The Fontana History of Chemistry

But as Mr Boyle never design’d to write a body of philosophy, only to bestow occasional essays on those subjects whereto his genius or inclination led him; ‘tis not to be expected that even the most exquisite arrangement should ever reduce them to a methodical and uniform system, though they afford abundant material for one.

Despite Shaw’s defensive remark, there was in fact a system in Boyle’s ‘ramblings’. Elsewhere Shaw himself identified it when he referred to Boyle as ‘the introducer, or at least, the great restorer, of the mechanical philosophy amongst us’. This claim that Boyle had restored the mechanical philosophy had first appeared in one of Richard Bentley’s Boyle lectures, or sermons, several years earlier.

The mechanical or corpuscular philosophy, though peradventure the oldest as well as the best in the World, had lain buried for many ages in contempt and oblivion, till it was happily restored and cultivated anew by some excellent wits of the present age. But it principally owes its re-establishment and lustre to Mr Boyle, that honourable person of ever blessed memory who hath not only shown its usefulness in physiology (i.e. physics) above the vulgar doctrines of real qualities and substantial forms, but likewise its great serviceableness to religion itself.

By the mid seventeenth century there was no longer any conceptual difficulty involved in the acceptance of minute particles, whether atomic or (less controversially) corpuscular, which, though invisible and untouchable, could be imagined to unite together to form tangible solids. No doubt the contemporary development of the compound microscope by Robert Hooke and others helped considerably in stimulating the imagination to accept a world of the infinitely small, just as the telescope had banished certain conceptual difficulties concerning the possibility of change in the heavens. If only Democritus had a microscope, Bacon said, ‘he would perhaps have leaped for joy, thinking a way was now discovered for discerning the atom’.

Boyle’s corpuscles were neither the atoms of Epicurus and Gassendi, nor the particles of Descartes and the Cartesians. They were at once more useful and more sophisticated than either of them. Boyle’s mechanical philosophy was built on the principles of matter and motion. The properties of bulk matter were explained by the size, shape and motion of corpuscles, and the interaction of chemical minima naturalia (molecules), the evidence for which lay in chemical phenomena. Like Bacon and his fellow members of the Royal Society, however, Boyle always claimed to dislike and distrust ‘systems’.

It has long seemed to me none of the least impediments of true natural philosophy, that men have been so forward to write systems of it, and have thought themselves obliged either to be altogether silent, or not write less than an entire body of physiology.

Yet, while he disagreed with Cartesian physics, he seems to have felt that Descartes’ picture of the world as an integrated system, or whole, was a fruitful one. He agreed that there were no isolated pieces in Nature; that every piece of matter in the universe was continually acted upon by diverse forces or powers. The world was a machine, ‘a self-moving engine’, ‘a great piece of clockwork’ comparable to ‘a rare clock such as may be seen at Strasbourg’, then the engineering marvel of Europe. God was the clock-maker, the universe was the clock.

All this sounds like a ‘system’, as indeed it was. What Boyle meant by opposing systems, as such, was that they were usually based upon an a priori, experimentally indefensible set of hypotheses. They had usually been assembled from hypotheses that were not verae causae (true causes), as Newton was to call the kind of hypothesis that ought to be acceptable in natural philosophy.

We can see now why Boyle could accept a mechanical, corpuscular system of philosophy. The corpuscular philosophy was a vera causa, which could explain a tremendous range of diverse phenomena, and which could be experimentally defended. At the same time, it avoided and did away with ‘inexplicable forms, real qualities, the four peripatetick elements … and the three chymical principles’. Hotness, coldness, colour and the many secondary qualities and forms of Aristotelian physics were swept aside and explained solely in terms of the arrangements, agglomerations and behaviour of chemical particles as they interacted. Boyle’s assertion of the corpuscular philosophy was like Galileo’s claim that the book of Nature was written in mathematical terms. Boyle’s book was ‘a well-contrived romance’ of which every part was ‘written in the stenography of God’s omnipotent hand’, i.e. in corpuscular, rather than geometrical, characters. By revealing its design, like Gassendi and Charleton earlier, Boyle reconciled what had formerly been perceived as an atheistical system with religion and, indeed, with the tenets of the Anglican church that had become the re-established Church of England following the Civil War.

