Adapted from Chapter 5 of “Weighing the Soul”

When I am introduced to strangers as a “chemist”, most of them conclude either that I dispense prescriptions or that I spend my time in a smelly laboratory mixing “chemicals” together to see what will happen.

If I were an alchemist, they would be right in both cases. According to the Oxford English Dictionary, the word derives from the ancient Greek for an infusion of plant juices, which is how the alchemists discovered and prepared many medicines, some of which we still use today. A strikingly similar word was used by the ancient Egyptians to describe a notorious Alexandrian sect that pursued chemical experimentation. The two words became one when they were taken over into the English language in the mediaeval word alchemy, which was concerned as much with the search for new drugs as it was with the pursuit of chemical transmutation. In both cases, though, the approach was the same – to mix, dissolve and heat different combinations of materials in the hope that they would produce something novel or useful.

Some modern experts dispute the above etymology but, whatever the origins of the word, chemists such as myself are still successors to the ancient alchemists. We still search for ways to make new materials (including medicines) or to improve the properties of old ones by chemical or physical manipulation. The professed difference between our trade and that of the alchemists was spelled out by Robert Boyle in his 1661 book The Sceptical Chymist. Most alchemists were guided in their experiments by a belief that nature was resolvable into three principles – salt, sulphur and mercury. Boyle claimed that this belief was unsupported by experimental evidence, and argued that progress could only be made by setting such beliefs to one side. We should be sceptical, he said, of all statements concerning nature that were not directly based on experimental evidence, and we should pay particular heed to quantitative evidence, involving the accurate measurement of weight, volume, temperature and other conditions.

This quantitative experimental approach has paid huge dividends. Boyle claimed that it put chemistry on a completely new footing, and derided the alchemists for their previous haphazard efforts. The truth, recently uncovered by the American historian Lawrence Principe, was that the alchemists were using quantitative experimentation hundreds of years before Boyle came on the scene, and that Boyle copied or appropriated from the alchemists many of the experiments and discoveries that he claimed as his own.

This is not to say that Boyle did not make many discoveries on his own account. He discovered new gases (including hydrogen and carbon dioxide) and the law named after him that predicts the relationship between their volumes and pressures. He proposed the idea of blood transfusion, and discovered a chemical indicator (syrup of violets) that would indicate whether a material was an acid or an alkali by its change of colour. Most importantly, from the point of view of his upper-class contemporaries, he discovered that ice could be made colder by mixing it with certain salts, and cardinals and others paid large amounts of money to have their drinks cooled by this method, which is still used in modern icecream makers.

Boyle wanted more, though. He wanted to be seen as “a virtuoso uninfluenced by older ideas”, even when those ideas involved the very core of alchemy – transmutation. Principe has shown that Boyle, while ridiculing alchemy in public, secretly spent much of his life in private pursuit of the philosophers’ stone, a mythical material that could supposedly “multiply” an initially small amount of gold indefinitely. Here I follow his quest, with its curious mixture of modern chemistry and ancient alchemy, and reveal the curiously parallel beginnings of my own life as a chemist.

The image of an alchemist in most people’s minds is that shown in Joseph Wright’s often-reproduced 1771 painting The Alchymist in Search of the Philosophers’ Stone discovers Phosphorus. The bearded alchemist (whose face bears a remarkable resemblance to that of Socrates in the Louvre portrait) is kneeling as if in worship, and gazing with Socratic intensity at a glass retort in which glowing white fumes are rising from a boiling liquid. The picture is imaginary, but the experiment and the alchemist were both real.

The alchemist was Henning Brandt, a failed Hamburg merchant who had turned to alchemy in an attempt to restore his fortunes. The experiment was his attempt in 1669 to manufacture the fabled philosophers’ stone from 60 buckets of human urine. Needless to say, he failed, but by mixing the concentrated urine with charcoal and applying strong heat he succeeded in producing a new material – white phosphorus, one of the first elements to be isolated by chemical means.

