Level: introductory
References: William Prout, "On the Relation between the Specific Gravities of Bodies in their Gaseous State and the Weights of their Atoms," Annals of Philosophy 6, 321-30 (1815); "Correction of a Mistake in the Essay on the Relation between the Specific Gravities of Bodies in their Gaseous State and the Weights of their Atoms," Annals of Philosophy 7, 111-3 (1816): both published anonymously.
Notes: William Prout (1785-1850) was a medical doctor with a great interest in chemistry. He developed analytical instruments and published chemical analyses of organic material. His analyses of food classified components which correspond essentially to proteins, carbohydrates, and fats. Prout was also recognized as a clinical authority on urinary and digestive disease.
By the end of the 19th century, however, Prout was best known for the hypothesis that atomic weights were multiples of that of hydrogen and the speculation that hydrogen was a fundamental building block of matter. These ideas were tremendously influential. They inspired ever more accurate determinations of atomic weights to test the multiples hypothesis. One of those sets of determinations, Lord Rayleigh's measurements of gas densities in the late 19th century, even led to the discovery of a new element (argon). As it became clear that the original multiples hypothesis was contradicted by experiment, various researchers modified rather than discarded it. In other words, even though the hypothesis suggested by Prout was not correct, it was too attractive to be regarded as entirely baseless. And indeed, there was something to the idea of simple building blocks of atoms after all: practically all the mass of atoms is in their nuclei, which are made of protons and neutrons, building blocks of nearly identical mass (one of which happens to be the nucleus of the dominant isotope of hydrogen).
This set of exercises shows that even the experimental data Prout cited gave very little support for his hypothesis. The data were generally not precise enough to decide whether or not gas densities were integer multiples of the density of hydrogen. As the solution points out, the uncertainty that may be inferred in the density of chlorine is about half the density of hydrogen--which means that one has no basis for saying that it is or is not an integer multiple. The density of azote appears not to be an integer multiple of that of hydrogen. For more on Prout and his hypothesis that just would not die, see W. H. Brock, From Protyle to Proton: William Prout and the Nature of Matter, 1785-1985 (Bristol: Adam Hilger, 1985).
Pedagogical note: The molar masses computed here look like atomic weights of the elements. Instructors may or may not wish to go into the fact that they are actually molecular weights but on an unfamiliar basis. All of the data refer to molecules, but they are on the unusual basis of H2 = 1. Since all of the molecules are diatomic, their molecular weights are in the same proportion as the respective atoms.
Solutions: To download solutions, go to:
http://web.lemoyne.edu/giunta/classicalcs/prout.doc
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