Boyle demonstrated the usefulness of chemistry not merely to medicine and technology (where it had long been accepted) but also to the natural philosopher, who had long despised it as the dubious activity of alchemists and workers by fire. Boyle aimed to show natural philosophers that it was essential that they took note of chemical phenomena, for the mechanical philosophy could not be properly understood otherwise. It was true, he admitted, that the theories of ordinary spagyrical chemists were false and useless; nevertheless, their experimental findings deserved attention, for if they could be disentangled from false interpretations, much would be found that would illustrate and support the corpuscular theory of matter.

In this way, Boyle strove to ‘begat a good understanding betwixt the chymists and the mechanical philosophers’. Chemists recognized him as a fellow chemist, even though he was a natural philosopher; while the natural philosophers recognized him as a respectable chemist because he was also a member of their company. By advocating a mechanical philosophy, Boyle would raise the social and intellectual status of ‘workers by fire’, reduce their proneness to secrecy and mysterious language, and make them into natural philosophers. As he wrote in another essay of 16612:

I hope it may conduce to the advancement of natural philosophy, if,… I be so happy, as, by any endeavours of mine, to possess both chymists and corpuscularians of the advantages, that may redound to each party by the confederacy I am mediating between them, and excite them both to enquire more into one another’s philosophy, by manifesting, that as many chymical experiments may be happily explicated by corpuscularian notions, so many of the corpuscularian notions may be commodiously either illustrated or confirmed by chymical experiments.

Boyle may be said to have united the proto-disciplines of chemistry and physics. But the partnership proved premature, for Boyle succumbed to the danger of not replacing the elements and principles of the chemists with a mechanical philosophy that was useful to the working chemist. This criticism can be most clearly made when discussing Boyle’s definition of the element in the sixth part of The Sceptical Chymist.

I now mean by elements, as those chymists that speak plainest do by their principles, 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 mixt bodies are immediately compounded, and into which they are ultimately resolved.

Leaving aside the fact that Boyle made no claim to be defining an element for the first time (as so many modern chemistry textbooks claim), in his next sentence he went on to deny that the concept served any useful function:

… now whether there be any one such body to be constantly met with in all, and each, of those that are said to be elemented bodies, is the thing I now question.

A modern analogy will make Boyle’s scepticism clear. If matter is composed ultimately of protons, neutrons and electrons, or, more simply still, of quarks, this, according to Boyle, should be the level of analysis and explanation for the chemist, not the so-called ‘elements’ that are deduced from chemical reactivity. To Boyle, materials such as gold, iron and copper were not elements, but aggregates of a common matter differentiated by the number, size, shape and structural pattern of their agglomerations. Although he clearly accepted that such entities had an independent existence as minima, he was unable to foresee the benefit of defining them pragmatically as chemical elements. For Boyle an ‘element’ had been irreversibly defined by the ancients and by his contemporaries as an omnipresent substance.

The seventeenth-century corpuscular, physical philosophy was all very well. It might explain chemical reactions, but it did not predict them, nor did it differentiate between simple and complex substances, the elementary and the compound. Nor, at this stage, did it align the supposed particles with weight and the chemical balance. Hence, although corpuscularianism was not overtly denied by later chemists, who were often content to accept it as an explanation of the physical character of matter, in chemical practice it was ignored. Chemists still needed the concept of an element and blithely returned to the four elements or to some other elementary concept. One thing had changed, however, as a result of Boyle’s criticisms. It was no longer possible to argue seriously that all of the possible elements, however many a chemist might postulate, were ubiquitously present in a particular material. Boyle’s scepticism suggested the possibility that some substances might contain less than the total number of elements; this made it possible for later chemists to be pragmatic about elements and to increase their number slowly and stealthily throughout the eighteenth century.