Brandt’s phosphorus glowed in the dark; an effect caused by its reaction with the small amount of oxygen in left in the evacuated retort. When air was admitted, and the reaction speeded up, the phosphorus caught fire. Brandt found that it would not catch fire if it was stored under water, which is the way that white phosphorus is still stored in laboratories today. The pure material was literally too hot to handle, but the gum that was left in the bottom of the retort contained only a small residue of phosphorus, and merely glowed when first exposed to air. It could be used to “write upon the Palm of your Hand, or upon Paper [d] what ever you write will appear all on fire, and the Letters may be read a long time after; but you must have a great care, that you do it softly, and to put it into Water, as soon as you have done, for if it happen to fire ‘twill burn the place most dreadfully.”

Brandt was keen to find a use for his miraculous new material. Its glow, and the associated heat, suggested medicinal properties, and it is possible that he saw it as a potential cure for syphilis. Whatever his reasons, they were sufficient for him to try an eye-watering experiment: “If the Privy Parts be therewith rubb’d”, he said “they will be inflamed and burning for a good while after.”

Brandt’s experiment seems foolhardy and ridiculous to modern eyes, but at least he tried it on himself, and did not make exaggerated claims based solely on the glow. Some of the ancient alchemists, however, did make exaggerated claims, but most of them observed a due caution that a modern scientist would be proud of, and sought to verify their observations as carefully as they could. Their main problem was the lack of a good guiding theory (almost inevitable when scientists are breaking totally new ground), so that they didn’t really understand what was going on. It was a problem that I, too, had, when I first began to learn chemistry at school, and came into contact for the first time with materials that would have delighted the ancient alchemists.

One of those materials was sulfuric acid, a powerful, corrosive solvent that was known to the ancients as “oil of vitriol”, and which was used by the alchemists in the “vitriolification” of metals and other materials. School children these days would never be allowed access to such a dangerous substance, but we were each given a bottle of it in the wooden rack of chemicals provided for our experimental class. The rack also contained a bottle full of purple crystals of potassium permanganate. My mother would have recognized these as “Condy’s crystals”, which were kept in the family medicine cabinet in case of snakebite, but my mother was not there when I took a seat at the back of the chemistry class and began surreptitiously to drop the crystals into a beaker of concentrated sulfuric acid. Like the ancient alchemists, I wanted to see what would happen. Like them, unfortunately, I did not know enough about the underlying chemical mechanisms to predict what the outcome might be.

I was delighted when the crystals left twisted purple trails in the acid as they slowly sank. My neighbours were more interested in the acrid purple fumes that the mixture was emitting, and egged me on to add more crystals. As our ancient chemistry master, “Pop” Jane, rambled on at the front of the class, ignorant of what was going on at the back, I proceeded to pour the rest of the crystals into the acid, eventually producing nearly half a litre of hot, fuming purple liquid.

The alchemists used “vitriolification” to change the properties of a material added to the acid. I had unwittingly done the same, transforming the relatively harmless potassium permanganate into permanganic acid, a highly unstable liquid that can explode spontaneously. I found out later that the amount that I had manufactured would have been enough to demolish the classroom. At the time, I was more concerned that the purple fumes would give me away and get me into trouble. I turned on the tap and cautiously flushed the acid down the sink, finishing just as “Pop” completed his spiel.

I had unknowingly taken a fearful risk because of my desire “just to see what would happen”. The leading alchemists sometimes took equally dangerous risks, but their experiments were not nearly as purposeless as mine. They hoped, by using fire and corrosive solvents, to refine, break up and recombine the component materials of the world, that they might produce other, more useful or valuable, materials. They were bold, and sometimes foolhardy, in the mixtures that they used and in testing the properties of the many new materials that they produced – but then, in the absence of a good guiding theory, they had to be. By taking such risks, they discovered the medicinal uses of many extracted plant materials, some of which we still use today. They purified antimony and other metals from their ores, manufactured calomel (sometimes still precribed as a purgative when I was a child), sal volatile (smelling salts), and caustic soda, and synthesised many of the pigments used by artists. Above all, they developed or refined most of the chemical processes that we still use, including amalgamation, crystallization, filtration, precipitation and distillation.

