A scientific association of great significance:
important paper by Niels Bohr
inscribed to colleague and friend, H.G. Moseley

"These questions and some further information about the constitution of atoms which may be got from experiments on the absorption of alpha-rays, will be discussed in more detail in a later paper."

BOHR, Niels. On the Theory of the Decrease of Velocity of Moving Electrified Particles on Passing Through Matter. Offprint from Philosophical Magazine, 6^th series, vol. 25, no. 145, pp.10-31 (January 1913). London: Philosophical Magazine, 1913. Octavo, original printed wrappers. $13,000.

First edition, scarce offprint issue, of one of Niels Bohr’s most important papers, inscribed on the front wrapper to scientist H.G. Moseley: “Mr. H.G. Moseley / with the best compliments / from the author”.

After receiving his PhD from the University of Copenhagen in 1911, Bohr went to Cambridge to work under J.J. Thomson. Discouraged, however, by Thomson’s lack of interest in Bohr’s work, Bohr moved to Manchester where Ernest Rutherford had just made his revolutionary discovery of the atomic nucleus. Under Rutherford, “from March to July 1912, working with utmost concentration, [Bohr] laid the foundations of his greatest achievement in physics, the theory of atomic constitution” (DSB). The present paper, Bohr’s first publication after this intense period of study and concentration, was “a brilliant piece of work, which he—working, as he said, ‘day and night’—completed with astonishing speed” (DSB).

“The problem was one of immediate interest for Rutherford's laboratory: in their passage through a material medium, alpha particles continually lose energy by ionizing the atoms they encounter, at a rate depending on their velocity. Their energy loss limits the depth to which the particles can penetrate into the medium, and the relation between this depth, or range, and the velocity offers a way of determining this velocity. What Bohr did was to analyze the ionizing process on the basis of the Rutherford model of the atom and thus express the rate of energy loss in terms of the velocity by a much more accurate formula than had so far been achieved—a formula, in fact, to which modern quantum mechanics adds only nonessential refinements” (DSB).  Bohr’s work provided key experimental evidence for the Rutherford atom and the conclusions were used by Bohr a few months later as justification for one of the central arguments and equations in his revolutionary “On the Constitution of Atoms and Molecules” (part I). Still relevant today, “On the Theory of the Decrease of Velocity” is listed as one of the 100 most cited (from a study of papers from 1945- 2002) papers from the Philosophical Magazine.

On the association between Bohr and Moseley:

Henry Gwyn Jeffreys Moseley began working under Rutherford at his Manchester laboratory in 1910 and it was there that he was introduced to Bohr, likely for the first time in 1913. The young Moseley, “at twenty-six was poised for great accomplishment. He needed only the catalyst of Bohr’s visit to set him off.” At the time of Bohr’s visit, Moseley had been making great advancements in the new field of X-ray crystallography and was gaining insights into the structural relationships between atoms. Bohr had previously suggested “that the order of the elements in the periodic table ought to follow the atomic number rather than, as chemists thought, the atomic weight” and Moseley, using the techniques of X-ray crystallography, was able to prove Bohr right experimentally. Later known as “Moseley’s Law,” the relationship between the frequencies of spectral lines and atomic numbers (a term he coined) became one of the foundations of our understanding of the periodic table and atomic structure, leading Moseley and others to identify the existence of “new” elements which were soon found. (Rhodes, The Making of the Atomic Bomb).

Moseley’s work “gave experimental confirmation of the Bohr-Rutherford atom.” As Bohr later commented, “the Rutherford work was not taken seriously. We cannot understand it today, but it was not taken seriously at all… The great change came from Moseley.” (Rhodes).