This more pragmatic view is seen clearly in Nicholas Lemery’s Cours de chymie (1675; English trans. 1686)3:

The word Principle in Chymistry must not be understood in too nice a sense: for the substances which are so-called, are Principles in respect to us, and as we can advance no further in the division of bodies; but we well know that they may be still further divided in abundance of other parts which may more justly claim, in propriety of speech, the name of Principles: wherefore such substances are to be understood as Chymical Principles, as are separated and divided, so far as we are capable of doing it by our imperfect powers.

This comes pretty close to Lavoisier’s operational definition of an element (Chapter 3).

It would be wrong to leave the impression that Boyle was a modern physical chemist, or, rather, chemical physicist. As a corpuscularian, Boyle had no difficulty in accepting the plausibility of transmutation of metals; indeed, a particle theory made ‘the alchymists’ hopes of turning other materials into gold less wild’. We know that Boyle took stories of magical events and of successful transmutations extremely seriously. In 1689 Boyle helped to secure the repeal of Henry IV’s Act against the multiplication of silver and gold, on the grounds that it was inhibiting possibly useful metallurgical researches. Throughout his life he investigated alchemists’ claims, albeit privately and cautiously and even secretly since, as recent research has shown, he clearly identified transmutation with the intervention of supernatural forces.

THE VACUUM BOYLIANUM AND ITS AFTERMATH

Boyle’s other principal contribution to natural philosophy was his investigation of the air, made possible by the invention of the air pump. The vacuum pump was first developed in Germany by Otto von Guericke, who demonstrated at Magdeburg in the 1650s how air could be pumped laboriously out of a copper globe to leave a vacuum. He then found that the atmosphere exerted a tremendous compressing force upon the globe. This was demonstrated theatrically in the famous Magdeburg experiment, which involved sixteen horses in trying to tear two evacuated hemispheres apart. Details of Guericke’s pumping system, which were published in 1657, rapidly awakened interest throughout Europe; for if a vacuum really was formed, this was prima facie evidence for the fallibility of Aristotelian physics and evidence in favour of the corpuscular philosophy.

Assisted by a young and talented laboratory assistant, Robert Hooke, Boyle built his own air pump in 1658 and began to investigate the nature of combustion and respiration with its aid. Some forty-three of his experimental findings, most of which he had had carefully witnessed by reputable friends and colleagues, were published in 1660 in New Experiments Physico-Mechanical touching the Spring of the Air and its Effects. Boyle’s law, linking pressure (the spring) and volume of the air, was developed from an experimental investigation provoked by a controversy after the book’s publication. For many years subsequently the British referred to the vacuum affectionately as the ‘vacuum Boylianum’.

Experiments with birds, mice and candles slowly led Boyle to conclude that air acted as a transporting agent to remove impurities from the lungs to the external air. (Incidentally, Boyle’s observation that insects do not die in a vacuum was confirmed in the twentieth century by Willis Whitney at the General Electric Company.) Like Helmont, Boyle never conceived of the air as a chemical entity; rather, it was a peculiar elastic fluid in which floated various reactive particles responsible for the phenomena of respiration, the rusting of iron, deliquescence and the greening of copper. On the other hand, Boyle clearly perceived that something in the air was consumed or absorbed during respiration and combustion, but he remained suitably cautious about its identification. His followers, including Hooke, who, as Curator of Experiments for the Royal Society, soon carved out an independent career for himself, were more confident.

During the English Civil War, Oxford was a Royalist stronghold. King Charles’ physician, William Harvey, who had demonstrated the circulation of the blood in 1628, was Warden of Wadham College, where he stimulated the development of co-operative investigations of physiology. The arrival of Boyle in Oxford in the 1650s further encouraged an interest in chemical questions among this community of undergraduates and Royalist exiles from London, including Richard Lower, John Mayow, John Wallis, John Wilkins and Christopher Wren. In 1659 Boyle hired an Alsatian immigrant, Peter Stahl, to teach chemistry publicly in Oxford. Those who were particularly interested in solving some of Harvey’s unanswered puzzles, including what happened to blood in the lungs or what was the origin of the blood’s warmth, took Stahl’s courses in the hope of finding chemical solutions. Among Boyle’s assistants at this time were Hooke and Mayow.

