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Uncle Tungsten
p149 By 1799, [Joseph-Louis] Proust had generalized his theory [that all chemical compounds were identical regardless of where they were found or how they were made ] into a law -- the law of fixed proportions. Proust’s analysis, and his mysterious law, excited attention among chemists everywhere, not least in England, where they were to inspire profound insights in the mind of John Dalton, a modest Quaker schoolteacher in Manchester.
Gifted in mathematics, and drawn to Newton and his “corpuscular philosophy” from an early age, Dalton had sought to understand the physical properties of gases -- the pressures they exerted, their diffusion and solution -- in corpuscular or “atomic” terms. Thus he was already thinking of “ultimate particles” and their weights, albeit in this purely physical context, when he first heard of Proust’s work, and by a sudden intuitive leap, saw how these ultimate particles might account for Proust’s law, and indeed the whole of chemistry.
For Newton and Boyle, though there were different forms of matter, the corpuscles or atoms of which they were composed were all identical. (Thus there was always, for them, the alchemical possibility of turning a base metal into gold, for this only entailed change of form, a transformation of the same basic matter.) But now the concept of elements, thanks to Lavoisier, was clear, and for Dalton there were as many kinds of atoms as there were elements. Every one had a fixed and characteristic “atomic weight,” and this was what determined the relative proportions in which it combined with other elements. Thus if 23 grams of sodium invariably combined with 35.5 grams of chlorine, this was because sodium and chlorine atoms had atomic weights of 23 and 35.5. (These atomic weights were not, of course, the actual weights of atoms, but their weights relative to that of a standard -- for example, that of a hydrogen atom.)
p150 Reading Dalton, reading about atoms, put me in a sort of rapture, thinking that the mysterious proportionalities and numbers one saw on a gross scale in the lab might reflect an invisible, infinitesimal, inner world of atoms, dancing, touching, attracting, and combining...
p151 But what Dalton intimated was infinitely more thrilling: for it was not just the atoms in the Newtonian sense, but atoms as richly individual as the elements themselves -- atoms whose individuality gave elements theirs.
Dalton later made wooden models of atoms, and I saw his actual models in the Science Museum as a boy. These, crude and diagrammatic as they were, excited my imagination, helped give me a sense that atoms really existed. But not everyone felt this... “Atoms,” the eminent chemist H. E. Roscoe was to write, eighty years later, “are round bits of wood invented by Mr. Dalton.”
It was indeed possible, in Dalton’s time, to regard the idea of atoms as implausible, if not outright nonsense, and it would be over a century before indisputable evidence for the existence of atoms was secured...
Isn’t it reasonable to think that -- just as Dalton leapt to this conclusion about atoms of each element being essentially different from each other -- there were many others who naturally leapt to other conclusions which they also attempted to prove. This natural variability is like the random mutations that drive evolution, but in science it is the scientific method of trial and proof that determines which ideas survive.
p152 Dalton’s theory of chemical atoms was detailed in his notebook on the 6th of September, 1803, his thirty-seventh birthday. He was at first too modest or too diffident to publish anything on his theory (he had, however, worked out the atomic weights of half a dozen elements -- hydrogen, nitrogen, carbon, oxygen, phosphorus, sulfur -- which he recorded in his notebook)...
p153 [Now we come to a common theme in this history, that the person making a breakthrough often has trouble going the next step. Dalton’s insight into atoms still left a confusion about the difference between atoms and molecules -- in this case the smallest unit of an element that was available to combine with other elements.] ...The Italian chemist [Amedeo] Avogadro, now hypothesized that equal volumes of gases contained equal numbers of molecules. For this to be so, the molecules of hydrogen and oxygen would have to have two atoms apiece...
But in an extraordinary way (at least so it seems in retrospect). Avogadro’s suggestion of diatomic molecules was ignored or resisted by virtually everyone, including Dalton... [I wonder if this had anything to do with Avogadro's being a Count -- from Turin no less]
p154 Finally in 1858, Avogadro’s countryman Stanislao Cannizzaro realized that Avogadro’s 1811 hypothesis provided an elegant way out of the decades-long confusion about atoms and molecules... at the close of 1860, chemists gathered at the first-ever international chemical meeting in Karlsruhe, it was Cannizzaro’s presentation that stole the show, and ended the intellectual agony of many years.
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