It is the last of these that is displayed in Wright’s painting. A retort is a spherical glass vessel provided with a long exit tube. The vessel, containing the boiling urine, is hot, but the tube is well away from the source of the heat, and remains cool, so that the phosphorus vapour condenses on it (this is why the tube is called a condenser). Brandt never did succeed in finding a practical use for the phosphorus that he had produced, but he did manage to restore his fortune by selling the secret of its manufacture to the German physician Johannes Daniel Krafft, who showed off the new wonder substance around the courts of Europe. The manner of its production did not remain secret for long. Before long, the secret had “leaked out” (to use the infelicitous phrase of one commentator), and others began to copy and improve the process. One who did so was Robert Boyle, the independently wealthy son of the Earl of Cork and founding member of the Royal Society. In 1680, 11 years after Brandt’s discovery, Boyle wrote a sealed memo where he described experiments in which he took “a considerable quantity of Man’s Urine” of which “a good part at least, had been for a pretty while digested before it was used”. It was then brought “to the consistence of a somewhat thick Syrup”, mixed with sand, and distilled under gradually increasing heat to produce phosphorus. The parallels with Brandt’s original process are striking, but nowhere in the memo (eventually published after his death in 1691) does Boyle acknowledge Brandt’s priority.

The production of phosphorus required the application of a fire “as intense as the Furnace was capable of giving”, a process that the alchemists called calcination. Fire was one of the strongest weapons in the armoury of the alchemists, and they discovered that many materials could be broken down by a sufficient application of heat. They also discovered that the action of fire could sometimes be enhanced if the materials were placed on a block of charcoal and the fire blown onto them through a blowpipe.

The charcoal block and blowpipe were still in regular use as analytical tools when I began to study chemistry. A particularly effective demonstration of their power was with the red powder that the alchemists called Mercurius praecipitatus, and which we know as mercuric oxide. When it was placed on a block of charcoal and heated with a blowpipe in our school chemistry classes, the hot flame from the blowpipe drove the oxygen to combine with the charcoal (carbon) to form carbon dioxide, which disappeared into the atmosphere, leaving shiny globules of mercury behind. I would have been less impressed by this demonstration had I known just how toxic mercury fumes can be. The old alchemists were particularly vulnerable to its effects, since mercury was believed to be one of the tria prima of substances from which all materials were composed, and the preparation of a supposedly special form of mercury called “philosophical mercury” was seen as an essential first step in the preparation of the philosopher’s stone.

It seems likely that the “severe nervous disorder” suffered by Newton in his later life was due to chronic mercury poisoning brought on by his alchemical experiments. In other cases, exposure to mercury fumes was deliberate. A common “cure” for syphilis in the seventeenth century consisted of placing the patient in a closed compartment, with only the head sticking out, and lighting a fire under a bowl of mercury over which the patient’s private parts were dangling. No wonder Brandt was so keen to find something that could simply be rubbed on.

The closed compartment was called the tub, and its use was so common that Shakespeare was able to make oblique reference to it in Measure for Measure and to be confident that his audience knew what he was referring to:

LUCIO: How doth my dear morsel thy mistress? Procures she still, ha?

POMPEY: Troth, sir, she has eaten up all her beef, and she is herself in the tub.

“Measure for Measure”, Act III, Scene 1

The effectiveness of the tub was judged by the amount of saliva produced by the victim. Three litres a day was regarded as a satisfactory amount; any less, and the length and strength of the treatment was increased.

Fire was not just used to heat mercury. It had many uses in alchemy, and almost all of them are still in use by chemists today. We use it, or its equivalent, in distillation “the separation of a volatile component from a substance by heating so as to drive off the component as a vapour which is condensed and collected in a cooler part of the apparatus.” We also use it in a variant of distillation invented by the alchemists and called “steam distillation” – a process in which plant materials are put in boiling water, with the steam carrying off the aromatic oils, which re-appear as a golden yellow layer floating on top of the condensate produced by cooling the steam.

