The 100 Most Influential Scientists of All Time (30 page)

Pierre Curie then joined her in the work that she had undertaken to resolve this problem and that led to the discovery of the new elements, polonium and radium. While Pierre Curie devoted himself chiefly to the physical study of the new radiations, Marie Curie struggled to obtain pure radium in the metallic state—achieved with the help of the
chemist André-Louis Debierne, one of Pierre Curie's pupils. On the results of this research, Marie Curie received her doctorate of science in June 1903 and, with Pierre, was awarded the Davy Medal of the Royal Society. That was also the year in which they shared with Becquerel the Nobel Prize for Physics for the discovery of radioactivity.

The birth of her two daughters, Irène and Ève, in 1897 and 1904 did not interrupt Marie's intensive scientific work. She was appointed lecturer in physics at the École Normale Supérieure for girls in Sèvres (1900) and introduced there a method of teaching based on experimental demonstrations. In December 1904 she was appointed chief assistant in the laboratory directed by Pierre Curie.

The sudden death of Pierre Curie (April 19, 1906) was a bitter blow to Marie Curie, but it was also a decisive turning point in her career. Henceforth she was to devote all her energy to completing alone the scientific work that they had undertaken. On May 13, 1906, she was appointed to the professorship that had been left vacant on her husband's death; she was the first woman to teach in the Sorbonne. In 1908 she became titular professor, and in 1910 her fundamental treatise on radioactivity was published. After being awarded a second Nobel Prize, for the isolation of pure radium, she saw the completion of the building of the laboratories of the Radium Institute (Institut du Radium) at the University of Paris.

Throughout World War I, Marie Curie, with the help of her daughter Irène, devoted herself to the development of the use of X-radiography. In 1918 the Radium Institute, the staff of which Irène had joined, began to operate in earnest, and it was to become a universal centre for nuclear physics and chemistry. Now at the highest point of her fame, Marie Curie devoted her researches to the study of the chemistry of radioactive substances and the medical applications of these substances.

In 1921, accompanied by her two daughters, Marie Curie made a triumphant journey to the United States, where President Warren G. Harding presented her with a gram of radium bought as the result of a collection among American women. She gave lectures, especially in Belgium, Brazil, Spain, and Czechoslovakia. She was made a member of the Academy of Medicine in 1922, and also was a member of the International Commission on Intellectual Co-operation by the Council of the League of Nations. In addition, she had the satisfaction of seeing the development of the Curie Foundation in Paris and the inauguration in 1932 in Warsaw of the Radium Institute, of which her sister Bronisława became director.

One of Marie Curie's outstanding achievements was to have understood the need to accumulate intense radioactive sources, not only to treat illness but also to maintain an abundant supply for research in nuclear physics. The resultant stockpile was an unrivaled instrument until the appearance after 1930 of particle accelerators. The existence of a stock of 1.5 grams of radium in Paris at the Radium Institute—in which, over a period of several years, radium D and polonium had accumulated—made a decisive contribution to the success of the experiments undertaken in the years around 1930, in particular those performed by Irène Curie in conjunction with Frédéric Joliot, whom she had married in 1926. This work prepared the way for the discovery of the neutron by Sir James Chadwick and, above all, for the discovery in 1934 by Irène and Frédéric Joliot-Curie of artificial radioactivity.

A few months after this discovery, Marie Curie died as a result of leukemia caused by the action of radiation. Her contribution to physics had been immense, not only in her own work, the importance of which had been demonstrated by the award to her of two Nobel Prizes, but
because of her influence on subsequent generations of nuclear physicists and chemists.

(b. July 4, 1868, Lancaster, Mass., U.S.—d. Dec. 12, 1921, Cambridge, Mass.)

A
merican astronomer Henrietta Swan Leavitt was known for her discovery of the relationship between period and luminosity in Cepheid variables, pulsating stars that vary regularly in brightness in periods ranging from a few days to several months.

Leavitt attended Oberlin College for two years (1886–88) and then transferred to the Society for the Collegiate Instruction of Women (later Radcliffe College), from which she graduated in 1892. Following an interest aroused in her senior year, she became a volunteer assistant in the Harvard Observatory in 1895. In 1902 she received a permanent staff appointment. From the outset she was employed in the observatory's great project, begun by Edward C. Pickering, of determining the brightnesses of all measurable stars. In this work she was associated with the older Williamina Fleming and the more nearly contemporary Annie Jump Cannon.

Leavitt soon advanced from routine work to a position as head of the photographic stellar photometry department. A new phase of the work began in 1907 with Pickering's ambitious plan to ascertain photographically standardized values for stellar magnitudes. The vastly increased accuracy permitted by photographic techniques, which unlike the subjective eye were not misled by the different colours of the stars, depended upon the establishment of a basic sequence of standard magnitudes for comparison. The problem was given to Leavitt, who began with a sequence of 46 stars in the vicinity of the
north celestial pole. Devising new methods of analysis, she determined their magnitudes and then those of a much larger sample in the same region, extending the scale of standard brightnesses down to the 21st magnitude. These standards were published in 1912 and 1917.

She then established secondary standard sequences of from 15 to 22 reference stars in each of 48 selected “Harvard Standard Regions” of the sky, using photographs supplied by observatories around the world. Her North Polar Sequence was adopted for the Astrographic Map of the Sky, an international project undertaken in 1913, and by the time of her death she had completely determined magnitudes for stars in 108 areas of the sky. Her system remained in general use until improved technology made possible photoelectrical measurements of far greater accuracy. One result of her work on stellar magnitudes was her discovery of 4 novas and some 2,400 variable stars, the latter figure comprising more than half of all those known even by 1930. Leavitt continued her work at the Harvard Observatory until her death.

