But if he was writing Vauquelin off, Davy was premature. The housekeeper’s truffle-paring may have been part of a chemical rather than a culinary exercise, for in 1813, the year of Davy’s visit, Vauquelin had isolated asparagine, an amino acid found in asparagus.
Fifteen years later, in June 1828, when Davy himself was nearing death, a spry Professor Vauquelin wrote to Faraday asking for some letters of recommendation for a young man intending to visit British cloth-bleaching factories.51 This letter carries clues that may shed some mild light on Davy’s growing attitude to Faraday during the continental tour of 1813. Vauquelin writes of Faraday’s ‘great reputation … justly acquired amongst chemists’, but begins, ‘although I have not yet been in direct contact with you …’. Vauquelin had forgotten that he and the younger Faraday had met long before, suggesting that Davy kept Faraday in the background, at best his amanuensis, at worst his invisible valet.
Nevertheless, Faraday had fond memories of his day in Vauquelin’s laboratory. He saw potassium chloride being manufactured by passing chlorine, held in earthenware vessels of ‘11 or 12 gallons capacity’, through a solution of potash in a six- or seven-gallon jar over a low heat. The chloride collected at the bottom of the solution, a different method, Faraday noted, to the one practised in England, where the chlorine was passed through several different portions of the potash solution. Talking with a laboratory workman, Faraday heard talk of Pierre Louis Dulong, the discoverer of the explosive nitrogen trichloride, who also worked with Vauquelin. Faraday, who had damaged his hand while experimenting with the explosive, could show his scars and relate how he, like Dulong, had been blooded for science.
Sir Humphry Davy’s arrival in Paris had been eagerly awaited. For weeks before he came French scientists had been discussing the visit, and making plans for the ceremony at the Institut de France on 2 November 1813 when he was to be awarded the Napoleonic gold medal. Ampère had been especially eager to meet the man he considered ‘the greatest chemist that had ever appeared’,1 and for his part Ampère was the first person Davy had wanted to meet. Davy was majestically received at the Institut de France, and, seated to the President’s right, was told during the éloge by the Secretary Georges Cuvier that the meeting was ‘honoured by the presence of Le Chevalier Davy’.2 He attended receptions and dinners in his honour: at the anniversary dinner of the Philomatic Society both he and Underwood were guests of honour. Toasts were drunk, but as a deference to the two Englishmen all declined to drink Napoleon’s health.3 Despite being a guest in a foreign country, Davy did not curb his opinions of people he met. John Ayrton Paris, his first biographer, reported that it had been observed that ‘during his residence … his likes and dislikes to particular persons were violent, and that they were, apparently, not directed by any principle, but were the effect of a sudden impulse’.4 Though Davy expressed dislikes privately, they did not appear in the character sketches of French scientists that he wrote some years later, and which were first published by his brother John: the sketches, of Guyton de Morveau, Vauquelin, Cuvier, Humboldt, Gay-Lussac, Berthollet, La Place and Chaptal, are invariably spirited and appreciative.5
On 23 November a deputation of three distinguished French scientists called at the Hôtel des Princes to see Sir Humphry, and set him a problem which not only gave renewed purpose and direction to his months in Paris, but delayed his departure for Italy and held him up in January 1814 in the south of France. André-Marie Ampère, Nicolas Clément and Charles Bernard Desormes were shown into Davy’s drawing room. One of them opened a box and took out a bottle of blackish flakes which had a shiny quality, deep violet in the light, lustrous, not unlike the lights that Davy had seen in Vauquelin’s chromium, though less iridescent. They called it ‘Substance X’. There was not much of a smell to it, and one of the scientists said it was quite brittle in larger lumps. The visitors looked enquiringly at Davy – Faraday was hovering behind trying to see but also trying to be invisible – and Davy looked at the flakes. Then one of the French scientists broke the silence, telling Davy that about two years earlier a gunpowder manufacturer, Bernard Courtois, had produced some crystals when making saltpetre at his works. He had had no idea what the stuff was, but when it was heated it gave off a sharp-smelling, poisonous, lurid violet smoke. The extraordinary thing was that it did not liquefy; it just disappeared on heating in a violet cloud.
There was a great deal of money in gunpowder manufacture in France at that time: there was a war on. Many thousands of barrels had been shipped out to supply the French armies in Spain, Portugal, Russia, Italy and the Austro-Hungarian Empire. Yet more was stockpiled in strategic dumps around France, much of it intended to damage English armies and interests. Gunpowder-making was a very sensitive industry, and the discovery of this strange by-product had to be handled carefully. The nature of the material had stumped even the flamboyant young French chemist Joseph Louis Gay-Lussac. He was a brave and daring figure, popular and famous for undertaking dangerous balloon ascents to gather samples of air for analysis and to take measurements of the strength of terrestrial magnetism. With Alexander von Humboldt he had formulated the law that oxygen and hydrogen combine precisely at the ratio of one to two by volume to make water, and that all gaseous reactions are in such simple proportions. These were revelations of the fundamental driving forces of life, and it was a matter of intense pride for Napoleonic France that a Frenchman was leading the way in analysing them. But even Gay-Lussac could not give a clear answer to what ‘Substance X’ was. He had found that it produced an acid very like hydrochloric acid, and both he and Nicolas Clément had ventured that it was indeed the same acid. And yet …
After two years without reaching any serious conclusion, Ampère seems to have decided that the only thing to do was to ask Sir Humphry Davy. There were clear risks; the dangers of asking a citizen of an enemy country to identify a by-product of gunpowder were obvious. But who else was there to ask? And so the deputation made its way to the Hôtel des Princes.
Sir Humphry asked his visitors how the material was obtained, but they could not or would not tell him. Faraday records: ‘The process by which it is obtained is not as yet publicly known. It is said to be obtained from a very common substance and in considerable quantities.’
Davy took out his travelling box of chemical equipment, and heated a few of the flakes. True to form they vaporised in a dramatic and quite beautiful but poisonous violet smoke. The men choked; someone ran to the window and flung it open. When the smoke had cleared, they took some more of the substance and heated it in a sealed jar. It did not need much heat to start to smoke, and very soon, as it cooled again, it condensed into purple crystals around the neck of the jar. They then dissolved some in alcohol, and formed a deep brown liquid which precipitated silver nitrate. Sir Humphry tipped a bit of this onto a sheet of paper and put it in the sun to dry, where it very quickly tarnished to a dirty black.
Then Sir Humphry tried some other tricks. He leant over his tubes and jars like a magician. He rubbed some of the mystery substance with zinc filings and found that a liquid formed. When it was put into a tube with potassium and heated it flared violently, and the men all backed off. It reacted even more violently when heated with phosphorus, and in combination with mercury a heavy metallic liquid formed which on heating became first orange, then black, then red. Faraday was taking notes of all this, as was his practice, and it is because of these notes, later transcribed into his Journal, that we know so much about this critical scientific meeting. In making the chemical combinations that Faraday described – and in a rented hotel room too – Davy was skimming the edges of extreme physical danger, not only from poisoning by the gas but from the effects of being showered by burning phosphorus or potassium or heated mercury. He was also risking expulsion from the hotel.
Over the next few days Davy made more experiments on the mysterious purple flakes. The visitors probably left him to it, but Faraday was present, as his notes, written out in the Journal under 1 December, make clear. There was much controversy in Paris over whether Davy should have been given a sample to work on alone – Thénard and