Although Dalton had presented some of his thoughts to the Literary and Philosophical Society in Manchester, they did not become known to a wider public until Thomson wrote of them. Thomson’s presentation was brilliant and persuasive, much more so than Dalton’s own exposition, which was crammed, awkwardly, into the final pages of his 1808 New System.
But Dalton himself realized that there were fundamental problems with his theory. For to pass from a combining or equivalent weight to an atomic weight required that one know the exact formula of a compound, for the same elements, in some cases, might combine in more than one way (as in the three oxides of nitrogen). So Dalton assumed that if two elements formed only a single compound (as hydrogen and oxygen appeared to do in water, or nitrogen and hydrogen in ammonia), they would do so in the simplest possible ratio: one to one. This ratio, he felt, would surely be the most stable. Thus he took the formula of water (in modern nomenclature) to be HO, and the atomic weight of oxygen to be the same as its equivalent weight, namely 8. Similarly, he took the formula of ammonia to be NH, and thus the atomic weight of nitrogen to be 5.
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+I02 → 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.
CHAPTER FOURTEEN
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 su
pported 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.
I wondered whether one could use the static electricity of the Wimshurst machine to light up one of Uncle Dave’s lightbulbs. Uncle said nothing, but provided me with some very fine wire made of silver and gold only a three-hundredth of an inch thick. When I connected the brass balls of the Wimshurst machine with a three-inch length of silver wire on a card, the wire exploded when I turned the handle, leaving a strange pattern on the card. And when I tried it with the gold wire, this was vaporized instantly, turning into a red vapor, gaseous gold. It seemed to me from these experiments that frictional electricity could be quite formidable – but that it was too violent, too intractable, to be of much use.
Electrochemical attraction, for Davy, was the attraction of opposites – the attraction, for example, of an intensely ‘positive’ metallic ion, a cation like that of sodium, to an intensely ‘negative’ one, an anion like that of chloride. But most elements, he thought, came between these on a continuous scale of electro-positivity or -negativity. The degree of electro-positivity among metals went with their chemical reactivity, hence their ability to reduce or replace less positive elements.
This sort of replacement, without any clear notion of its rationale, had been explored by the alchemists in the production of metallic coatings or ‘trees.’ Such trees were made by inserting a stick of zinc, say, into a solution of another metallic salt (a silver salt, for example). This would result in the displacement of the silver by the zinc, and metallic silver would be precipitated from the solution as a shining, almost fractal, arborescent growth. (The alchemists had given these trees mythical names, so the silver tree was called Arbor Dianae, the lead tree Arbor Saturni, and the tin tree Arbor Jovis.)«34»
I had hoped, at one point, to make such trees of all the metallic elements – trees of iron and cobalt, and bismuth and nickel, of gold, of platinium, of all the platinium metals; of chromium and molybdenum, and (of course!) tungsten; but various considerations (not least, the prohibitive cost of the precious metal salts) confined me to a dozen or so basic ones. But the sheer aesthetic delight of these – no two trees ever looked the same; they were as different, even with the same metal, as snowflakes or ice crystals, and different metals, one could see, were deposited in different ways – soon gave way to a more systematic study. When did one metal lead to the deposition of another? And why? I used a zinc rod, sticking it first into a solution of copper sulfate, and got a gorgeous encrustation, a copper plating, all around it. I then experimented with tin salts, lead salts, and silver salts, putting a zinc rod into solutions of these, and produced shining, crystalline trees of tin, lead, and silver. But when I tried to make a zinc tree, by sticking a copper rod into a solution of zinc sulphate, nothing happened. Zinc was clearly the more active metal, and as such could replace the copper, but not be replaced by it. To make a zinc tree, one had to use a metal even more active than zinc – a magnesium rod, I found, worked well. Clearly all these metals did form a sort of series.
Davy himself pioneered the use of electrochemical displacement for protecting the copper bottoms of ships from corrosion in seawater, attaching to them plates of more electropositive metals (such as iron or zinc), so that these would become corroded instead, a so-called cathodic protection. (Though this seemed to work well under laboratory conditions, it did not work well at sea, because the new metal plates attracted barnacles – and thus Davy’s suggestion was ridiculed. Yet the principle of cathodic protection was brilliant, and eventually became, after his death, a standard way of protecting the bottoms of ocean-going vessels.)
Reading about Davy and his experiments stimulated me to a variety of other electrochemical experiments: I put an iron nail in water, attaching a piece of zinc to it to protect it from corrosion. I removed the tarnish from my mother’s silver spoons by putting them in an aluminium dish with a warm solution of sodium bicarbonate. She was so pleased by this that I decided to go further and try electroplating, using chromium as the anode and a variety of household objects as the cathode. I chromium-plated everything I could lay hands on – iron nails, bits of copper, scissors, and (this time to my mother’s considerable annoyance) one of the silver spoons that I had previously cleaned of tarnish.