Steam distillation was one of the many chemical processes that I tried at home as a child. I put some eucalyptus leaves in boiling water in a saucepan on our electric stove and catching the steam on the lid so that the droplets would run down into a glass, with a layer of “eucalyptus oil” floating on the top. I was gratified by the production of the oil, but not nearly as gratified as I was by the result of another experiment which concerned “waterglass”, a thick syrupy solution of sodium silicate whose main use was in preserving eggs. My science magic book, borrowed from the local library, suggested taking some unsuspecting person’s box of matches, dip the wooden stems in waterglass, allowing them to dry, and then replace them in the box. The unsuspecting person in this case was my father, who was a heavy smoker, but whose matches on this occasion simply refused to light, proving that sodium silicate is a very effective flame retardant, and also (from my father’s reaction) that it is sometimes wise to keep one’s mouth shut even when an experiment has turned out successfully.

Waterglass holds a special place in the history of chemistry. It was discovered by the German alchemist Johann Rudolph Glauber (after whom Glauber’s Salts are named). He manufactured it by fusing sand with sodium carbonate (i.e. washing soda) and then dissolving the glassy mass in water. A favourite (and, for once, safe!) experiment among schoolchildren of my generation was to put crystals of materials such as blue copper sulphate or red cobalt nitrate in the bottom of a bottle filled with waterglass. Over the course of several days, the crystals would produce fantastic growths that looked very much like coloured plants or corals.

The person who first performed this remarkable experiment was Glauber, and it was Glauber’s description of the results that stimulated Robert Boyle’s interest in chemistry. He became fascinated by this “Liquor in which all Metalls [sic] grow into Lovely Trees compos’d of Roote and Branches, & the usuall Parts constituent of those Plants” and performed many experiments with it. Boyle would later experiment with real coral which, he found, would produce gas bubbles when he poured vinegar onto it. The gas was carbon dioxide, one of Boyle’s truly original discoveries. It was produced from coral because coral is mostly calcium carbonate, which releases carbon dioxide when it is exposed to an acid (in this case, the acetic acid in the vinegar). Boyle found that the gas would not support the life of some unlucky flies that he put into it. He also discovered that it was heavier than air. My father once showed me how a bottle full of carbon dioxide could be “poured” over a candle to put it out (which it does for the same reason as it kills flies, by denying them oxygen). I used to delight in performing this simple trick as a child for the benefit of my assembled aunts and uncles.

The “corals” produced by placing crystals in waterglass are not carbonates, but silicates. Boyle later made out that he had discovered them, and failed to reveal his alchemical source. He also repeated Glauber’s erroneous claim that alluvial gold was produced in sand, again without acknowledgment. The closest that Boyle came to admitting his debt to alchemy was in his famous “Essay on Nitre”, which he said was “an attempt to make Chymical Experiments useful to Illustrate the Notions of the Corpuscular Philosophy.” In other words, according to Boyle, the old alchemists might have discovered a few things by accident, but it took a “Philosopher” such as himself to devise and interpret experiments that would reveal what was going on underneath, and in particular to show that materials are made up of very small particles that he called “corpuscles” and which we would now call atoms or molecules.

These acts of concealment were perpetuated by Boyle’s followers after his death. His biographer, Thomas Birch, carefully omitted Boyle’s alchemical correspondence from his list of collected works, and may even have destroyed some of it to preserve Boyle’s reputation as a “virtuoso uninfluenced by older ideas”. Lawrence Principe has assembled an impressive amount of evidence (including the Sennert story quoted above) which shows that both Boyle and modern chemistry owed a great deal to the alchemists. One thing that they owe is the idea of weighing materials before and after they have been subjected to a process. The idea was developed into a rule by the alchemist Joan Baptista van Helmont, who believed that however much weight goes into a reaction at one end must come out at the other end – a belief that we now know as the “Law of Conservation of Mass”, one of the most fundamental and useful laws of modern chemistry. Van Helmont’s most spectacular application of it was when he planted a willow tree weighing 5 pounds in a tub of earth that weighed 200 pounds. After 5 years of watering with specially pure water, the tree weighed 163 pounds and 3 ounces, while the earth still weighed very close to 200 pounds. Van Helmont concluded that the increase in weight was entirely due to water that had been transmuted into bark, leaves, etc. It was a very reasonable conclusion, and only much later did scientists discover that much of the increase in weight came from the absorption of atmospheric carbon dioxide, transmuted by the plant (with the aid of sunlight) into organic molecules such as cellulose and chlorophyll.

