escaped from the rock. But the lead method, developed by Boltwood in 1907, proved satisfactory and is still in use today In effect. Boltwood reversed his procedure of confirming the accuracy of lead:uranium ratios by the accepted geological ages of the source rocks, and used these ratios to date the rocks. Because most geologists, under the influence of Lord Kelvin’s nineteenth-century pronouncements, inclined toward an age of the earth measured in tens of millions of years, Boltwood’s claim for a billion-year span was met with some skepticism. However, the subsequent work of Arthur Holmes, an understanding of isotopes, and the increasing accuracy of decay constants and analyses finally brought widespread acceptance of this method in the 1930’s

Boltwood’s major contributions lay in the understanding of the uranium decay series. Still, he was able to suggest, with Rutherford in 1905, that actinium is genetically related to uranium, though not in the same chain as radium, while in the thorium series he almost heat Hahn to the discovery of mesothorium in 1907. His other significant service to the study of radioactivity was to bring greater precision and advanced techniques into the laboratory, as in his insistence that only by complete dissolution and boiling of the mineral could all the emanation be extracted from radioactive bodies.

Boltwood remained at Yale the rest of his life, except for the academic year 1909–1910, when he accepted an invitation to Rutherford’s laboratory at the University of Manchester. Yale, fearing that he would remain in England indefinitely, offered Boltwood a full professorship in radiochemistry I his appointment brought him back to New Haven, but it also marked the end of his research career. Heavy academic duties, including supervision of construction of the new Sloane Physics Laboratory and unsuccessful efforts to obtain large quantities of radioactive minerals for research, seem to have taken all his time and energy. His stature as the foremost authority on radioactivity in the United States brought him membership in the National Academy of Sciences, the American Philosophical Society, and other organizations, but it also brought him numerous requests from prospectors, mine owners, speculators, chemical refiners, and wholesalers to analyze samples, devise separation processes, and find financial backing (from wealthy Yale alumni) for various projects. These efforts probably helped stimulate the production of radium, in which the United States led the world by about 1915, although they did not appreciably aid the progress of science.

In 1918 Boltwood was appointed director of the Yale College chemical laboratory and presided over the consolidation of the Yale and Sheffield chemistry departments. To cement this union, the new Sterling Chemistry Laboratory was proposed, and Boltwood was placed in charge of its design. He completed it successfully but the strain of this effort caused a breakdown in his health from which he never fully recovered. Periods of severe depression alternated with his more customary cheerful spirits, and resulted in his suicide during the summer of 1927.

Boltwood’s influence in radioactivity was widespread—through his published papers, correspondence, and personal contacts, for he trained surprisingly few research students. Part of his success stemmed from his close association with Rutherford, but like Rutherford’s other chemical collaborators, Soddy and Hahn, he was eminently capable of major contributions in his own right.

BIBLIOGRAPHY

I. Original Works. A reasonably complete list of Boltwood’s publications is in Alois F. Kovarik’s sketch of him in Biographical Memoirs of the National Academy of Sciences, 14 (1930). 69–96. His unpublished correspondence, papers, and laboratory notebooks are preserved in the Manuscript Room. Yale University Library His extensive correspondence with Rutherford is in the Rutherford Collection. Manuscript Room, Cambridge University Library.

II. Secondary Literature. In addition to Kovarik’s memoir (see above), the following obituary notices offer information about Boltwood: Yale Alumni Weekly, 37 (7 Oct. 1927), 65; Kovarik, in Yale Scientific Magazine, 2 (Nov. 1927), 25, 44, 46: Rutherford, in Nature, 121 (14 Jan. 1928), 64–65; Kovarik, in American Journal of Science, 15 (Mar. 1928). 188–198.

Lawrence Badash

Bertram Borden Boltwood

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.

A Scholar and Businessman

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.

Search Continued at Yale

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.

Further Reading

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). □

Boltwood, Bertram Borden (1870–1927) Professor of radiochemistry at Yale University, in 1907 Boltwood made early determinations of the age of the Earth using uranium: lead ratios. He was able to show a long geologic time-scale of up to 2000 Ma. See RADIOMETRIC DATING.