In the Micrographia (1665), a pioneering treatise on microscopy and many other subjects, Hooke developed a theory of combustion that owed something to the two-element acid-alkali theory of Sylvius, and even more to a widely known contemporary meteorological theory that was based upon a gunpowder analogy. According to this ‘nitro-aerial’ theory, thunder and lightning were likened to the explosion and flashing of gunpowder, whose active ingredients were known to be sulphur and nitre. By analogy, therefore, a violent storm was explained as a reaction between sulphureous and nitrous particles in the air. Since it was also known that nitre lowered the temperature of water and fertilized crops, it could be argued that the nitrous particles of air were probably responsible for snow and hail and for the vitality of vegetables. Such ideas can be traced back to Paracelsus and to alchemical writers such as Michael Sendivogius.

Hooke laid out his version of this theory in the form of a dozen propositions. He assumed that air was a ‘universal dissolvent’ of sulphureous bodies because it contained a substance ‘that is like, if not the very same, with that which is fixt in saltpetre’. During the solution process a great deal of warmth and fire was produced; at the same time, the dissolved sulphureous matter was ‘turn’d into the air, and made to fly up and down with it’.

The nitro-aerial theory received its fullest development in the writings of the Cornish Cartesian physician, John Mayow (1641–79) in Five Medico-Physical Treatises published in 1674. How much of his work was mere summary of the ideas of Boyle, Hooke and the Oxford physician, Richard Lower, has been the subject of dispute. Even if his work was syncretic, it was of very considerable interest and influence. Mayow used the theory to explain a very wide range of phenomena, including respiration, the heat and flames of combustion, calcination, deliquescence, animal heat, the scarlet colour of arterial blood and, once more, meteorological events. He showed that, when a candle burned in an inverted cupping glass submerged in water, it consumed the nitrous part of the air, which thereupon lost its elasticity, causing the water to rise. The same thing happened when a mouse replaced the candle.

Hence it is manifest that air is deprived of its elastic force by the breathing of animals very much in the same way as by the burning of flame.

Calcination involved the mechanical addition of nitro-aerial particles to a metal, which, as he knew from some of Boyle’s findings, brought about an increase of weight – an explanation also propagated by Mayow’s obscure French contemporary, Jean Rey. This explanation seemed confirmed by the fact that antimony produced the same calx when it was heated in air as when it was dissolved in nitric acid and heated.

Early historians of chemistry liked to find a close resemblance between Mayow’s explanation and the later oxygen theory of calcination. But it is only the transference properties that are similar. Quite apart from different theoretical entities being used in the two theories, we must note that Mayow’s theory was a mechanical, not chemical, theory of combustion. A more serious historiographical point is that Mayow’s theory essentially marked a return to a dualistic world of principles and occult powers. Sulphur and nitre now replaced the tria prima of Paracelsus.

Nitro-aerial spirit and sulphur are engaged in perpetual hostilities with each other, and indeed from their mutual struggles they meet, and from their diverse states when they succomb by turns, all changes of things seem to arise.

Neither Boyle nor Hooke appears to have referred to Mayow’s work in their writings. In any case, Boyle was sceptical of the ‘plenty and quality of the nitre in the air’.

For I have not found that those that build so much upon this volatile nitre, have made out by any competent experiment, that there is such a volatile nitre abounding in the air. For having often dealt with saltpetre in the fire, I do not find it easy to be raised by a gentle heat; and when by a stronger fire we distil it in closed vessels, it is plain, that what the chemists call the spirit of nitre (nitric oxide), has quite differing properties from crude nitre, and from those that are ascribed to the volatile nitre of the air; these spirits being so far from being refreshing to the nature of animals that they are exceeding corrosive.

