Prout’s hypothesis

Paper nine is another one that appeared in American Scientist. In it I took a philosophical look at two important ideas that contributed to the evolution of the periodic system. These two ideas are Prout s hypothesis and the notion of triads, which was the subject of paper eight. Both hypotheses are interesting because they were extremely productive even though they both turned out to be refuted some time later. The fact that this should happen lends some support to the views of Karl Popper who always claimed that refutability was the all important aspect of good hypotheses and theories and not whether they turn out to be correct or not.23 For Popper, all that we really have is tentative theories and not theories that last forever. 

Prout s hypothesis that the atoms of all elements were composites of hydrogen. Further, Mendeleev was particularly fond of emphasizing that smooth curves should not be drawn through the points representing numerical data on the elements, since such curves would imply continuity instead of strict individuality. 

Chemists still didn t know why there were so many different chemical elements or whether any patterns could be found in them. In 1815 the English physician William Prout had proposed the hypothesis that all of the elements were condensed hydrogen. For example, the atomic weight of oxygen was 16. According to Prout, this indicated that 16 volumes of hydrogen had condensed to form this element. Of course Prout s hypothesis was incorrect. However, during the nineteenth century there was no empirical evidence that either supported or contradicted the idea. Consequently, some chemists adopted the theory while many others opposed it. 

Marignac s life work, which, like that of Stas, consisted in making many precise determinations of atomic weights in order to test William Prout s hypothesis (71), won Berzelius sincerest praise, for he wrote ... 

Berzelius, who died in 1848, wrote a critique of Prouts hypothesis in 1845. He noted that transmutation had never been observed in the laboratory. If Prout s hypothesis was right, transmutation would be at least a theoretical possibility, and failure to observe it argued against Prout. The experimental data about atomic weights that best supported Prouts hypothesis were those of Thomas Thomson, and Berzelius had nothing but scorn for Thomsons abilities as an analyst. Clearly, Berzelius regarded it as merely a coincidence that many atomic weights had either whole-number or half-whole-number values. 

Two other, partially successful models to account for elemental periodicity, were proposed before and forgotten after the advent of quantum theory. An anonymous proposal, later ascribed to Prout, was based on the assumption that all atoms are composites of hydrogen. The purpose of this proposal was to account for the statistically improbable distribution of relative atomic weights, close to integer values. Following the discovery of isotopes Prout s hypothesis gained some new respectability, but it has never been fully exploited. Another theory was summarized by its author [20] in the statement ... 

Quoted in Prout s Hypothesis, Alembic Club Reprints, no. 20 (Edinburgh The Alembic Club, 1932), 17. 

Prout s hypothesis was revived by the discovery of isotopes thus chlorine con sists of two natural isotopes and CP" , and boron of two isotopes B and in each case with nearly integral atomic weights and present in such relative amounts as to give the chemical atomic weight. It is now seen that Prout s idea e ntained an element of truth. 

A second difficulty is that of anonymity. Journals of the time frequently published anonymous scientific communications, especially on controversial subjects. An example of such was published in Thomson s Annals of Philosophy in 1815, entitled On the Relations between the Specific Gravity of Bodies in Their Gaseous State and the Weight of Their Atoms. This was later shown to have been written by Prout, and has led, of course, to Prout s hypothesis (which does not anywhere occur in his paper, but originates rather in the editorial comments of Thomson). To discover the authors of such contributions is itself a special field of bibliographical endeavor and is outside the scope of the ordinary searcher it involves an intimate knowledge of historical minutiae and, moreover, an access to letters and documents not usually available in libraries other than those of the Royal Society and one or two similar ancient institutions on the continent of Europe. 

Benfey, O.T. 1952. Prout s Hypothesis. Journal of Chemical Education 29 78-81. 

However, the coffin of Prout s hypothesis was nailed shut around 1865 by the careful analytical studies of Jean Servais Stas. Figure 308 depicts a magnificent apparatus for total analysis of silver iodate (Agl03). How could one possibly argue with a Rube Goldberg-looking apparatus like that The gasometer... 

These simplified concepts tend to exemplify the apparent utility of Prout s hypothesis as an organizing principle. The atomic mass number, often incorrectly truncated to atomic mass, treats protons and neutrons as equals. Taken too literally, the mass of uranium-238 would appear to be roughly equal to that of 238 hydrogens (protium or hydrogen-1 atoms) with some tiny discrepancy understood as arising from the 0.1% difference in mass between protons and neutrons. In fact, if we take the masses of 92 protons, 146 neutrons, and 92 electrons, the total mass is 240.0 amu. 

---