Boyle’s main debt was to one alchemist in particular – the Harvard-trained American George Starkey, whom Boyle met early in 1651, and whose recently discovered notebooks reveal just how close old alchemical practice was to modern chemical practice. Starkey pursued the doctrine that Boyle later claimed for his own, that “experimental results afford final judgment on the truth of conjectures”; in other words, fact is the death of hypothesis. Principe gives many examples where Starkey and other alchemists designed and performed critical experiments to test their hypotheses, just as Boyle did and as modern scientists still do. Starkey’s notes, unlike the directions presented in print by many of his alchemical contemporaries, describe his experiments so precisely that others can replicate them – another characteristic of modern science. One of the experiments that he described was the formation of a “Philosophical Tree” from gold and mercury. The process began with the mixing of liquid mercury and amorphous gold in the bottom of a flask, and continued with heating the mixture in a specified way that caused the two substances to amalgamate and form a beautiful feathery tree that filled the flask. Principe has been able to replicate the process by following Starkey’s directions, and there is no doubt that Boyle would have been able to do the same when Starkey sent a description of the experiment to him. Boyle’s early fascination with “Metalls that grow into lovely Trees” was thus renewed, and the unusual effect of the mercury on the gold may have been one of the stimuli that accelerated his efforts to produce “philosophical mercury”.

“Philosophical mercury” was not what we now understand as mercury – that silvery liquid that my schoolfellows and I used to chase around the laboratory bench and try unsuccessfully to pick up with our fingers. “Philosophical mercury” was supposed to be an especially pure and very active form of mercury, contained in all materials and responsible for their properties. The “philosophical mercury” extracted from gold, for example, was responsible for its gold-like properties, and would convey those properties to other materials to which it was added. No wonder the alchemists were anxious to get their hands on it!

One of the properties that alchemical texts claimed for “philosophical mercury” was that it grew hot when amalgamated with gold. Boyle obtained a sample from “the only Operator I trusted in the making of it” (probably George Starkey), but only ten years after Starkey’s death in 1665 did he eventually report that he had

“made trial of our Mercury, when I was all alone. For when no Body was by me, nor probably dreamt of what I was doing, I took to one part of the Mercury, sometimes half the weight and sometimes an equal weight of refin’d Gold reduced to a Calx or subtle Powder. This I put into the palm of my left hand, and putting the Mercury upon it, stirr’d it and press’d it a little with the finger of my right hand, by which the two Ingredients were easily mingled, and grew not only sensibly but considerably hot”

It is hard to know what to make of Boyle’s description. Modern measurements show that the amalgamation of gold with mercury absorbs a small amount of heat (rather than emitting it), so that the amalgam should become slightly cooler as it is formed. This suggests that the mercury may have contained an impurity, which Newton suggested could have given the gold “a greater shock, & so put it into brisker motion”. Modern chemistry (or, at least, the chemistry that I was taught) provides no clue as to what such an impurity might have been. It is unlikely that Boyle was fooling himself, but he published no information on how the mercury might have been made. Professor Principe says that “The mystery of the mercury has sufficed to make it an object of curiosity for three centuries” and has informed me in that he is using clues from Boyle’s private correspondence in an attempt to solve the mystery, with promising results.

Mystery or no, the “incalescence” that Boyle experienced when he performed the amalgamation experiment left him in no doubt that he had the key to the philosopher’s stone in his hand. Unfortunately, he didn’t know how to use it. This was why he was forced to breach the secrecy that usually enshrouded his alchemical activities, and to publish the result of his experiment. This action alarmed Sir Isaac Newton sufficiently for him to write to Boyle urging him to “preserve high silence” on this sensitive topic. But Boyle had no choice. Publication was his only way to attract the attention of the clandestine society of adepts whom Boyle believed to hold the secret of transmutation.