Despite the speculative character of the nitro-aerial theory, there is much to admire concerning Mayow’s experimental ingenuity. Although he did not develop the pneumatic trough, he devised a method for capturing the ‘wild spirits’ that Helmont had found so elusive by arranging for pieces of iron to be lowered into nitric acid inside the inverted cupping glass. As we can see, however, the results were inevitably baffling to Mayow, for although the water level in the cup eventually rose (as the nitro-aerial theory predicted), it was initially depressed. (Insoluble hydrogen would have been the first product of this displacement reaction; secondary reactions would have then produced soluble nitrogen dioxide.)

NEWTON’S CHEMISTRY

Newton’s interest in chemistry was life-long and reputedly aroused when, as a schoolboy at Grantham Grammar School, he lodged with an apothecary. He wrote only one overtly chemical paper, but important and influential chemical statements are to be found in the Principia Mathematica (1687) and the Opticks (1704). As mentioned in Chapter 1, there also exist in manuscript thousands of pages of chemical and alchemical notes, much of them identifiable as transcriptions from contemporary printed or manuscript works. Newton seems to have been interested in both exoteric and esoteric alchemy, that is, his interest extended beyond the empirical and experimental information that might be gleaned from alchemical texts to the ‘mysteries’ and secrets that were imparted in metaphor and allegory.

Newton was principally influenced by Helmont and Boyle; he also found the nitro-aerial theory attractive as a sustaining principle reminiscent of Helmont’s blas.

I suspect, moreover, that it is chiefly from the comets that spirit comes, which is indeed the smallest but the most subtle and useful part of the air, and so much required to sustain the life of all things with us.

And in the Principia Newton more than hinted that all matter took its origin in water.

The vapours which arise from the sun, the fixed stars, and the tails of comets, may meet at last with, and fall into, the atmospheres of the planets by their gravity, and there be condensed and turned into water and humid spirits; and from thence, by a slow heat, pass gradually into the form of salts, and sulphurs, and tinctures, and mud, and clay, and sand, and stones, and coral, and other terrestrial substances.

Nature was a perpetual worker; all things, he wrote in the Opticks, grow out of, and return by putrefaction into, water.

Nevertheless, Newton subscribed wholeheartedly to Boyle’s corpuscular philosophy, to which he added the mechanisms of attraction and repulsion to explain not merely the gravitational phenomena of bulk planetary matter, but also the chemical likes (affinities) and dislikes (repulsions) that individual substances displayed towards one another. Such inherent powers of matter, which Newton attributed to a subtle ether that bathed the universe, replaced the astral influences of Paracelsus and the blas of Helmont as the causes of motion and change. Newton made this the subject of his only published chemical paper, ‘De natura acidorium’, written in 1692 but not published until 1710, as well as the ‘Queries’ 31 and 32 of the 1717 edition of the Opticks. In these writings Newton suggested that there were exceedingly strong attractive powers between the particles of bodies, which extended, however, only a short distance from them and varied in strength from one chemical species to another. This hypothesis of short-range force led him to speculate about what eighteenth-century chemists called ‘elective affinities’ and the reason why, for example, metals replaced one another in acid solutions. He gave the replacement order of six common metals in nitric acid.

The investigation of chemical affinity became one of the absorbing problems of chemistry. In 1718, Étienne Geoffroy (1672–1731) produced the first table of affinities, and more elaborate ones were produced by Torbern Bergman (1735–84) and others from the 1750s onwards. As the Newtonian world picture grew in prestige, chemists and natural philosophers also began to interpret these tables in terms of short-range attractions. In 1785 Buffon even identified the laws of affinity with gravitational attraction; but all attempts to satisfy what has been described as the ‘Newtonian dream’ to mathematize (i.e. quantify) affinity ended in failure. It was left to Claude Berthollet to point out in 1803 that other factors, such as mass (concentration), temperature and pressure, also decided whether or not a particular reaction was possible.