I did not realize at first that there was any connection between these experiments and the batteries I was playing with at the same time, although I thought it an odd coincidence that the first pair of metals I used, zinc and copper, could produce either a tree or, in a battery, an electric current. I think it was only when I read that, to get a higher voltage, batteries used nobler metals such as silver and platinum that I started to realize that the two series – the ‘tree’ series and Volta’s series – were probably the same, that chemical activity and electrical potential were in some sense the same phenomenon.
We had a large old-fashioned battery, a wet cell, in the kitchen, hooked up to an electric bell. The bell was too complicated to understand at first, and the battery, to my mind, was more immediately attractive, for it contained an earthenware tube with a massive, gleaming copper cylinder in the middle, immersed in a bluish liquid; all this inside an outer glass casing, also filled with fluid, and containing a slimmer bar of zinc. It looked like a miniature chemical factory of sorts, and I thought I saw little bubbles of gas, at times, coming off the zinc. This Daniell cell (as it was called) had a thoroughly nineteenth-century, Victorian look about it, and this extraordinary object was making electricity all by itself – not by rubbing or friction, but just by virtue of its own chemical reactions. That this was quite another source of electricity, not frictional or static, but a radically different sort of electricity, must have seemed astounding in the extreme, a new force of nature, when Volta discovered it in 1800. Previously there had been only the fugitive discharges, the sparks and flashes, of frictional electricity; now one could have at one’s disposal a steady, uniform, unvarying current. One only needed two different metals – copper and zinc would do, or copper and silver (Volta worked out a whole series of metals, differing in the ‘voltage,’ the potential difference, between them), immersed in a conducting medium.
The first batteries I made myself used fruit or vegetables – one could stick copper and zinc electrodes into a potato or a lemon and get enough current to light a tiny 1-volt bulb. And one could wire half a dozen lemons or potatoes together (in series to get a higher voltage, or in parallel to get more power) to make a biological ‘battery.’ After the fruit and vegetable batteries, I turned to coins, using alternating copper and silver coins (one had to use silver coins made before 1920, for later ones were debased) with moistened (usually saliva-moistened) blotting paper between them. If I used small coins, farthings and sixpences, I could get five or six such couples in an inch, or I could make a pile a foot high, with sixty or seventy couples, enclosed in a tube, which could give quite a sharp, 100-volt shock. One could go on, I thought, to make an electric stick filled with narrow couples of copper and zinc foil, a lot thinner than coins. Such a stick, with five hundred or more couples, might generate a thousand volts, more even than an electric eel, enough to frighten off any assailant – but I never got as far as making one.
I was fascinated by the huge range of batteries developed in the nineteenth century, some of which I could see in the Science Museum. There were ‘single fluid’ batteries, like Volta’s original cell, or the Smee, or the Grenet, or the massive Leclanche, or the slim, silver battery of de la Rue; and there were two-fluid batteries, like our own Daniell, and the Bunsen, and the Grove (which used platinum electrodes). Their number seemed endless, but all were designed, in their different ways, to secure a more reliable and constant flow of current, to protect the
electrodes from the deposition of metal or the adherence of gas bubbles, and to avoid (as some batteries caused) the emission of noxious or inflammable gases.
These wet cells had to be topped up with water from time to time; but the little dry cells in our torches were clearly different. Marcus, seeing my interest, dissected one for me, using his powerful scout knife, showing me the outer case of zinc, the central carbon rod, and the rather corrosive and strange-smelling conducting paste between them. He showed me the massive 120-volt battery in our portable radio (this was a necessity in the war, when the electricity supply was so erratic) – it contained eighty linked dry cells, and weighed several pounds. And once he opened the bonnet of the car – we had the old Wolseley at the time – and showed me the accumulator, with its lead plates and acid, and explained how this had to be charged, and could carry a charge repeatedly, but not generate one itself. I adored batteries, and they did not have to be live; when my interest was made known to the family, used batteries of all shapes and sizes poured in, and I rapidly accumulated a remarkable (though wholly useless) collection of the things, many of which I opened and dissected.
But my favorite remained the old Daniell cell, and when we went modern and got a natty new dry cell for the bell, I appropriated the Daniell for myself. It had only a modest voltage of 1 or 1½ volts, but the current, several amperes, was considerable in view of its size. This made it very suitable for heating and lighting experiments, where one needed a substantial current, but the voltage hardly mattered.
Thus I could readily heat wire – Uncle Dave had supplied me with a whole bandolier of fine tungsten wire of all different thicknesses. The thickest wire, two millimeters in diameter, became mildly warm when I connected a length of it across the terminals of the cell; the thinnest wire grew white-hot and incinerated in a flash; there was a comfortable in-between wire that one could maintain for a little while at red heat, though even at this temperature it soon oxidized and disintegrated into a fluff of yellowish white oxide. (Now I knew why it had been crucial to remove the air from lightbulbs, and why incandescent lighting was not possible unless the bulbs were evacuated or filled with an inert gas.)