Publication, though, had its dangers, not the least of which was that Boyle might become identified as an alchemist himself. His answer was to publish his observations under a pseudonym, but the pseudonym that he chose was unbelievably transparent. It simply consisted of his initials in reverse order, and it was as “B.R.” that Boyle published his observations in the Philosophical Transactions of the Royal Society. He also took the unusual step of presenting his paper in two columns of parallel text, one in English, and the other in Latin, presumably on the assumption that all alchemists would be able to read Latin, even if their native language was not English. To make doubly sure, he also published his article in German, the native language of many of the alchemists.

It seems extraordinary that the man who set out in public to demolish alchemy was so bent on pursuing it in private. A part of the answer was that Boyle didn’t really set out to demolish alchemy, although later historians interpreted The Sceptical Chymist as the book which established a clear and immediate distinction between ancient, “unscientific” alchemy and modern scientific chemistry. Boyle was not challenging the alchemists’ observations and measurements, however; only their interpretation in terms of the tria prima. His interpretation of his own and the alchemists’ experiments was that materials are made up of elements, although his definition of an element was very far from the modern one. He wrote in The Sceptical Chymist that “I now mean by elements . . . certain primitive and simple, or perfectly unmingled bodies; which not being made of any other bodies, or of one another, are the ingredients of which all those called perfectly mixed bodies [chemical compounds] are immediately compounded, and into which they are ultimately resolved.” In other words, he believed that his “elements” were all aggregates of the same sorts of corpuscle, arranged in different ways, so that there seemed to be no barrier to the transmutation of one element into another by rearranging the corpuscles. The modern definition of an element also sees them as being constructed from “corpuscles” (at a simple level, electrons, protons and neutrons), but these are arranged with the protons and neutrons in a tiny core, surrounded by a cloud of electrons whose number (which is exactly equal to the number of protons ) defines which element it is. There is no possibility of transmutation unless the nucleus is disrupted by forces beyond the reach of chemists, whose effects are achieved only by manipulating the outermost of the electrons.

Boyle never did identify any particular material as an element. He even failed to identify hydrogen, the simplest of materials, as an element, although he was the first to discover this inflammable gas when he observed it bubbling up from the surface of some iron nails that he had put into dilute sulfuric acid. I improved on his experiment when I was at school, and listening to another of “Pop” Jane’s interminable rambles. I put some iron filings into dilute acid and added detergent to the mixture to capture the bubbles as they rose to the surface. Where I went wrong was in blowing the raft of bubbles through the air and towards a Bunsen burner, on the assumption that each individual bubble would explode with its own gentle pop. I should have foreseen that the whole lot would go off at once with a devastating bang, an event that brought the senior master running down the corridor, and which may have been one cause of “Pop’s” premature retirement.

Another cause for Pop’s retirement may have been the gradual diminution of the school’s stock of chemicals after a friend and I had volunteered to help in the school’s chemicals store. Our reward to ourselves was a tithe of every chemical, which we split 50:50. I was very envious when my friend was able to augment his stock with a one-pound jar of arsenic trioxide that his grandfather had kept to kill ants. We used some of it to develop our own chemical test for arsenic, on the assumption that we would both grow up to be chemical detectives. We also laid our hands on a small bottle of mercury, and tested its amalgamating properties on lumps of zinc, of which we had a plentiful supply.

We knew enough about the toxic properties of mercury vapour to perform our experiments in the open air, with the wind blowing away from us. Boyle recommended a similar approach to Starkey when the alchemist began to complain of “very horrid and seemingly Pestilential Symptomes” following experiments with mercury, antimony, and arsenic. Boyle’s suggestion was that Starkey should remove the glass from the windows in his laboratory. He would have been better advised to suggest that Starkey should be more careful, since the alchemist often had accidents where glasses and crucibles were spilled or cracked, with the contents covering the walls and floor of his furnace. Starkey’s financial position was such that he was forced to try to recover the material, which on one occasion involved disassembling the entire brick furnace

In the main, though, Starkey was a very careful experimenter. One of Boyle’s main reasons for pursuing the elusive “Philosopher’s Stone” was a painstaking demonstration in front of some of his friends in which Starkey succeeded in producing from antimony ore a material that looked very much like gold. It may even have been gold, since antimony ore often contains trace amounts of that element. Boyle also believed that he had himself achieved a form of reverse transmutation in his own laboratory, turning gold into silver by dissolving it in a highly corrosive mixture of nitric acid and antimony trichloride, and then fusing the resultant white powder with borax to produce silvery metallic globules. He called the acidic mixture his menstruum peracutum, or “universal solvent”, although a translation as “ultra-effective solvent” might be more accurate; if it was a truly universal solvent, it is hard to see what he could have kept it in. The metal that he produced was probably pure antimony, or an alloy of antimony and gold. One thing that we can be sure of is that it wasn’t silver, a pure element in its own right.