Leavitt's outstanding achievement was her discovery in 1912 that in a certain class of variable stars, the Cepheid variables, the period of the cycle of fluctuation in brightness is highly regular and is determined by the actual luminosity of the star. The subsequent calibration of the period-luminosity curve allowed American astronomers Edwin Hubble, Harlow Shapley, and others to determine the distances of many Cepheid stars and consequently of the star clusters and galaxies in which they were observed. The most dramatic application was Hubble's use in 1924 of a Cepheid variable to determine the distance to the great nebula in Andromeda, which was the first distance measurement for a galaxy outside the Milky Way. Although it was later discovered that there are actually two different types of Cepheid variable, the same method can still be applied separately to each type.

(b. Aug. 30, 1871, Spring Grove, N.Z.—d. Oct. 19, 1937, Cambridge, Cambridgeshire, Eng.)

N
ew Zealand-born British physicist Ernest Rutherford was considered the greatest experimentalist since Michael Faraday (1791–1867). Rutherford was the central figure in the study of radioactivity, and with his concept of the nuclear atom he led the exploration of nuclear physics. He won the Nobel Prize for Chemistry in 1908, was president of the Royal Society (1925–30) and the British Association for the Advancement of Science (1923), was conferred the Order of Merit in 1925, and was raised to the peerage as Lord Rutherford of Nelson in 1931.

In 1895 Rutherford won a scholarship that had been created with profits from the famous Great Exhibition of 1851 in London. He chose to continue his study at the Cavendish Laboratory of the University of Cambridge, which J.J. Thomson, Europe's leading expert on electromagnetic radiation, had taken over in 1884. At Cambridge, Rutherford determined that a magnetized needle lost some of its magnetization in a magnetic field produced by an alternating current. This made the needle a detector of electromagnetic waves, a phenomenon that had only recently been discovered. Rutherford's apparatus for detecting electromagnetic waves, or radio waves, was simple and had commercial potential. He spent the next year in the Cavendish Laboratory increasing the range and sensitivity of his device, which could receive signals from half a mile away.

Rutherford accepted Thomson's invitation to collaborate on an investigation of the way in which X-rays changed
the conductivity of gases. This yielded a classic paper on ionization—the breaking of atoms or molecules into positive and negative parts (ions)—and the charged particles' attraction to electrodes of the opposite polarity. He then pursued other radiations that produced ions, looking first at ultraviolet radiation and then at radiation emitted by uranium. Placement of uranium near thin foils revealed to Rutherford that the radiation was more complex than previously thought: one type was easily absorbed or blocked by a very thin foil, but another type often penetrated the same thin foils. He named these radiation types alpha and beta, respectively, for simplicity.

Rutherford's research ability won him a professorship at McGill University, Montreal, which had an exceptionally well-equipped laboratory. Turning his attention to another of the few elements then known to be radioactive, he and a colleague found that thorium emitted a gaseous radioactive product, which he called “emanation.” This in turn left a solid active deposit, which soon was resolved into thorium A, B, C, and so on. Curiously, after chemical treatment, some radioelements lost their radioactivity but eventually regained it, while other materials, initially strong, gradually lost activity. This led to the concept of half-life—in modern terms, the interval of time required for one-half of the atomic nuclei of a radioactive sample to decay—which ranges from seconds to billions of years and is unique for each radioelement and thus an excellent identifying tag.

Rutherford recognized his need for expert chemical help with the growing number of radioelements. Sequentially, he attracted the skills of Frederick Soddy, a demonstrator at McGill; Bertram Borden Boltwood, a
professor at Yale University; and Otto Hahn, a postdoctoral researcher from Germany. With Soddy, Rutherford in 1902–03 developed the transformation theory, or disintegration theory, as an explanation for radioactivity—his greatest accomplishment at McGill. Rutherford and Soddy claimed that the energy of radioactivity came from within the atom, and the spontaneous emission of an alpha or beta particle signified a chemical change from one element into another.

Before long it was recognized that the radioelements fell into three families, or decay series, headed by uranium, thorium, and actinium and all ending in inactive lead. Rutherford determined that the alpha particle carried a positive charge, but he could not distinguish whether it was a hydrogen or helium ion.

In 1907 Rutherford accepted a chair at the University of Manchester, whose physics laboratory was excelled in England only by Thomson's Cavendish Laboratory.

With the German physicist Hans Geiger, Rutherford developed an electrical counter for ionized particles; when perfected by
Geiger, the Geiger counter became the universal tool for measuring radioactivity. Rutherford and his student Thomas Royds were able to isolate some alpha particles and perform a spectrochemical analysis, proving that the particles were helium ions. Boltwood then visited Rutherford's laboratory, and together they redetermined the rate of production of helium by radium, from which they calculated a precise value of Avogadro's number.

In 1911, Rutherford disproved William Thomson's model of the atom as a uniformly distributed substance. Because a few of the alpha particles in his beam were scattered by large angles after striking the gold foil, Rutherford knew that the gold atom's mass must be concentrated in a tiny, dense nucleus. Continuing his long-standing interest in the alpha particle, Rutherford studied its slight scattering when it hit a foil. Geiger joined him, and they obtained ever more quantitative data. In 1909 when an undergraduate, Ernest Marsden, needed a research project, Rutherford suggested that he look for large-angle scattering. Marsden found that a small number of alphas were turned more than 90 degrees from their original direction.