Harold 5. King U. S. Army Chemical Corps Board
Army Chemical Center, Maryland
It is difficult to transport one's self hack forty years and to see the problems of that time through the eyes of those who were then in the vanguard of science. The very existence of isotopes, substances belonging in the same place in the periodic table hut with different atomic weights, was being questioned. People were asking whether samples of an element derived from various geologic sources had different atomic weights and whether it could he possible to fractionate a single sample into portions differing in density. It seems appropriate to present early developments and concepts in the words and context of the period. To set the stage I quote from the Presidential Address before the American Association for the Advancement of Science, delivered by T. W. Richards in December 1918: If ordinary lead should be found to be complicated, many, if not all, other elements should be tested. The outcome, while not in the least affecting our table of atomic weights as far as practical purposes are concerned, might lead to highly intereding theoretical conclusions. How can such remote scientific knowledge, even if it satisfies our ever-insistent intellectual curiosity, he of any practical use? W h o can tell? It must be admitted that the relationship is apparently slight as regards any immediate application, but one can never know how soon any new knowledee concerning the nature of things may bear unexpected fruit. Faradsy had no coneeption 01 the electric locomotive or the power plants of Niagsra when he performed those crucial experiments with magnets and wires that laid the basis for the dvnamo. Nearly fifty years elapsed before his experiments on electric induction in moving wires bore fruit in s. practical electric lighting system; and yet more years before the trolley car, depending equally upon the principles discovered by Faraday, became an evervday ooceurrence.
It took only 25 years before another element, uranium, was being fractionated on a large scale to enrich it in respect to its content of Usas. Today the term "isotope" is at least as familiar to the school boy as "trollev car." I n this presentation, we deal with the two main series of investigations conducted at the Wolcott Gibbs Memorial Laboratory a t Harvard by T. W. Richards and his co-workers. First there was a series of determinations of the atomic weights of lead from various sources to settle the question of whether isotopes really differ in respect to their atomic weights. Secondly a series of investigations was made on various properties of these several leads to determine what properties are dependent on a difference in weight or mass. These explorations developed into an intensive theoretical Presented as part of the Symposium on Inorganic Chemists in the Nuclear Age before the Division of History of Chemistry and the Division of Inorganic Chemistry a t the 134th Meeting of the American Chemical Society, Chicago, September, 1958.
Pioneering Research on Isotopes at Harvard and laboratory study of the separation of isotopes of an "elementary substance" from each other. The Atomic Weight of Lead of Radioactive Origin
Boltwood, Ramsay, Rutherford, Fajans, and other workers upon radioactivity pointed out that the most conclusive test concerning the then recent theory of the degeneration of radioactive elements was to be found in the determination of the atomic weights. A theory, proposed by Fajans and by Soddy, indicated that some of the places in the periodic table should perhaps include several elements different in atomic weight but very similar in other properties. Thus in the place assigned to lead we should expect to find ordinary lead and the leads produced by the disintegration of uranium, thorium, and actinium. The problem was one capable of a decisive gravimetric test; specimens of lead, consisting of different mixtures obtained from different sources, should have different atomic weights. The determination of atomic weights involves experimental work of great refinement. Richards, building on the foundations laid by Stas, carried this branch of analytical chemistry to a point where be and his pupils, Baxter a t Harvard and Honigschmid a t Munich, were recognized as the world authorities. As Richards in his Nobel Lecture on Atomic Weights stated: I have tried alwavs to be sure t h a t the substance being weighed represented all the substance which I was seeking to weigh, and nothing more; and wherever possible I have not contented myself with a hypotheticnl presumption that such was the cane, hut have endeavored to prove by special experiments, first, that nothing was lost, and, secondly, that no foreign substance had been unintentionally included.
In his devotion to accurate knowledge, Richards often quoted Plato: If from any art that which concerns weighing and meitswing and arithmetic is taken away, how little is left of that art.
Richards said: Today we may paraphrape this silying as follmw: "If from chemistry are tsken away the atomic weights (or other numerical data representing the same definite proportions), little will remain of that science." As a science becomes more scientific it becomes more quantitative, and greater accuracy in the determination of its fundamental mathematical banis is required.
In the 40 years of labor on atomic weight determinations, Richards and his associates evaluated, revised. and substantiated the values for 60% ," of the then recognized elements. In the course of his revisions and refinements of earlier methods, Richards developed new methods of approach and types of apparatus. The nephelometer facilitated the exact determination of the end point of the reaction between silver and Volume 36, Number 5, May 1959
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Wolcott G i b b r Memorial Loborotoiy personnel in 1920. From left to right: Front row, Or. Henry Krepelko. Mirr Edith H. Lanman, Proferror Theodore W . Ri