Boyle was persuaded by these experiments that he was on the track of the elusive “philosopher’s stone”, but the main reason for his pursuit of this goal seems to have been that he had seen a demonstration of the “stone” in action. According to his description, a traveling adept filled a crucible with lead in his presence, heated the crucible in a furnace until the lead had melted, and then cast some red powder on to the surface of the molten lead. The adept then covered the crucible and replaced it in the furnace for fifteen minutes. When it was removed and allowed to cool, the lead had gone, to be replaced by a mass of pure gold. From that day on, Boyle knew (or believed that he knew) that transmutation was possible, and set himself to discover the secret.

The event probably occurred in 1679, some four years after the publication of Boyle’s “philosophical mercury” paper, which seems to have achieved its intended purpose in attracting the attention of the charlatan. It seems hard to believe now that a scientist of Boyle’s stature could have been so easily deceived by what must have been a trick, but scientists are as gullible as anyone else when it comes to being persuaded by evidence for something that they want to believe in. Boyle never did realise that he had been fooled, and he never did discover what the itinerant alchemist’s secret was. Possibly it was a material that combined with silver, mercury or lead to produce something that looked like gold. Many of the alchemists’ preparations used sulphur at some stage, and many sulphides have a golden appearance. One such is iron pyrites (iron sulphide), which is known as “fool’s gold” because of its ability to fool the uninitiated eye. Chalcopyrite (copper iron sulphide) is also a mineral which has a brassy lustre, while antimony sulphide is the principal component of a golden pigment used by painters. There was plenty of scope, then, for the eye to be fooled by purely chemical changes in the alchemist’s mixture.

By 1689, Boyle must have believed himself to be very close to discovering the secret, because he sought and obtained the repeal of the three-hundred-year-old “Act Against Multipliers”, which outlawed transmutation “on pain of felony”. He did actually succeed in producing a red powder that he believed had some of the requisite properties, and when he died in 1691, an anxious Sir Isaac Newton wrote to his executor, asking if the powder had been discovered among Boyle’s effects. The executor, John Locke, unearthed the powder and sent it to Newton, but without any instructions for its use. Newton wrote again, asking for the “receipt”, which Locke also managed to find. It was heavily coded, but Newton seems to have decoded it and tried it out. His attempt failed, which is hardly surprising, and Newton was thereafter dismissive of the whole idea of “transmutation”.

Boyle’s recipe for transmutation were bound to fail, because the energies that he was using (those of fire or chemical change) were only powerful enough to affect the electrons at the surface of the atom. To achieve true transmutation, higher energies are needed that are sufficient to produce changes in the atom’s nucleus. To date, the only way to effect such changes has been to bombard the nucleus with energetic particles produced by nuclear reactions or by giant particle accelerators.

Scientists have now found that a sufficiently powerful laser can also be used to do the job. Professor Ken Ledingham at the University of Strathclyde has used this method to turn gold atoms into mercury atoms – a joke on the old alchemists, but one with a serious purpose, since he has now adapted the process to make radioisotopes for use in medical diagnosis (a similar process could also be used in principle to reduce the half-life of long-lived radioactive waste).

One of the goals of the ancient alchemists, that of transmutation, has thus been realized in a way that they could not possibly have foreseen. Their other goal – that of producing new materials by manipulating old ones – has been fulfilled many times by modern chemists, who owe a debt to Robert Boyle for emphasizing and codifying the need for reproducible, quantitative experimentation. It is a debt which is almost as great as that which Boyle and his modern successors owe to the ancient alchemists.

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