Boltwood, Bertram Borden

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