And yet, as was demonstrated by the French chemist Gay-Lussac, in the very year that Dalton published his New System, if one measured volumes and not weights one found that two volumes, not one, of hydrogen combined with one volume of oxygen, to yield two volumes of steam. Dalton was skeptical of these findings (although he could have confirmed them himself with great ease), skeptical because he felt they would entail the breaking of an atom into two, to allow the combination of a half-atom of oxygen with each atom of hydrogen.
Although Dalton talked about ‘compound’ atoms, he had not clearly distinguished (any more clearly than his predecessors) between molecules – the smallest amount of an element or compound that could exist free – and atoms – the actual units of chemical combination. The Italian chemist Avogadro, reviewing Gay-Lussac’s results, 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. Their combination to form water, therefore, could be represented as 2H2+1O2 → 2H20.
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. There remained great confusion between atoms and molecules, and a disbelief that atoms of the same sort could link together. There was no problem in seeing water, a compound, as H20, but a seemingly insuperable difficulty in allowing that a molecule of pure hydrogen could be H2. Many atomic weights of the early nineteenth century were thus wrong by simple numerical factors – some seemed to be half what they should be, some twice, some a third, some a quarter, and so on.
Griffin’s book, my first guide in the laboratory, was written in the first half of the nineteenth century, and many of his formulas, and hence many of his atomic weights, were as erroneous as Dalton’s. Not that any of this mattered too much in practice – nor, indeed, did it affect the great virtue, the many virtues, of Griffin. His formulas and his atomic weights might indeed have been wrong, but the reagents he suggested, and their quantities, were exactly right. It was only the interpretation, the formal interpretation, that was askew.
With such confusion about elemental molecules, added to uncertainty about the formulas of many compounds, the very notion of atomic weights started to be discredited in the 1830s, and indeed the very notion of atoms and atomic weights fell into disrepute, so much so that Dumas, the great French chemist, exclaimed in 1837, ‘If I were master I would efface the word atom from science.’
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, atomic and equivalent weights. Cannizzaro’s first paper was as ignored as Avogadro’s had been, but when, 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.
This was some of the history I nosed out when I emerged from my lab and got a ticket to the library of the Science Museum in 1945. It was evident that the history of science was anything but a straight and logical series, that it leapt about, split, converged, diverged, took off at tangents, repeated itself, got into jams and corners. There were some thinkers who paid little attention to history (and it may be that there are many original workers who are much better off for not knowing their precursors or antecedents – Dalton, one feels, might have had more difficulty in proposing his atomic theory had he known the huge and confused history of atomism for the two thousand years that preceded him). But there were others who pondered the history of their subjects continually, and whose own contributions were integrally related to their pondering – and it is clear that this was the case with Cannizzaro. Cannizzaro thought intensely about Avogadro; saw the implications of his hypothesis as no one else had; and with them, and his own creativity, revolutionized chemistry.
Cannizzaro felt very passionately that the history of chemistry needed to be in the minds of his students. In a beautiful essay on the teaching of chemistry, he described how he introduced his pupils to its study by ‘endeavouring to place them… on the same level with the contemporaries of Lavoisier’, so that they might experience, as Lavoisier’s contemporaries did, the full revolutionary force, the wonder of his thought; and then a few years ahead, so that they could experience the sudden, blinding illumination of Dalton.
‘It often happens’, Cannizzaro concluded, ‘that the mind of a person who is learning a new science, has to pass through all the phases which the science itself has exhibited in its historical evolution.’ Cannizarro’s words had a powerful resonance for me, because I, too, in a way, was living through, recapitulating, the history of chemistry in myself, rediscovering all the phases through which it had passed.
14. Lines of Force
When I was very young I had been intrigued by ‘frictional’ electricity, of the sort that made rubbed amber attract bits of paper, and when I returned from Braefield, I began to read about ‘electrical machines’ – discs or globes of some nonconducting material, turned by a crank and rubbed against the hand, or a cloth, or a cushion of some sort – which would produce powerful sparks or shocks of static electricity. It seemed easy enough to make such a simple machine, and in my first attempt at making one I used an old record as the disc. Gramophone records at the time were made of vulcanite and easily electrified; the only problem was that they were thin and fragile, easily shattered. For a second, more robust machine, I used a thick glass plate and a cushion covered with leather and coated with zinc amalgam. I could get handsome sparks from this, more than an inch long, if the weather was dry. (Nothing worked if the weather was damp, for then everything conducted.)
One could connect the electrical machine to a Leyden jar – basically a glass jar coated with tinfoil on both sides, and a metal ball at the top, connected to the inside foil by a metal chain. If one connected several such jars together, they could hold a formidable charge. It was such a ‘battery’ of Leyden jars in the eighteenth century, I read, which had been used in one experiment to give an almost paralyzing shock to a line of eight hundred soldiers, all of them joined by holding hands.
I also got a small Wimshurst machine, a beautiful thing with revolving glass discs and radiating metal sectors that could yield massive sparks up to four inches long. When the plates of the Wimshurst machine were revolving fast, everything around it became highly charged: tassels became electrified, their threads straining apart; pithballs would fly apart, and one felt the electricity on one’s skin. If there was a sharp point nearby, electricity would stream from it in a luminous brush, a little corposant, and one could blow out candles with the outstreaming ‘electric wind’, or even get this to turn a little rotor on its pivot. Using a simple insulating stool – a wooden board supported by four tumblers – I was able to electrify my brothers so their hair stood on end. These experiments showed the repulsive power of like electric charges, each thread of the tassel, each hair, acquiring the same charge (whereas my first experience, with rubbed amber and bits of paper, had shown the power of electrically charged bodies to attract). Opposites attracted, likes repelled.